The Geologic Basis for a Reconstruction of a Grounded Ice Sheet in Mcmurdo sound,
Antarctica, at the Last Glacial Maximum
Author(s): G. H. Denton and D. R. Marchant
Source: Geografiska Annaler. Series A, Physical Geography, Vol. 82, No. 2/3, Glacial and
Paleoclimatic History of the Ross Ice Drainage System of Antarctica (2000), pp. 167-211
Published by: Blackwell Publishing on behalf of the Swedish Society for Anthropology and
Geography
Stable URL: http://www.jstor.org/stable/520991
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THE GEOLOGIC BASIS FOR A RECONSTRUCTION OF
A GROUNDED ICE SHEET IN MCMURDO SOUND,
ANTARCTICA, AT THE LAST GLACIAL MAXIMUM
BY
G.H. DENTON' AND D.R. MARCHANT2
1
Department of Geological Sciences and Institute for Quaternary Studies,
Bryand Global Sciences Center, University of Maine, Orono, Maine, USA
2Department of Earth Sciences, Boston University,
Boston, Massachusetts, USA
Denton, G.H. and Marchant, D.R., 2000: The geologic basis for
a reconstruction of a grounded ice sheet in McMurdo Sound, Antarctica, at the last glacial maximum. Geogr. Ann., 82 A (2-3):
167-211.
ABSTRACT. A grounded ice sheet fed from the Ross Embayment filled McMurdo Sound at the last glacial maximum (LGM).
This sheet deposited the little-weathered Ross Sea drift sheet, with
far-traveled Transantarctic Mountains (TAM) erratics, on lower
slopes of volcanic islands and peninsulas in the Sound, as well as
on coastal forelands along the TAM front. The mapped upper limit of this drift, commonly marked by a distinctive moraine ridge,
shows that the ice-sheet surface sloped landward across McMurdo Sound from 710 m elevation at Cape Crozier to 250 m in the
eastern foothills of the Royal Society Range. Ice from the Ross
Embayment flowed westward into the sound from both north and
south of Ross Island. The northern flowlines were dominant, deflecting the southern flowlines toward the foothills of the southern
Royal Society Range. Ice of the northernflowlines distributed distinctive kenyte erratics, derived from western Ross Island, in Ross
Sea drift along the TAM front between Taylor and Miers Valleys.
Lobes from grounded ice in McMurdo Sound blocked the mouths
of TAM ice-free valleys, damming extensive proglacial lakes. A
floating ice cover on each lake formed a conveyor that transported
glacial debris from the grounded ice lobes deep into the valleys to
deposit a unique glaciolacustrine facies of Ross Sea drift.
The ice sheet in McMurdo Sound became grounded after
26,860 14C yr BP. It remained near its LGM position between
23,800 14Cyr BP and 12,700 14Cyr BP. Recession was then slow
until sometime after 10,794 14Cyr BP. Grounded ice lingered in
New Harbor in the mouth of Taylor Valley until 8340 14Cyr BP.
The southward-retreatingice-sheet grounding line had penetrated
deep into McMurdo Sound by 6500 14Cyr BP.
The existence of a thick ice sheet in McMurdo Sound is strong
evidence for widespread grounding across the Ross Embayment
at the LGM. Otherwise, the ice-sheet surface would not have
sloped landward, nor could TAM erratics have been glacially
transportedwestward into McMurdo Sound from fartheroffshore
in the Ross Embayment.
Introduction
This paper presents glacial-geologic and chronologic data from the McMurdo Sound region (Figs
1 and 2) that constrain the areal distribution, surGeografiska Annaler - 82 A (2000) · 2-3
face elevation, and chronology of ice grounded in
the western Ross Sea during the last glacial maximum (LGM). Place names used in the text are
shown in Figs 1, 2, and 12.
McMurdo Sound is the only region along the
Transantarctic Mountains (TAM) front where ice
grounded in the Ross Embayment at the LGM
flowed landwards, lapping onto volcanic islands in
the sound and terminating in arcuate lobes that projected into coastal ice-free valleys. This westwardflowing grounded ice left extensive drift sheets and
well-developed moraine ridges on the volcanic islands. It produced pronounced moraines on coastal
headlands of the TAM, and it dammed proglacial
lakes in ice-free valleys that opened to the west
coast of McMurdo Sound (Hendy et al. 2000; Hall
et al. 2000). Radiocarbon-dated algal beds from
drained kettles and from deltas that formed at the
margin of proglacial lakes in Taylor Valley (Hall
and Denton 2000a) and in the Royal Society foothills (Clayton-Greene et al. 1988) provide a chronology for this ice sheet. The overall results in this
paper afford a background for interpreting the glacial chronology of eastern Taylor Valley (Hall and
Denton 2000a) and for reconstructing a grounded
ice sheet in the overall Ross Embayment at the
LGM (Denton and Hughes 2000).
Geographic setting
The TAM form the faulted, uplifted, and tilted
shoulder of the asymmetric West Antarctic intracontinental rift system (Figs 1 and 2). Near western
McMurdo Sound they rise steeply from the coast to
elevations of 2000 m (Dry Valleys block) to 4000
m (Royal Society block). The TAM feature gently
tilted sedimentary units (Devonian-Triassic Bea167
G.H. DENTON AND D.R. MARCHANT
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Fig. 1. Locality map of the Ross Embayment. DR, Dominion Range; TV, Taylor Valley; MS, McMurdo Sound; TNB, Terra Nova
Bay; VLB, Victoria Land Basin.
168
Geogratiska Annaler - 82 A (2000) - 2-3
RECONSTRUCTION OF GROUNDED ICE SHEET
8°S
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0
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Fig. 2. Index map of the McMurdo Sound region.
Geografiska Annaler · 82 A (2000) · 2-3
169
G.H. DENTON AND D.R. MARCHANT
con Supergroup) that overlie Precambrian-Devonian basement and are intruded and capped by
Jurassic dolerite and basalts (Ferrar Supergroup).
McMurdo Sound features volcanic islands (Ross,
White, Black) and peninsulas (Brown and Mount
Morning/Mount Discovery/Minna Bluff), all part
of the Cenozoic-age McMurdo Volcanic Group.
The volcanic bedrock includes subaerial lava flows,
agglomerates, flow breccia, and tuffs. Mount Erebus on Ross Island is the only volcano still active.
Typical water depths in McMurdo Sound range
from 500 to 800 m.
The extensive ice-free areas in the McMurdo
Sound region are due to two major factors. First,
headward cutting of huge East Antarctic outlet glaciers that pass through the TAM to the north and
south has captured much of the inland ice that once
flowed into the Dry Valleys. As a result, only the local Taylor Dome now feeds small outlet tongues
into the western Dry Valleys. Ice from Taylor Dome
also flows southeastward into Skelton Glacier and
northeastwardinto Mackay Glacier outlet (Drewry
1982). The second important factor was the emplacement of the high Mount Morning/Mount Discovery/Minna Bluff volcanic complex, which extends from the southernmost Royal Society Range
of the TAM eastward into the Ross Embayment.
This volcanic topographic barrier,unique along the
TAM front, deflects the northward-flowing Ross
Ice Shelf aroundMcMurdo Sound and eastern Ross
Island.
The overall result is that the inland ice sheet and
the Ross Ice Shelf now feed very little ice into McMurdo Sound. This leaves the prevailing southwesterly winds as the major factor in determining
the distribution of local glaciers and ice-free areas.
In passing across major mountain barriers, these
winds deposit snow on the upwind (southwestward-facing) slopes and ablate snow and ice from
downwind (northeastward-facing) slopes and valleys; most of this ablation is from sublimation by
adiabatically warmed air descending the lee slope
of topographic obstacles. In the Dry Valleys the
prevailing winds produce a patchwork of glaciers,
with surface blue-ice ablation regions determined
by topography and elevation. The largest complex
is the combined Taylor and FerrarGlaciers, which
have extensive blue-ice areas downwind from the
high Quartermain Mountains and Royal Society
Range. Within the interior valleys, the wind pattern
is important in determining the pattern of snow accumulation and blue-ice ablation areas on smaller
alpine glaciers.
170
In the southern McMurdo Sound region, the prevailing winds promote snow accumulation on the
upwind (southwest) flanks, and extensive ice-free
areas on downwind (northeast) slopes of major
topographic barriers. Thus the ice-free eastern
foothills of the Royal Society Range are downwind
from the high topographic barrierbetween Mount
Rucker and upper Koettlitz Glacier. Likewise, the
Blue Glacier piedmont system has extensive blueice ablation areas.
The prevailing southwesterly winds leave a surface ablation zone on the floating Koettlitz Glacier
tongue in the lee of Mounts Morning and Cocks.
Brown Peninsula, the northernslope of Mount Discovery, and the surface dirty-ice zone of the McMurdo Ice Shelf all lie downwind of Mount Discovery. Descending southwesterly winds leave the
north slope of Minna Bluff largely ice free and create blue-ice ablation stripes on the adjacent ice
shelf. Southwesterly winds promote snow accumulation on the south-facing slope of Black Island, but
leave the northern slope ice free and create an ablation zone on the ice shelf north of the island. Likewise, White Island is ice covered on the windward
side and has ice-free terrainon the leeward side. Although Ross Island is largely ice covered, ice-free
terrain occurs at Capes Barnes, Royds, Evans, and
Crozier. There is also an extensive ice-free coastal
area on the western flank of Mount Bird south of
Cape Bird. As did Dochat et al. (2000), we here refer to this ice-free area as Cape Bird, even though
such a designation is not strictly correct.
The blue-ice ablation zones downwind from
topographic obstacles cause striking differences in
the character of ice shelves floating on southern
McMurdo Sound and the adjacent Ross Sea. The
northward flow of the western Ross Ice Shelf is
largely deflected east of Ross Island by Minna
Bluff, although some ice leaks around the end of
the bluff and then westward toward the sound. The
calving front of the Ross Ice Shelf extends from
Cape Crozier on Ross Island eastward across the
Ross Embayment. The surface of the thick Ross Ice
Shelf is an accumulation area. In sharp contrast,
much of the surface of the contiguous McMurdo
Ice Shelf has extensive ablation areas because it lies
downwind of volcanic topographic barriers.
Glacial deposits
The extensive ice-free areas produced by the combination of topography and prevailing winds make
the McMurdo Sound region the most important loGeografiska Annaler · 82 A (2000) · 2-3
OF GROUNDEDICESHEET
RECONSTRUCTION
cality along the TAM front for investigating the
configuration of grounded ice in the Ross Embayment at the LGM. The former ice extent can be reconstructed from the areal distribution of drift and
moraine ridges. Former flow directions can be determined from the slope of bounding moraines and
from the distribution of erratics. Because the offshore islands and peninsulas are composed of
Cenozoic-age volcanics, the tracing of erratics is a
powerful tool for differentiating onshore from offshore flow along the western McMurdo Sound
coastline at the LGM.
Unconsolidated and poorly sorted diamictons
mantle lower slopes of volcanic islands and peninsulas in McMurdo Sound, as well as the mouths of
ice-free valleys of the TAM that open to the western
McMurdo coast. Some diamictons are local in origin, derived from mass wasting of volcanic bedrock. These diamictons lack exotic lithologies, striated clasts, and reworked marine shells. Other
diamictons are glacial drifts that feature 5-10% exotic clasts, include lenses of glacial ice, and at some
localities contain fragmented marine shells. These
glacial drifts are subdivided by surface and internal
weathering characteristics, outcrop pattern, petrology, and, in some cases, by stratigraphicposition.
In places, we recognize two (or more) distinct drifts
on the flanks of volcanic islands/peninsulas in McMurdo Sound. The youngest is a ubiquitous low-elevation drift that commonly extends without morphological breaks from the coast or ice shelf up to
a limiting moraine ridge or erratic line. The second
is an older, high-elevation drift(s) that consists of
isolated and weathered erratics above the upper
limit of the low-elevation drift. Because they are
rich in non-volcanic erratics, these drifts could not
have been produced by expansion of local alpine
glaciers and ice caps on volcanic islands and peninsulas in McMurdo Sound. Below, we describe the
areal distribution, texture, and mineralogy of these
drifts.
Minna Bluff
This long (30 km) and narrow (5 km) volcanic peninsula extends seaward from Mount Discovery for
nearly 40 km into the Ross Embayment. Maximum
elevations along the crest of Minna Bluff are between 800 and 1000 m. Minna Saddle at about 600
m elevation separates Minna Bluff from adjacent
Mount Discovery. Minna Bluff is a significant obstacle for the northward-flowing Ross Ice Shelf
(Figs 2 and 3).
Geografiska Annaler · 82 A (2000) · 2-3
The southern flank of Minna Bluff is obscured
beneath coalescing alpine glaciers, perennial snow
fields, and the Eady Ice Piedmont. In contrast, the
northern flank of Minna Bluff features three separate regions of predominantly ice-free terrain,each
more than 10 km2 in area. An unweathered and erratic-rich drift mantles volcanic bedrock in each of
these three areas. Midway between Minna Saddle
and the eastern tip of Minna Bluff, the drift extends
unbroken from the ice shelf up to a sharp moraine
ridge at 637 m elevation (Fig. 3). This moraine
ridge extends laterally for a distance of 7 km and
ranges from 590 m to 637 m elevation. Likewise,
about 5 km east of Minna Saddle, a nearly continuous drift sheet extends from the ice shelf up to
(and underneath) the terminus of coalescing alpine
glaciers at about 490 m elevation. Finally, at about
15 km northwest of the tip of Minna Bluff, an unweathered and erratic-rich drift extends continuously from the ice shelf up to an erraticboulder line
between 560 and 630 m elevation.
At each locality the younger, lower drift has similar texture, composition, and morphologic form. It
comprises subrounded cobbles of granite, dolerite,
gneiss, and sandstone, all set within a matrix of
loose, unoxidized, and slightly stratified sand. The
drift at all mapped localities exhibits well-developed constructional morphology and commonly
includes an ice core. Far-travelederratics lack ventifaction, pitting, and quartzifiedrinds; about 5% of
the surface clasts are striated.The frequency of surface erratics is between 100 and 200 clasts/150 m2.
Although they are separated by alpine glaciers
and perennial snowbanks, all three drift patches on
northernMinna Bluff are correlated on the basis of
similar surface weathering texture, mineralogy,
morphologic form, and erratic boulder frequency.
Collectively, these drift patches represent deposition by glacier ice that reached from the McMurdo
Ice Shelf up to at least 637 m elevation on northern
Minna Bluff. The limited exposure of the drift
patches precludes reconstruction of a detailed icesurface profile.
A second and higher glaciogenic deposit made up
solely of isolated and scattered erratic cobbles occurs on the east flank of a large cinder cone 15 km
west of the outer tip of Minna Bluff. This cinder
cone is the only place on Minna Bluff where we
found glacial erratics above the sharp drift limit described previously. The scattered erratics include
gneiss, granite, and dolerite resting on cavernously
weathered volcanic bedrock. The frequency of erratic cobbles is less than 1 clast/150 m2 (Table 1).
171
G.H. DENTON AND D.R. MARCHANT
Fig. 3. Geologic map of the southern McMurdo Sound region, showing the McMurdo Ice Shelf, the northern
flank of Mount Discovery, Brown Peninsula, Black Island and Minna Bluff. See Fig. 12 for location.
172
Geografiska Annaler . 82 A (2000)
2-3
RECONSTRUCTION OF GROUNDED ICE SHEET
Table 1. Physical characteristics of Ross Sea Drift (RSD).
Location
Areal extent and morphology
Upper-elevation limit
Lithology
Soil development
Minna
Bluff
RSD forms a nearly continuous drift sheet
on the north flank of Minna Bluff; in places, this little-weathered drift with kame
and kettle topography is concealed beneath alpine glaciers and perennial snow
banks. Midway between Minna Saddle
and the eastern tip of Minna Bluff, the
drift sheet extends unbroken from the iceshelf surface to a sharp moraine ridge that
extends laterally over a distance of 7 km.
This moraine ridge marks the upper limit
of RSD on Minna Bluff. An older drift,
comprising largely weathered erraticcobbles on volcanic bedrock and rubble,
crops out above RSD.
Maximum limit of RSD on
Minna Bluff is 637 m. Maximum limit of the older drift on
Minna Bluff (represented by
isolated, caverously weathered erratics on weathered volcanic bedrock 15 km west of
the outer tip of Minna Bluff) is
810 m elevation.
Erratics in RSD include
dolerite, granite, and
gneiss; 5% are striated.
Surface concentration
averages 150-200 erratics/150 m2. On the older
drift, surface erratics
average <1/150 m2.
Lithologies in RSD and
the older drift are the
same.
RSD shows a loose,
sandy, volcanic matrix with little oxidation; in places, it is
slightly stratified;it is
commonly cemented
by ice at depths >35
cm. It lacks visible
salts. The older drift
shows a greater degree of oxidation
(where matrix sands
are present) and cavernous weathering
than RSD.
Mount
RSD forms a continuous drift sheet (> 1 m
Discovery thick) that extends from the ice-shelf surface up to a sharp maximum limit at 519
m elevation. From this maximum limit
(about 2 km north of Minna Saddle), the
drift limit descends westward and can be
traced laterally for over 20 km. On all but
the west flank of Mt Discovery, this drift
is commonly cored by ice and shows extensive thermokarst and post-depositional gelifluction and soliflution. Surface erratic stripes trend east-west on the southeastern flank of Mt Discovery; several
stripes cross the tide crack and pass onto
the McMurdo Ice Shelf.
RSD reaches a maximum elevation on the SE flank of Mt
Discovery at 519 m. Here,
RSD overlies an older drift
with a sharp, planar contact.
From this limit at 519 m elevation, RSD descends to 327 m
on the northernflank of a bedrock ridge opposite Brown Peninsula Saddle and to 256 m
elevation on the west flank of
Mt Discovery east of Hahn Island. The older drift on Minna
Bluff reaches a maximum elevation of 810 m. Just as the
younger RSD, the upper limit
of this older drift can be traced
laterally for 20 km and slopes
from east to west, from 810 m
elevation opposite Minna Saddle to 463 m elevation along
the western slope of Mt Discovery.
Maximum limit of RSD on
Brown Peninsula is 365 m elevation (east-facing slope of a
cinder cone 1 km north of
Brown Saddle). In general, the
upper limit elevation ranges
between 340 m and 360 m on
the east flank of Brown Peninsula and between 245 m and
265 m on the western flank of
the peninsula. An older drifton
Brown Peninsula reaches a
maximum of 400 m elevation
(where it crops out above the
maximum limit of RSD at 360
m elevation). Unlike RSD, this
older drift lacks a well-defined
moraine limit.
Erraticsin RSD include
little weathered granite,
dolerite, sandstone, and
marble. The frequency
of surface erratics averages 100/150 m2.On the
older drift, erratics
show some cavernous
weathering and, in places, well-developed
desert varnish and ventifaction. The frequency
of surface erratics on
the older drift ranges
from 10-100/150 m2.
RSD shows a sandy,
volcanic matrix with
little oxidation. Ice is
common at 35-50 cm
depth; there are no
visible salts. On the
western flank of Mt
Discovery, RSD
lacks matrix sands
and instead consists
solely of erratics on
volcanic regolith.
Erratics in RSD include
granite, sandstone, and
dolerite; 5% show striations and smooth, glacially polished surfaces.
Surface clast concentration averages 150-200
erratics/150 m2. Erratics on older drift show a
surface concentration of
about 50/150 m2; these
erratics show pitting
and rest on weathered
volcanic bedrock.
RSD shows loose,
sandy volcanic matrix with little oxidation; slightly stratified in places; no visible salts; silt concentrationdecreases with
increasing elevation.
Older drift shows
weathered surface erratics, modified by
wind abrasion, salt
weathering and
desert varnish.
Brown
Peninsula
RSD forms a continuous drift sheet (>1 m
thick in places) that extends from the iceshelf surface up to a well-defined moraine
ridge that nearly encircles Brown Peninsula. The elevation and morphology of
this moraine is thickest (>3 m), widest (>4
m) and highest (>320 m) on east-facing
slopes. Surface stripes of concentrated erratics trend nearly north-south on the
western flank of Brown Peninsula; at
their southern margin, the stripes curve
westward across Brown Saddle. Several
stripes pass from Brown Saddle to the
McMurdo Ice Shelf, where they form part
of the ablation till on the ice-shelf surface.
Geografiska Annaler · 82 A (2000) · 2-3
173
G.H. DENTON AND D.R. MARCHANT
Location
Areal extent and morphology
Upper-elevation limit
Lithology
Soil development
Black
Island
RSD forms a nearly continuous drift sheet
that covers two-thirds of the ice-free region on Black Island. The drift extends
from the ice-shelf surface up to a sharp
upper limit that is commonly markedby a
well-defined moraine ridge. The drift limit is highest on east-facing slopes and dips
westward around cinder cones. On the
western flank of Black Island the drift
limit is marked by a continuous moraine
ridge (2-3 m in relief) that is 3.5 km long;
a narrow delta borders this moraine ridge
northwest of Mt Aurora. At Scallop Hill,
this drift overlies glacially striated and
molded volcanic bedrock that indicates
ice flow was from southeast to northwest
at about 15°W. The drift is commonly
cored by ice and shows extensive
thermokarst.
At the northeastern margin of
Black Island about 4.5 km SSE
of Mt Melania, RSD reaches a
maximum upper limit at 522 m
elevation. Here, a flat-topped
moraine ridge (10 m wide)
wraps around the base of two
cinder cones. This same drift
reaches a maximum of 390 to
400 m elevation on the western
flank of Black Island. An older
drift, consisting of discrete
patches of volcanic rubble
with 1% erratic cobbles, is exposed above RSD on the
northeasternflank of Mt Aurora. This older drift lacks a welldefined upper limit. Weathered erratics associated with
this drift reach a maximum elevation of 710 m.
Erraticsin RSD include
little-weathered granite,
sandstone, and dolerite.
Striated cobbles are
common within drift at
Scallop Hill. Concentration of surface erratics in RSD ranges between 35 and 50 clasts/
150 m2. Concentration
of surface erraticson the
older driftranges from 5
to 15 clasts/150m2.
RSD shows loose,
sandy, volcanic matrix with little oxidation. Ice common at
>50 cm depth; no visible salts. On western
flank of Black Island
RSD is commonly
stratified and, near
the upper limit, is
interbedded with discontinuous algal
mats. Older drift
shows greater oxidation and surface
weathering, and lacks
associated algal mats.
White
Island
RSD crops out in the lee of topographic
obstacles such as at Mt Henderson, Isolation Point, Mt Hayward, and Mt Heine;
elsewhere White Island is largely covered
with glacial ice. The largest exposure of
RSD occurs on the NE slope of Mt Heine,
where an unbroken sheet without morphological breaks or recessional moraines extends from the margin of perennial snowbanks near the ice shelf up to
561 m elevation.
Maximum limit of RSD is 561
m elevation on the surface of a
prominent platform between
two flat-topped cinder cones
about 500 m east of the summit
of Mt Heine. A line of erratic
cobbles and boulders on volcanic-rich colluvium marks
the upper limit. Erraticsdo not
crop out on White Island
above 561 m elevation (although if present they may be
buried beneath glacier ice).
Erratics in RSD include
granite, gneiss, sandstone, and dolerite. Striated cobbles are rare.
Surface concentration
ranges from 20 to 100
erratics/150 m2.
Cape
Crozier
RSD at Cape Crozier is thickest at the
base of broad valleys between 300 and
500 m elevation. It reaches a diffuse upper limit, without a moraine ridge or sharp
line of erraticboulders, at 710 m elevation
in the center of a broad, east-west trending valley near the center of the ice-free
area at Cape Crozier. In places, the upper
limit is delineated by patterned ground
(patterned ground is well-developed on
the drift surface, but less developed/absent above and beyond RSD). At the base
of steep cinder cones, RSD shows extensive solifluction and gelifluction. RSD is
thickest (>1 m) between 350 and 650 m
elevation. An older drift comprising
largely weathered erratic cobbles crops
out above RSD to 825 m elevation.
Unlike nearby sites, the upper
limit of RSD on Cape Crozier
lacks a well-defined upper moraine. The maximum limit of
RSD is placed at the highest
unweathered erratics associated with matrix sands; this
maximum limit is at 710 m elevation and lies parallel with
the coastline. An older drift
crops out as isolated erratics
up to 825 m elevation near the
rim of Bomb Peak.
174
RSD shows loose,
granular volcanic
matrix with little oxidation. Shallow icecemented sands are
common below 300
m elevation. RSD on
White Island is thickest near its upper limit
and thins to <30 cm
on steep, volcanic
slopes. Ice-free terrain above 650 m elevation shows cavernous weathering.
Erratics in RSD include RSD shows a loose,
granite, dolerite, sand- sandy volcanic mastone, and marble; stria- trix; fines increase in
tions are common in
matrix with decreasdrift between 350 and
ing elevation; RSD at
650 m elevation. The
Cape Crozier lacks
frequency of surface er- visible salts. The oldratics is between 25 and er drift lacks matrix
50/150 m2, increasing
sands and instead
consists of weathered
downslope. The freerratics on volcanic
quency at the mapped
upper limit of the older bedrock and oxidized
drift is less than 1 clast/ regolith.
150 m2. Erratics on the
upper drift (granite and
dolerite) show cavernous weathering and extensive wind abrasion.
Geografiska Annaler
82 A (2000) · 2-3
RECONSTRUCTION OF GROUNDED ICE SHEET
Location
Areal extent and morphology
Upper-elevation limit
Lithology
Soil development
Cape
Bird
RSD mantles most of the ice-free terrain
at Cape Bird. North of Shell Glacier, the
drift extends from near sea level up to
(and underneath) the edge of an ice cap at
590 m elevation. In places, the drift reaches a maximum thickness of 30 m; it is
commonly ice cored and shows active
thermokarst and solifluction. In streamcut sections, RSD shows alternating layers of gravel, sand, and mud that fine upward to the top of the deposit. There is no
discernible break in weathering characteristics or in surface morphology from
sea-level to the upper drift limit.
RSD crops out as isolated erratics near
Backdoor Bay, Horseshoe Bay, and at
Cape Bare. Southeast of Backdoor Bay,
these erratics are admixed with unoxidized, fine- to coarse-grained sands and
gravel. Conical mounds of RSD (up to 2
m high) dot the landscape southeast of
Backdoor Bay. These mounds have a core
of ice and a veneer (up to 75 cm thick) of
unweathered, stratified RSD.
Thermokarst is common. RSD at Cape
Royds and Cape Bare lacks a well-defined moraine at its upper-elevation limit.
Maximum limit of RSD at
Cape Bird is at least 590 m elevation. At 590 m elevation,
RSD passes beneath the margin of an ice cap centered on
Mt Bird; thrust moraines
alongside the ice cap have reworked RSD. RSD south of
Shell Glacier crops out as discontinuous patches that, as is
the case northof Shell Glacier,
pass beneath the margin of the
ice cap centered on Mt Bird.
Erraticsin RSD include
granite, sandstone, marble, dolerite and trace
amounts of kenyte (five
mapped clasts); 2 to 3%
of the erratics are striated. RSD at Cape Bird
includes numerous reworked shell fragments,
sponge spicules, and
foraminifera.
RSD shows a semicompact, silty volcanic matrix (although patches of
sandy/gravelly matrix occur in places
near the upper drift
limit). The matrix is
unoxidized, commonly stratified, and
lacks visible salts.
Solifluction is common.
At Backdoor Bay, a nearly
continuous sheet of RSD extends from near sea level to
220 m elevation. At elevations
above 220 m, RSD thins progressively inland, until at 280
m elevation it consists solely
of scattered erratics on weathered kenyte bedrock. The
highest mapped erratic (granite) at Cape Royds occurs at
329 m elevation, near the ice
apron on Mt Erebus.
Erratics in RSD include
granite, dolerite, marble, and sandstone. The
frequency of surface erratics ranges from less
than 5/150 m2 near its
upper limit to 20/150 m2
nearthe conical mounds
of ice-coredRSD. Some
erratics reach up to 2 m3
in size.
Where present, the
matrix of RSD is
loose, granular, and
unoxidized; it is stratified in places. At two
localities, marine fossils are interbedded
with coarse- grained
matrix sands. Surface
clasts on RSD show
little salt weathering
or ventifaction, even
when isolated and
perched on weathered kenyte bedrock.
Cape
Royds
Cape
Barne
(channels)
A zone of glacially eroded (areally
scoured) bedrock channels extends from
Cape Royds to Cape Bare. All channels
show up and down long profiles; intervening divides lack RSD with matrix, but
instead show perched erratics on volcanic
bedrock. Near Backdoor Bay, the channels are 100 to 200 m long; all channels
trend north-south over most of their
length, but curve westward toward McMurdo Sound at their southern ends.
The channels, some of which
are up to 30 m deep, become
progressively deeper and wider from Cape Royds to Cape
Bame (from north to south).
All channels occur below 200
m elevation. Two major channels at Cape Bare are situated
within 0.5 km of the present
coastline.
The channels are incised in kenyte lava
flows, affording 40Ar/
39Arages from 32 to 90
kyr BP (Esser et al.
1994).
In the floor of one
channel, fossiliferous
marine deposits occur along with mirabilite on an ice-cored
hillock. Mollusc
shells from this deposit gave a 14C date
of >49,000 yr BP
(Stuiver et al. 1981).
Cape
Evans
RSD at Cape Evans consists only of iso- The maximum limit of isolated The frequency of erratlated erratics perched on weathered vol- erratics at Cape Evans is 100 m ics at Cape Evans is less
than 1/150 m2. Kenyte,
canic bedrock. These erratics occur only elevation.
the distinctive bedrock
at a few localities below 100 m elevation.
at Cape Barne (10 km to
The bedrock channels, so common at
the north), does not ocCape Royds and Cape Barne, do not occur
cur as blocks at Cape
at Cape Evans.
Evans.
The few erratics at
Cape Evans show
varying degrees of
wind erosion and
desert varnish. None
is striated.
Cape
Royds
Cape
Barne
Turks
RSD does not crop out at Turks Head or Not applicable.
at Hut Point Peninsula. The bedrock
Head
Hut Point channels, so common at Cape Evans and
Peninsula at Cape Royds, likewise do not occur at
Turks Head or at Hut Point Peninsula.
Geografiska Annaler · 82 A (2000) · 2-3
Not applicable.
Not applicable.
175
G.H. DENTON AND D.R. MARCHANT
Location
Areal extent and morphology
Upper-elevation limit
Lithology
Soil development
Scott
Coast
between
Cape
Bernacchi
and
Koettlitz
Glacier
A nearly continuous, matrix-supported
and commonly ice-cored drift sheet mantles the eastern slopes and valley mouths
of the coastal foothills. The drift (thickest
near the coast, >2.0 m) wraps around
coastal headlands as ground moraine and
extends inland on intervening valley
floors as glaciolacustrine sediment
(Hendy et al. 2000). A well-defined moraine limit on the headlands slopes inland
from a uniform elevation of 250 to 260 m
between Hobbs Glacier in the north to
Howchin Glacier to the south.
Although the maximum drift
limit along the headlands averages 250 to 260 m elevation,
RSD rises to 315 m elevation
adjacent to Walcott Bay. At
Heald Island, the drift limit
slopes from 310 m in the
southwest to 290 m elevation
in the northeast. Numerous
folds in drift distal to Hobbs
Glacier reflect recent advance
of the Hobbs terminus.
Erratics in RSD include
far-traveled volcanic
cobbles, as well as cobbles and boulders of
granite, dolerite, and
sandstone. Kenyte erratics are common in
the drift between Blue
Glacier and Miers Valley; kenyte is absent
from RSD south of
Miers Valley.
Matrix sands in RSD
are oxidized slightly
to 15 cm depth; it
lacks visible salts; in
places, the matrix of
RSD is largely volcanic.
Ferrar
Glacier
Valley
A thin (<1 m), little-weathered, and matrix-supported drift sheet mantles the
southern wall of FerrarGlacier valley.
The drift lacks an outer moraine ridge and
instead thins to feather edges on weathered bedrock and/or colluvium.
The upper limit of RSD is
highest near the coast, where it
reaches an elevation of 400 m,
and lowest in the central valley
near Overflow Glacier, where
it reaches an elevation of 200
m, only about 50 m above the
present surface of FerrarGlacier.
The frequency of erratic
clasts (granite and dolerite) in RSD is < 5/150
m2 in FerrarGlacier valley. Kenyte, so common
in RSD on the Scott
Coast to the north and
south of FerrarGlacier
valley, is rare;we found
only three kenyte erratics in RSD in Ferrar
Glacier valley.
A little-weathered, kenyte-rich drift sheet The well-defined limit of
RSD in Taylor Valley
mantles the valley floor at the mouth of
RSD, marked by a prominent features east-west
Taylor Valley and extends upward with- moraine ridge, dips inland
trending strips of conout break to a prominent moraine ridge on from a maximum of 350 m ele- centrated kenyte erratHjorth Hill. Inland, along the valley floor vation on the flank of Hjorth ics. Graniteand dolerite
and adjacent valley walls, 189 perched la- Hill. This upper-limit moraine clasts are common in
custrine deltas document the existence of diminishes in size farther to
RSD, as are far-traveled
a contemporaneous proglacial lake (Gla- the west (inland) and merges volcanic clasts in addicial Lake Washburn) dammed by the
tion to kenyte.
imperceptibly with glaciolagrounded ice lobe at the mouth of Taylor custrine sediments associated
with Glacial Lake Washburn.
Valley (Hall and Denton 2000a). Morphologic forms on the valley floor (lateral
benches, cross valley- and sinuous-longitudinal ridges, and conical mounds) are
all interpreted as having been formed by
sediment transported into Taylor Valley
by a lake-ice conveyor on Glacial Lake
Washburn (Hall et al. 2000a).
Matrix sands in RSD
are slightly oxidized
to 15 cm depth. Clasts
are angular to subangular; we did not find
striated clasts in RSD
in FerrarGlacier valley, though they are
common in RSD on
the Scott Coast to the
north and south.
Taylor
Valley
Scott
Coast
between
Cape
Bernacchi
and Cape
Roberts
176
Wilson drift (a correlative of RSD; Hall
and Denton 2000b) crops out in isolated
ice-free areas from the northernslopes of
Hjorth Hill (where it merges with kenyterich RSD) to Spike Cape. From Hjorth
Hill, an interlobate moraine extends
northeast for about 2 km, marking the
junction of two separate ice lobes that deposited RSD and Wilson drift. On Marble
Point, Kolich Point, and Spike Cape, Wilson drift extends discontinuously from
the uppermost limit of Holocene raised
beaches (16.5 -21 m elevation) to the
present-day Wilson Piedmont Glacier.
Wilson Drift is not preserved on Cape
Roberts and Dunlop Island because icefree areas here are below the marine limit
(Hall and Denton 2000b).
Although discontinuous, Wilson drift extends from the marine limit (16.5 to 21 m elevation) to the eastern margin of
the Wilson Piedmont Glacier.
At its southern boundary Wilson drift merges with the welldefined moraine comprised of
RSD at Hjorth Hill.
Wilson drift contains
exclusively local lithologies, including gneiss,
granite, schist, marble
and dolerite. Kenyte, so
common in RSD in
Taylor Valley and near
Cape Bernacchi, does
not occur in Wilson
drift. Numerous striations occur on molded
bedrock knobs that
project through Wilson
drift. These features indicate deposition of
Wilson drift by seaward-flowing ice of the
Wilson Piedmont Glacier.
RSD is matrix-supported and oxidized
slightly down to 10
cm depth. The glaciolacustrine facies features sorted silty sand
with dropstones ranging from gravel to
boulders, many striated. Fossil algae are
present, as well as
broken pieces of barnacles, sponge spicules, corals, and
shells.
Wilson drift is matrix-supported and
unoxidized. Clasts
are predominantly
angular, although
striated cobbles are
locally abundant.Unlike coeval kenyterich drift in Taylor
Valley, Wilson drift
does not contain reworked shells, corals,
or other marine organisms (Hall and
Denton 2000b).
Geografiska Annaler · 82 A (2000) · 2-3
RECONSTRUCTION OF GROUNDED ICE SHEET
Dolerite erratics show numerous pits, well-developed wind-polished facets, and desert varnish. A
caverously weathered granite cobble at 810 m elevation marksthe upper limit of this glaciogenic unit.
Mount Discovery
Mount Discovery is a large stratovolcano that
forms a substantial topographic obstacle to the prevailing southwesterly winds that drain across McMurdo Ice Shelf and Brown Peninsula. The northern (downwind) flank of Mount Discovery, and
nearly all of adjacent Brown Peninsula, are nearly
free of glacier ice (Fig. 3). In contrast, perennial
snowbanks and alpine glaciers cover the southern
(upwind) slope of Mount Discovery.
A thick (>1.0 m) and nearly continuous drift
sheet extends from the McMurdo Ice Shelf to a
sharp upper limit at 519 m elevation on Mount Discovery (Fig. 3). The drift can be traced almost continuously for 30 km along the northern flank of
Mount Discovery (recent downslope movement
has in places obscured the upper limit of the drift).
The drift surface is uniform, without morphological breaks, from the ice shelf to its upper limit. The
drift limit slopes westward from a maximum elevation of 519 m about 2 km north of Minna Saddle,
to 327 m elevation on the north flank of a bedrock
ridge opposite Brown Peninsula saddle, and finally
to a minimum elevation of 256 m on the west flank
of Mount Discovery east of the Koettlitz Glacier
and Hahn Island (Fig. 3). Locally, the drift limit is
consistently highest on the eastern flanks of cinder
cones; this pattern suggests westward flow such
that glacier ice was banked up against the eastern
flanks of the cones.
This drift sheet is nearly everywhere ice cored
and shows well-developed kettles and thermokarst
features. It is composed of a mixture of loose cobble- and gravel-sized clasts set within a matrix of
slightly bedded, unoxidized sands. A notable exception is on the western flank of Mount Discovery,
where, in places above about 200 m elevation, the
drift consists solely of scattered erratics on volcanic bedrock or regolith. Elsewhere, surface erratic
stripes (granite, dolerite, sandstone, and marble)
trend east-west in the terrain between low-elevation (<500 m) cinder cones on the southeastern
flank of Mount Discovery. The orientation of these
stripes is consistent with the inferred east to west
flow of glacial ice across the lower flanks of Mount
Discovery based on the slope of the drift limit. Several stripes cross the tidal crack and pass onto the
Geografiska Annaler * 82 A (2000) · 2-3
surface of the McMurdo Ice Shelf (Fig. 3). One of
these stripes on the northern slope of Mount Discovery extends downward from an elevation of 190
m onto the ice-shelf surface. Stuiver et al. (1981)
and Kellogg et al. (1990) suggested that such bands
are remnants from a former grounded ice sheet in
southern McMurdo Sound. There are about 100 erratics/150 m2 on the surface of this younger drift on
Mount Discovery (Table 1).
A higher and older drift reaches a maximum elevation of 805 m on the southeastern flank of Mount
Discovery (Fig. 3). This older drift shows a continuous upper limit that can be traced laterally for nearly 30 km. The upperdriftlimit slopes westward from
its maximum elevation of 805 m opposite Minna
Saddle, to 710 m on bedrock cliffs opposite Brown
Peninsula, and finally to 463 m along the northwestern slope of Mount Discovery. This older drift exhibits a greaterdegree of surface and internalweathering than the younger, low-elevation drift. Surface
erratics show cavernous weathering, and matrix
sands are slightly oxidized to 15 cm depth. At its
limit near Minna Saddle, this older drift features a
well-defined, ice-cored moraine that overlies weathered volcanic bedrock and regolith. The frequency
of surface erratics (granite, dolerite, sandstone, and
gneiss) on this moraine is about 100 clasts/150 m2.
Elsewhere, this older drift is clast-supported,lacks a
core of glacial ice, and displays a low frequency of
surface erraticsof about 10-25 clasts/150 m2.Along
the western flank of Mount Discovery (opposite
Hahn Island and Koettlitz Glacier), the drift limit is
marked by a well-defined line of weathered erratic
boulders and cobbles at 463 m elevation. This erratic
line is about 200 m higher in elevation than the
younger drift limit. In hand-dug sections 2 km west
of Minna Saddle, the older driftunderlies the younger drift with a sharp planar contact.
Brown Peninsula
Brown Peninsula extends northward from Mount
Discovery for about 25 km into the McMurdo Ice
Shelf and the floating Koettlitz Glacier ice tongue
(Figs 2 and 3). The low-elevation (50-100 m), flattish Brown Saddle separates Brown Peninsula
from adjacent Mount Discovery. Parasitic cinder
cones occur along the flanks and crest of Brown
Peninsula, forming an overall undulating surface
topography. The largest cinder cone is situated at
the center of Brown Peninsula and reaches 804 m
elevation.
A poorly sorted and unconsolidated drift sheet
177
G.H.DENTONAND D.R. MARCHANT
with erratics of granite, dolerite, and sandstone is
exposed continuously from the ice-shelf surface up
to a well-defined moraine ridge that nearly encircles Brown Peninsula (Figs 3 and 10b). This moraine reaches a maximum elevation of 365 m on a
small cinder cone about 1 km north of Brown Saddle. In general, the elevation and morphology of
this moraine varies consistently as it curves around
cinder cones; it is thickest (>3 m), widest (>4 m),
and highest (>320 m) on east-facing slopes. The
upper elevation of the drift ranges between 340 and
360 m on the eastern flank of Brown Peninsula and
between about 245 and 265 m on the western flank
of the peninsula.
The drift on Brown Peninsula is nearly everywhere cored by ice, shows well-developed constructional morphology, and thickens near the
coast. It is composed of moderately well-rounded
cobbles set within a loose matrix of slightly oxidized and poorly stratified fine- to coarse-grained
sand. About 90% of the cobbles are local volcanic
clasts and about 10% are erratics of granite, sandstone, and dolerite foreign to Brown Peninsula.
About 5% of these erratics show well-developed
glacial striations and smooth, glacially polished
surfaces. Just as at Mount Discovery, the drift on
Brown Peninsula exhibits several surface stripes of
concentrated erratics. Along the western flank of
Brown Peninsula, these stripes trend nearly northsouth; at their southern margin, they curve eastward across Brown Saddle. Stripes that pass from
Brown Saddle onto the McMurdo Ice Shelf are part
of the debris bands on the ice-shelf surface described previously. The orientation of the erratic
stripes at Brown Saddle is consistent with the inferred east-to-west glacier flow across the lower
flanks of Brown Peninsula based on the westward
slope of the drift limit.
A second and older glaciogenic deposit is exposed along the crest of a large cinder cone located
2 km south of the tip of Brown Peninsula. It reaches
a maximum elevation of 400 m, which is about 60
m above the low-elevation drift on this same cinder
cone (Fig. 3). It is comprises isolated, cobble-sized
erratics of dolerite and granite that rest on cavernously weathered volcanic bedrock. These erratics
show a greater development of wind abrasion, salt
weathering, and desert varnish than those at the surface of the lower-elevation drift.
Black Island
Situated 15-20 km east of Brown Peninsula, Black
178
Island is today largely free of perennial snow banks
and glacier ice (Figs 2 and 3). The island is roughly
circular in plan view, with three high promontories:
one is at the southern tip of the island (unnamed),
another is along the east coast (Cape Spirit, including Scallop Hill at 223 m elevation), and a third is
at the northwest margin of the island (Cape Hodgson, including Mount Melania at 330 m elevation).
Mount Aurorais the highest peak and reaches an elevation of 1041 m at the center of Black Island. The
narrow White Strait (2.5 km in width) separates
Black Island from White Island (Fig. 2).
A poorly sorted and unconsolidated drift with
granite, dolerite, and sandstone erratics covers twothirds of the ice-free terrain on Black Island (Fig.
3). The drift extends continuously from the ice
shelf to a sharp upper limit commonly marked by
a well-defined moraine ridge. At the northeastern
margin of Black Island about 4.5 km SSE of Mount
Melania, the drift reaches a maximum upper limit
at 522 m elevation. Here, a flat-topped moraine
ridge, up to 10 m wide, wraps around the base of
two cinder cones. On the western flank of Black Island, the drift limit is at 390 to 400 m elevation, and
is marked by a continuous moraine ridge (2-3 m in
relief) that is 3.5 km long. Striations are incised in
glacially molded trachyte bedrock to about 220 m
elevation on Scallop Hill. These erosional features
indicate that ice flow was from southeast to northwest at about N15°W.
The drift sheet on Black Island is ice cored, has
well-developed constructional morphology (debris
bands and hummocks), and is thickest near the
coast. It comprises little-weathered, cobble-sized
erratics (granite, dolerite, and sandstone) set within
a stratified matrix of loose and unoxidized fine- to
coarse-grained sands. The frequency of surface erratics is between 35 and 50 clasts/150 m2. A large
flat-topped deposit (500 m long and 75 m wide)
made up of layered fine- to coarse-grained sand occurs at the upper limit of the drift sheet on western
Black Island. Sand layers within this deposit dip inland at about 5-10°E; grain size also decreases inland. This deposit is interpreted as a delta formed
alongside a former ice margin.
Consisting of discrete patches of volcanic rubble
with about 1% erratic cobbles, a second and older
drift is exposed above the limit of the younger drift
on the northeastern flank of Mount Aurora. This
older drift reaches a maximum elevation of 710 m.
Its erratic cobbles show moderate-to-extensive
cavernous weathering, its matrix sediments lack a
core of ice, and its surface is smooth, without humGeografiska Annaler · 82 A (2000) · 2-3
RECONSTRUCTION OF GROUNDED ICE SHEET
mocks, debris bands, or moraines. Small lines of
granite, sandstone, and dolerite erratics (<100 m
long) occur parallel with, and 20-50 m above, the
sharp limit of the low-elevation (younger) drift on
the western flank of Black Island. These erratic
stripes rest on bedrock and, unlike the low-elevation drift, lack a sand matrix.
White Island
Although White Island is largely ice covered, small
patches of ice-free terrain occur near the northern
tip of the island, in the lee of high-elevation cinder
cones (for example, Mount Heine at 710 m elevation), and along the western coast near Isolation
Point and Mount Henderson (Fig. 2). In these icefree areas, a poorly sorted and unconsolidated glacial drift with granite, dolerite, and sandstone erratics occurs alongside volcanic-rich colluvium
and weathered volcanic bedrock. Many of the dolerite erratics are striated. Poorly sorted fine- to
coarse-grained sand (nearly 100% volcanic grains)
and gravel-sized volcanic rubble form the drift matrix. The drift sheet reaches its maximum elevation
of 561 m at the surface of a prominent platform between two flat-topped cinder cones about 500 m
east of Mount Heine. On this platform the drift
shows a sharp upper limit marked by a line of erratic cobbles and boulders on volcanic-rich colluvium. The drift extends as an unbroken sheet without morphological breaks or recessional moraines
from the margin of perennial snowbanks near the
ice shelf up to its maximum limit at 561 m elevation.
Ross Island
Ross Island is made up of four separate volcanoes:
Mounts Terror(3230 m), TerraNova (2130 m), Erebus (3794 m), and Bird (1800 m) (Fig. 2). The
largest ice-free areas on Ross Island occur at Cape
Crozier, Cape Bird, Cape Evans, Cape Bare,
TurksHead, and Hut Point Peninsula (Fig. 4). Elsewhere on Ross Island, glaciers, fed by icefields on
the flanks of these volcanos, flow across the coastline into the Ross/McMurdo Ice Shelf or into the
Ross Sea (Fig. 2).
Ross Island features an unusual bedrock assemblage. Kenyte (peralkaline phonolite) crops out extensively on the western flank of Mount Erebus at
Cape Royds, Cape Evans, Cape Barne, and Turks
Head, but not at Cape Crozier, Cape Bird, or Hut
Point Peninsula (or elsewhere in the McMurdo
Geografiska Annaler · 82 A (2000) · 2-3
Sound region). The limited areal extent of this distinctive bedrock lithology on Ross Island allows reconstruction of former ice-flow directions based on
the distribution of kenyte erratics in drift along the
western coast of McMurdo Sound.
Cape Crozier. The overall topography at Cape Crozier, located at the eastern tip of Ross Island (Fig.
4a), is largely controlled by volcanic bedrock. Seaward-dipping lava flows (10-15°) and conical cinder cones form the dominant landforms. The upper
limit of ice-free terrain on Cape Crozier is marked
by the termini of local alpine glaciers that flow
down the eastern flank of Mount Terror.At the center of this ice-free terrainis a broad, flat-floored bedrock-controlled valley (trending east-west) that extends from about 600 m elevation to the base of a
prominent cinder cone at about 940 m elevation.
Superimposed on this valley, as well as on undulating bedrock-controlled topography elsewhere on
Cape Crozier, is a thin and unconsolidated drift
made up of granite, sandstone, dolerite, and marble
erratics set within a matrix of unoxidized, volcanic
sand and gravel (Fig. 4a). Some erratics reach 3 m3
in size, though most are about 50 cm3. On steeper
bedrock slopes (>10-15°), the drift has been reworked into solifluction and gelifluction lobes. The
drift at Cape Crozier is thickest (>75 cm) on nearhorizontal benches and between about 300 and 500
m elevation. It reaches a diffuse upper limit, without a moraine ridge or sharpline of erraticboulders,
at 710 m elevation in the center of the broad valley
described above. This limit is placed at the highest
mapped erratic associated with matrix sands and
gravels in the valley. This upper limit is parallel
with the coastline. Recessional moraines are not associated with this drift sheet. Rather, drift extends
unbroken from the coast up to the mapped limit at
710 m elevation without morphological change.
Erratics on the drift sheet are fresh, without extensive desert varnish, ventifaction, pitting, or spallation; erratic frequency is between 25 and 50 clasts/
150 m2, increasing downslope. The presence of fartraveled erratics foreign to Ross Island, together
with the areal distribution of the drift sheet, indicates deposition from glaciers other than those
flowing off the western slopes of Mount Terror.
Rather, just as at Minna Bluff, Brown Peninsula,
Mount Discovery, Black Island, and White Island,
the widespread drift on Cape Crozier must have
been deposited by a grounded ice sheet that advanced landward.
On Cape Crozier we recognize a second and old179
G.H. DENTON AND D.R. MARCHANT
(a)
Fig. 4. Distribution of Ross Sea
drift on Cape Crozier (a), Cape
Bird (b) (from Dochat et al. 2000),
and Capes Royds/Barne (c) on
Ross Island. See Fig. 12 for locations. The circled numbers in (b)
refer to radiocarbon sample sites in
Dochat et al. (2000, table 2). The
circled numbers in (c) refer to radiocarbon sample sites in Table 2.
sion Hill, andCinderHill) andby steepcliffs along
the southerncoast (Figs 2 and4b). Ice-freeterrain
extendsfrom the coast up to the marginof an ice
capcenteredon MountBird.Thisice capfeeds severaloutletglaciersthatflowaroundparasiticcinder
cones andterminateon land.ShellGlacier,thelargest of these outlets,descendsto about50 m elevationanddividesCapeBirdintonorthernandsouthern sectors(Fig. 4b). Stream-cutchannelsandravinesoccurwheremeltwaterrunoffis concentrated
features
of
Bird
Bird.
The
Cape
topography Cape
gentle (15° to 20°) seawardslopes brokenby three in topographiclows.
A single, unweathered,nearlycontinuousdrift
centralparasiticcindercones (TrachyteHill, Inclu-
er glacialunithigheron the slope thanthe younger
driftdescribedabove.This olderunit is composed
solely of weatheredgranite,dolerite,andsandstone
erraticson weatheredvolcanic bedrock/regolith.
The highesterratics(graniteanddolerite)occurat
825 m elevationnearthe rim of a volcanic crater.
The erraticfrequencyat the upperlimitis less than
1 clast /150 m2 (Table1).
180
Geografiska Annaler * 82 A (2000) · 2-3
OF GROUNDEDICESHEET
RECONSTRUCTION
(b)
Geografiska Annaler · 82 A (2000)
2
2-3
181
G.H.DENTONAND D.R. MARCHANT
(C)
182
Geografiska Annaler · 82 A (2000) · 2-3
RECONSTRUCTION OF GROUNDED ICE SHEET
sheet mantles most of the ice-free terrain at Cape
Bird. North of Shell Glacier, this drift extends from
near sea level up to (and underneath)the edge of the
Mount Bird ice cap at 590 m elevation. South of
Shell Glacier the drift occurs in isolated patches
with feather edges on adjoining volcanic bedrock
knobs. The drift is generally matrix-supported,
stratified, and light gray in color (2.5Y 5/4). Where
it is exposed along the walls of deep ravines 100 m
south of Shell Glacier, the drift reaches a thickness
of 30 m. Just as is the case with all the other thick
drift sheets described previously, there is no discernible break in weathering characteristics or in
surface morphology from sea level to the upper
drift limit. Erratics are fresh, without ventifaction
or spallation, and matrix sands are loose and unoxidized. A detailed description of the surficial geology and geomorphology of Cape Bird is presented
in Dochat et al. (2000).
In general, the drift sheet at Cape Bird rests unconformably on weathered volcanic bedrock. The
drift shows alternating layers of gravel, sand, and
mud that fine upward to the top of the deposit. The
drift contains erratic cobbles of granite, marble,
dolerite, and sandstone, as well as local volcanic
bedrock (principally basalt). The erratics constitute
about 1-5% of the cobble fraction. Some 2-3% of
the basalt cobbles are striated. We found only five
small cobbles of kenyte on Cape Bird (<15 cm in
diameter) (Dochatet al. 2000). The drift at Cape
Bird includes numerous reworked shell fragments (mostly barnacles Bathylasma corolliforme
(Hoek)), corals, sponge spicules, echinoderm
spines, and foraminifera, as well as thin lenses of
debris-laden ice (2-5 cm in thickness). Foraminifera within the diamicton include Cibicides refulgens, Ehrenberingina glabra, Trifarina, Cassidulina, and Cassidulinoides porrecta (T. Kellogg, personal communication, 1994). The marine shell
fragments and far-traveled erratics together indicate that the drift on Cape Bird was not related to
an advance of the local ice cap on Mount Bird.
Rather, it must have been deposited by a grounded
ice sheet that advanced from adjacent McMurdo
Sound landward onto Cape Bird up to at least 590
m in elevation.
Linear ridges, as much as 40 m long and 10 m
high, occur parallel both to Shell Glacier and to the
margin of the ice cap centered on Mount Bird.
These ridges are within 5 to 10 m of the present-day
glacier margins and they feature an ice core. Interpreted as active thrust moraines (Dochat et al.
2000), these ridges indicate that local glaciers on
Geografiska Annaler · 82 A (2000) · 2-3
Mount Bird are now at their most extensive position since deposition of the erratic-bearing drift
sheet that mantles Cape Bird.
Cape Royds and Cape Barne. The ice-free terrain
at Cape Royds and Cape Barne forms a long and
gentle ramparton the lower slopes of Mount Erebus
on western Ross Island. The edge of an extensive
ice apron on the western flank of Mount Erebus defines the upper limit of this ice-free terrain (Fig.
4c). Isolated erratics of granite, dolerite, sandstone,
and marble (up to 2 m3) occur on weathered kenyte
bedrock near Backdoor Bay, Horseshoe Bay, and at
Cape Barne (Fig. lOo). Southeast of Backdoor Bay,
these erratics are admixed with fine- to coarsegrained sand and gravel. Here, buried algal mats
(0.5-2 cm thick and alternating with near-horizontal layers of fine- to coarse-grained sands down to
75 cm depth) are widespread. Also common to the
east and southeast of Backdoor Bay are conical
drift mounds with a core of ice and an outer surface
layer of loose sand, gravel, and erratic clasts (total
sediment thickness is between 25 cm and 75 cm).
These ice-cored mounds have as much as 2 m of relief. Debris bands within the ice cores commonly
dip eastward (inland). Numerous reworked shell
fragments and corals (0.5 to 1.5 cm long) occur at
two localities along the southwestern edge of the
drift sheet between 136 and 143 m elevation (David
and Priestley 1914; Debenham 1920, 1921) (Fig.
4c). These marine fossils are mixed with coarsegrained debris that rests on ice. Shells from one site
afforded a date of >47,700 14Cyr BP (Y-2642), and
those from another site gave an age of 37,700+2100
14C yr BP (QL-83) (Stuiver et al. 1981) (Table 2). At
elevations above 220 m, the drift at Cape Royds
thins progressively inland, until at 280 m elevation
it consists solely of scattered erratics on weathered
kenyte bedrock. The highest mapped erratic (granite) at Cape Royds is at 329 m elevation, near the
ice apron on Mount Erebus.
A zone of glacially eroded (areally scoured) volcanic bedrock extends from Cape Royds to Cape
Bare (Fig. 4c). The scoured bedrock is a complex
maze of winding ridges, isolated bedrock knobs,
and anastomosing channels. Most channels follow
local bedrock structure; some are bounded by narrow, sinuous lava flows that today rise 3-5 m in relief above channel floors. All channels show upand-down long profiles. The channels and intervening divides lack a veneer of fine-grained drift. Instead, isolated erratics of granite, dolerite, and
sandstone rest on eroded kenyte bedrock. The
183
G.H. DENTON AND D.R. MARCHANT
Table 2. Radiocarbon dates associated with glacial deposits in the McMurdo Sound region from the University of Washington Quaternary Isotope Laboratory (QL), the University of Waikato Radiocarbon Laboratory (Wk), the Yale University Radiocarbon Laboratory (Y), and the NSF-Arizona Accelerator Mass Spectrometry (AMS) Facility (AA). The locations of the sample sites are given in
Figs 4c and 12. Two dates are given for each sample of marine organisms. The top date for each of these pairs does not have a reservoir
correction for the deficiency in 4C of Antarctic marine waters. The bottom date for each pair incorporates a reservoir correction of 1300
14Cyears for the deficiency in Antarctic marine waters (Berkman and Forman 1996). See Hall and Denton (2000a) for additional dates
of deposits in Taylor Valley, Hall and Denton (2000b) for additional dates of raised beaches along Scott Coast north of Taylor Valley,
and Dochat et al. (2000) for additional dates of beaches and glacial deposits at Cape Bird on Ross Island.
Site
no.
Lab.
no.
Age
613C
(14C yr BP)
(%o) Description and context
1
QL-995
9860 + 160
2
QL-1036
3
QL-1146
10,000 + 40
12,330 + 50
4
Y-2399
9490 + 140
5
Y-2401
6100 + 140
6
QL-1160
8800 + 50
7
QL-1221
6580 + 50
8
QL-80
6190 + 80
9
QL-1584
13,300 + 150
10
QL-1590
12,530 +40
11
QL-1587
8710 + 50
12
QL-910
AA-28631
AA-30285
AA-30286
AA-30287
19,770 + 110
19,690 + 180
19,530 + 160
19,060 + 240
19,370 + 240
Wk-609
22,950 + 360
12
184
-
Reference
Lower FerrarGlacier valley. Layer of algae in place in silt bed Stuiver et al. (1981)
in delta deposited in ice-marginal lake at 62 m elevation.
Gives the age of marginal lake dammed on south valley wall
after ice dropped from LGM maximum position but was still
grounded near mouth of FerrarGlacier valley.
Stuiver et al. (1981)
Ditto, except delta at 43 m elevation.
in
m
delta
at
elevation.
Gives
255
the
Stuiver et al. (1981)
Algal layer perched
age of a small proglacial lake dammed against the outer moraine of Ross Sea drift.
Mat of algae that formed in small kettle lake on ice-cored Stuiver et al. (1981)
Ross Sea drift, and was left strandedon drift surface at 134
m elevation when kettle lake drained. Date is a minimum
value for the lowering of the Ross Sea ice surface to this locality.
Stuiver et al. (1981)
Ditto, except sample at 100 m elevation
Algae in delta at 122 m elevation in Salmon Valley. Date Stuiver et al. (1981)
is a minimum value for recession of Ross Sea ice lobe from
this locality.
Algae in delta at 2 m elevation in Garwood Valley. Mini- Stuiver et al. (1981)
mum age for recession of lobe of Ross Sea ice sheet from
sample locality.
Mat of algae that formed in small kettle lake on ice-cored Stuiver et al. (1981)
Ross Sea drift and was left stranded on drift surface at 24
m elevation when lake drained. Date is a minimum value
for lowering of Ross Sea ice surface to this locality.
Algal layer at 189 m elevation in stratigraphicsequence of Dagel (1984)
the glaciolacustrine facies of Ross Sea drift deposited in
Marshall Valley in a proglacial lake from a floating ice cover. Ross Sea ice sheet still at, or very close to, its LGM position.
-4.74
-4.12
-4.79
-4.61
Algal layer in surface lacustrine sediments deposited in
proglacial lake in Marshall Valley at 125 m elevation. Ice
lobe from Ross Sea ice sheet still near LGM position
Mat of algae that formed in small kettle lake on ice-cored
Ross Sea drift in Marshall Valley and was left stranded on
drift surface at 101 m elevation when lake drained. Date is
a minimum value for lowering of Ross Sea ice surface to
this locality.
Fibrous algal mats at 81 m elevation in fine sand unit below
carbonate bed, which in turnunderlies the gypsum moraine
Miers Valley. Date gives age of proglacial lake dammed in
Miers Valley by a lobe of the grounded Ross Sea ice sheet,
and is a maximum value for the gypsum moraine, which is
thought to mark a grounding-line position.
Lacustrine carbonate bed at 81 m elevation above fibrous
algal mats in fine sand unit, and just below the gypsum moraine in Miers Valley. The date gives age of proglacial lake,
and is maximum value for gypsum moraine. It is not yet
known why this date is older than that of the fibrous algal
mats given immediately above.
Dagel (1984)
Dagel (1984)
Clayton-Greene (1986),
Clayton-Greene et al. (1988),
and this paper
Geografiska Annaler * 82 A (2000) · 2-3
OF GROUNDEDICESHEET
RECONSTRUCTION
Site
no.
Lab.
no.
Age
('4C yr BP)
13
Wk-716
19,150 180
14
Wk-627
14,200 + 120
15
Wk-720
13,700 + 180
Ditto, except lacustrine carbonate plate from 156 m elevation.
16
Wk-719
14,800 + 170
Ditto, except lacustrine carbonate plate from 80 m elevation.
17
Wk-718
10,300 + 900
Ditto, except lacustrine carbonate plate from 121 m elevation.
18
Wk-717
14,200 + 120
Ditto, except lacustrine carbonate plate from 88 m elevation.
19
Wk-582
16,000 + 140
Ditto, except lacustrine carbonate plate from 142 m elevation.
20
Wk-629
17,950 + 160
Ditto, except lacustrine carbonate plate from 100 m elevation.
21
Wk-628
18,600 + 170
Ditto, except lacustrine carbonate plate from 140 m elevation.
22
Wk-630
16,750 + 260
Ditto, except lacustrine carbonate plate from 125 m elevation.
23
Wk-814
14,400 + 130
Upper layer of lacustrine carbonate plate with three layers,
at 113 m elevation.
Wk-813
18,100 + 200
Wk-812
19,850 + 220
24
QL-166
6600 + 60
5300 + 60
Ditto, except middle layer of same lacustrine carbonate
plate
Ditto, except lower layer of same lacustrine carbonate
plate.
Mixture of marine shells, largely barnacles, from surface of
McMurdo Ice Shelf.
25
QL-1126
26
QL-85
6510 ± 50
5210+ 50
4140 + 60
Stuiver et al. (1981)
Kellogg et al. (1990)
Mat of non-marine algae strandedon ice-cored debris band Stuiver et al. (1981)
on surface of McMurdo Ice Shelf, affords minimum age for
ice shelf.
27
QL-84
5670 + 100
4370 + 100
Mixture of marine shells from surface of McMurdo Ice
Shelf.
28
QL-1127
4630 + 80
3330 + 80
Barnacles from surface of McMurdo Ice Shelf.
29
QL-1128
Barnacles from surface of McMurdo Ice Shelf.
30
QL-79
1260 + 30
modern
1290 + 50
moder
Mixture of marine shells from surface of McMurdo Ice
Shelf.
Stuiver et al. (1981)
Kellogg et al. (1990)
Stuiver et al. (1981)
Kellogg et al. (1990)
Stuiver et al. (1981)
Kellogg et al. (1990)
Stuiver et al. (1981)
Kellogg et al. (1990)
31
QL-1223
3630 + 90
2330 + 90
Ditto
Kellogg et al. (1990)
32
QL-1222
Barnacles from surface of McMurdo Ice Shelf.
33
QL-1132
3590 + 80
2290 + 80
3610 + 40
2310 +40
34
QL-97
Mixture of marine shells from surface of McMurdo Ice
Shelf.
35
QL-77
3370 + 80
2070 + 80
1370 + 50
moder
Stuiver et al. (1981)
Kellogg et al. (1990)
Stuiver et al. (1981)
Kellogg et al. (1990)
Stuiver et al. (1981)
Kellogg et al. (1990)
Stuiver et al. (1981)
Kellogg et al. (1990)
Geografiska Annaler * 82 A (2000) · 2-3
613C
(%o)
Description and context
Reference
Lacustrine carbonate plate at 78 m elevation in Miers Valley. Plate formed from calcite deposition onto the floor of
a former proglacial lake.
Ditto, except lacustrine carbonate plate from 101 m elevation.
Clayton-Greene (1986) and
Clayton-Greene et al. (1988)
Clayton-Greene (1986) and
Clayton-Greene et al. (1988)
Clayton-Greene (1986) and
Clayton-Greene et al. (1988)
Clayton-Greene (1986) and
Clayton-Greene et al. (1988)
Clayton-Greene (1986) and
Clayton-Greene et al. (1988)
Clayton-Greene (1986) and
Clayton-Greene et al. (1988)
Clayton-Greene (1986) and
Clayton-Greene et al. (1988)
Clayton-Greene (1986) and
Clayton-Greene et al. (1988)
Clayton-Greene (1986) and
Clayton-Greene et al. (1988)
Clayton-Greene (1986) and
Clayton-Greene et al. (1988)
Clayton-Greene (1986) and
Clayton-Greene et al. (1988)
Clayton-Greene (1986) and
Clayton-Greene et al. (1988)
Clayton-Greene (1986) and
Clayton-Greene et al. (1988)
Stuiver et al. (1981)
Kellogg et al. (1990)
Barnacles from surface of McMurdo Ice Shelf.
Serpulids from surface of McMurdo Ice Shelf.
Ditto
185
G.H. DENTON AND D.R. MARCHANT
Site
no.
Lab.
no.
Age
(t4C yr BP)
36
QL-1225
37
863C
Description and context
Reference
1340 + 30
modem
Bryozoa from surface of McMurdo Ice Shelf.
Stuiver et al. (1981)
QL-77
1370 + 50
70 + 50
Ditto
38
QL- 1226
3280 + 300
1980 + 300
Mixture of marine shells from surface of McMurdo Ice
Shelf.
Stuiver et al. (1981)
Kellogg et al. (1990)
Kellogg et al. (1990)
39
QL-4024
3050 + 30
1750 + 30
Barnacles from surface of McMurdo Ice Shelf.
Kellogg et al. (1990)
40
QL-4025
30,900 + 160
29,600 + 160
Ditto
Kellogg et al. (1990)
41
QL-1446
45,500 +1500
42,200 +1500
Ditto
Kellogg et al. (1990)
42
QL-1445
1890 + 60
590 + 60
Ditto
Kellogg et al. (1990)
43
QL-4026
Ditto
Kellogg et al. (1990)
44
QL-4027
>46,000
>44,700
22,070 + 140
20,770 + 140
Ditto
Kellogg et al. (1990)
45
QL-1444
2710 + 60
1410 + 60
Ditto
Kellogg et al. (1990)
46
QL-4028
20,760 + 80
19,460 + 80
Ditto
Kellogg et a. (1990)
47
QL-4029
4880 + 30
3580 + 30
Ditto
Kellogg et al. (1990)
48
QL- 1443
7750 + 90
6450 + 90
Ditto
Kellogg et al. (1990)
49
QL-4032
4310 +30
3010+30
Ditto
Kellogg et al. (1990)
50
QL-4031
2790 + 40
1490 + 40
Ditto
Kellogg et al. (1990)
51
QL-1128
1260 + 30
modem
Ditto
52
QL-1447
570 + 60
modem
Bryozoa from surface of McMurdo Ice Shelf.
Stuiver et al. (1981)
Keilogg et al. (1990)
Kellogg et al. (1990)
53
QL-4030
4430 + 30
3130+30
Barnacles from surface of McMurdo Ice Shelf.
Kellogg et al. (1990)
54
QL-1130
3770 + 40
2470 + 40
Barnacles from surface of McMurdo Ice Shelf.
55
QL-1448
4410 + 70
3110 +70
Ditto
Stuiver et al. (1981)
Kellogg et al. (1990)
Kellogg et al. (1990)
56
QL-1449
3190 + 50
1890 + 50
Ditto
Kellogg et al. (1990)
57
QL-1453
720 + 60
modem
Bryozoa from surface of McMurdo Ice Shelf.
Kellogg et al. (1990)
58
QL-4034
4900 + 30
3600 + 30
Barnacles from surface of McMurdo Ice Shelf.
Kellogg et al. (1990)
59
QL- 1450
5250 + 50
3950 + 50
Ditto
Kellogg et al. (1990)
60
QL-1454
5610 + 50
4310+ 50
Ditto
Kellogg et al. (1990)
61
QL-4033
3820 + 30
2520 ± 30
Ditto
Kellogg et al. (1990)
186
(%c)
Geografiska Annaler · 82 A (2000) · 2-3
RECONSTRUCTION OF GROUNDED ICE SHEET
Site
no.
Lab.
no.
Age
(14C yr BP)
62
QL-167
63
613C
Description and context
Reference
3130 + 90
1800 + 90
Mixture of marine shells from surface of McMurdo Ice
Shelf.
QL-4023
2160 + 50
860 + 50
Barnacles from surface of McMurdo Ice Shelf.
Stuiver et al. (1981)
Kellogg et al. (1990)
Kellogg et al. (1990)
64
QL-1224
4410 + 90
3110 +90
Ditto
Kellogg et al. (1990)
65
QL-1452
2150 + 40
850 + 40
Ditto
Kellogg et al. (1990)
66
QL-4022
2530 + 50
1230+ 50
Ditto
Kellogg et al. (1990)
67
QL-4035
4150 + 30
2850 + 30
Ditto
Kellogg et al. (1990)
68
QL- 1129
>51,000
>49,700
Ditto
Kellogg et al. (1990)
69
QL-83
o
36,300 _1000
36,300
Shells (largely Serpulae) mixed into Ross Sea drift resting
on an ice core. The shells are not in place, but are ice transported. These dates afford maximum ages for the emplacement of Ross Sea drift at this locality.
Marine shells (largely Serpulae) mixed into Ross Sea drift
on an ice core. The shells are not in place, but are ice transported. These dates afford maximum ages for the emplacement of Ross Sea drift at this locality.
Mixture of marine shells set in a layer of sponge spicules
about 30 cm thick resting on an ice core and overlain by
kenyte-rich drift at 59 m elevation. The entire area exhibits
ice-cored Ross Sea drift. These shells are not in place but
are ice transported.Therefore the date affords a maximum
age for deposition of Ross Sea drift in this area.
Stuiver et al. (1981)
Debenham (1921)
David and Priestley (1914)
+ 1200
35,000
QL-83
+1200
1000
4+2100
39,000 -1700
+2100
37,700 -1700
70
Y-2642
>49,000
>47,700
71
Y-2641
>47,000
>45,700
(%o)
Stuiver et al. (1981)
David and Priestley (1914)
Mixture of marine shells set in a sponge mat at 28-32 m el- Stuiver et al. (1981)
evation that rests on an ice core and is overlain by ablation Debenham (1920)
drift rich in kenyte clasts. The whole deposit represents
Ross Sea drift, and occurs at 28-32 m elevation in a subglacial meltwater channel. A mollusc shell from this deposit afforded a U/Th age of 120,000 + 6000 years (Stuiver et
al. 1981). These shells are not in place, but are ice transported. These dates are all maximum ages for the deposition of Ross Sea drift on Cape Barne.
channels near Backdoor Bay are 100 to 200 m long.
They become progressively deeper and better developed from Cape Royds to Cape Bame. Two major channels at Cape Barne are situated within 0.5
km of the present coastline. These channels are incised to a depth of 30 m. They trend north-south
over most of their length and are separated by a
broad, flat divide about 100 m wide. At their southern ends, the channels curve westward toward McMurdo Sound. Perennially frozen lakes occupy the
deepest depressions in the floors of these channels.
In the floor of one of these channels, fossiliferous
marine deposits, 30-60 cm thick, occur along with
mirabilite and drift on an ice-cored hillock (Debenham 1920, 1921) (Fig. 4c). Mollusc shells from this
deposit gave a radiocarbon date of >45,700 14Cyr
Geografiska Annaler · 82 A (2000) · 2-3
Stuiver et al. (1981)
Debenham (1921)
David and Priestley (1914)
date of 120,000 yr BP
BP (Y-2641) and a 234U/230Th
(Stuiver etal. 1981) (Table 2). Stuiver etal. (1981)
agreed with Debenham (1920) that these deposits
were frozen into an ice shelf, as is currently happening on the McMurdo Ice Shelf, and then were
transportedto their present position when the shelf
grounded and spread over the adjoining terrain.
Thus the dates of shells on both Cape Royds and
Cape Bame afford maximum values for impingement of grounded ice onto western Ross Island.
The walls of the easternmost channel at Cape
Bame expose three near-horizontal, phonolite lava
flows in stratigraphic section. 40Ar/39Ardates of
anorthoclase crystals from a bedrock sample about
2 m above the channel floor range from 88 ±3 to 91 ±
2 kyr (Esser et al. 1994). 40Ar/39Aranalyses of
187
G.H.DENTONAND D.R. MARCHANT
anorthoclase crystals from kenyte bedrock between
channels at Cape Royds afforded an age of 73 + 5 kyr
(Esser et al. 1994). These dates are maximum values for channel incision (and hence for ice overriding) at Cape Barne and Cape Royds, respectively.
The overall trend and progressive deepening and
widening of the channels from Cape Royds to Cape
Barne strongly suggest that glacier ice here flowed
from north to south. As discussed below, numerous
kenyte erratics in drift along the western McMurdo
coast were most likely derived from the glacially
eroded kenyte bedrock between Cape Royds and
Cape Bare.
Cape Evans, TurksHead, and Hut Point Peninsula.
Ice-free terrainat Cape Evans, Turks Head, and Hut
Point Peninsula extends from the coast up to the
lower edge of an ice apron that mantles the slopes
of Mount Erebus. Alpine glaciers that drain this ice
apron and flow across the coast into McMurdo
Sound or the Ross Ice Shelf define the lateral extent
of ice-free terrain at each area.
The topography at Cape Evans, Turks Head, and
Hut Point Peninsula reflects cinder cones and sinuous lava flows. The bedrock channels so common
at Cape Royds and Cape Barne do not occur at Cape
Evans, Turks Head, and Hut Point Peninsula. At
Cape Evans, a low-lying plain extends from the
coastline up to the margin of the Erebus ice apron
at about 195 m elevation. Granite and dolerite erractics occur only at a few localities below 100 m
elevation at Cape Evans, resting directly on weathered kenyte bedrock dated at 32+6 kyr (Esser et al.
1994). Although it is the major bedrock lithology at
Cape Evans and Turks Head, kenyte does not crop
out, or occur as glacial erratics, at Hut Point Peninsula.
Sandstone, dolerite, marble, and granite erratics
occur at Cape Bird, Cape Royds, Cape Barne, and
Cape Evans, but not at Turks Head or Hut Point Peninsula. In fact, the concentration of these fartraveled erratics along the western coast of Ross Island decreases from about 45-100 clasts/150 m2 at
Cape Bird in the north to less than 1 clast/150 m2
at Cape Evans in the south. The implication is that
glacier ice carrying TAM erratics on the western
coast of Ross Island flowed southward from Cape
Bird to Cape Evans, and then was deflected westward across the sound. Such a flow path is consistent with the orientation of glacially carved channels on the western coast of Ross Island that indicate southward ice flow between Cape Royds and
Cape Barne. Moreover, because kenyte erratics do
188
not occur at Hut Point Peninsula, the glacier ice that
eroded kenyte bedrock at Cape Barne and Cape
Evans must have been deflected westward.
McMurdo Ice Shelf
The McMurdo Ice Shelf, an extension of the Ross
Ice Shelf, now floats on southern McMurdo Sound
(Figs 2, 3, 5 and 10a). Its eastern sector is currently
fed by ice inflow from the Ross Ice Shelf, as well
as by glaciers flowing southward from Mount Erebus and Mount Terror (Heine 1967). In the southwest it coalesces with the floating tongue of Koettlitz Glacier. The surface equilibrium line, which is
determined by the prevailing southwesterly winds
passing over volcanic topographic barriers,extends
southward from near Hut Point Peninsula to White
Island (Paige 1968; Swithinbank 1970); satellite
images suggest that this line is irregular between
White Island and Minna Bluff, and that it trends
east-west across Koettlitz Glacier near Heald Island. The ice-shelf surface east and south of the
equilibrium line is an accumulation area.
An extensive ablation area occurs north and west
of the equilibrium line. Surface ablation is 0.5-1.0
m per year, from sublimation and melting (Swithinbank 1970). This melting produces a surface
stream network that in mid-summer drains northward off the ice-shelf edge into McMurdo Sound
(Fig. 5). The shelf thickness in the ablation area
ranges from about 100 m near Mount Discovery in
the south to 15 m near the Dailey Islands in the
north (Swithinbank 1970). Except for relatively
clean ice derived from Koettlitz Glacier, the upper
surface of the shelf is covered by a thin debris mantle that commonly forms bands and swirls. Moreover, it has long been known that well-preserved
marine macrofossils occur, sometimes in abundance, on the ice-shelf surface (Scott 1905; Ferrar
1907; David and Priestley 1914; Debenham 1920).
Debenham (1920) suggested that shelf ice in the
ablation area was maintained by basal freezing and
surface ablation. In this case, marine sediment and
macrofossils froze onto the base of the shelf where
it touched the sea floor, subsequently moving upward through the shelf as surface ice ablated. This
mechanism explains both the surface shelf sediment and the delicate marine fossils preserved intact on the shelf.
The shelf exhibits several major types of surface
deposits that are commonly intertwined. One is
windblown sediment from other surface deposits
and from adjacent ice-free terrain;another is wideGeografiska Annaler · 82 A (2000) · 2-3
RECONSTRUCTION OF GROUNDED ICE SHEET
Fig. 5. Aerial view of McMurdo Ice Shelf. View is toward the south. A semi-permanent rift in the ice shelf runs from left to right across
the center of the photograph. Behind the rift are surface debris bands. In front of the rift is a combination of debris in surface streams,
ice pinnacles aligned to prevailing wind directions, and remnants of detached debris bands (Kellogg et al. 1990). In right center is Brown
Peninsula, in right background is Mount Discovery, in far background is Minna Bluff, and in left background is Black Island. To the
right is Koettlitz Glacier. See Fig. 12 for location of this photograph.
spread volcanic-rich surface debris, commonly
mixed with shells; and a thirdinvolves extensive
bands of surface debris, in places mixed with
shells, thathas a grain-sizedistributionsimilarto
till and thatcontainsgranite,gneiss, schist, dolerite, sandstone, siltstone, and quartzite erratics,
some striated.
One debrisbandbordersthe northeasternshore
of Black Island, and anotherextends northward
Geografiska Annaler · 82 A (2000) · 2-3
from nearBlack Islandto the outertip of the McMurdoIce Shelf (Stuiveret al. 1981;Kellogget al.
1990). These debrisbands consist largely of volcanicmaterial,buttheyalsocontainnumerousnonvolcanicerraticsoriginallyderivedfromtheTAM.
Moreover,marinesedimentsand fossils are commonly enclosed in both bands.Uncorrecteddates
of shells from the debrisband along the northern
shoreof BlackIslandare 1290+ 50 14Cyr BP (QL189
G.H. DENTON AND D.R. MARCHANT
Fig. 6. Oblique aerial view of ice-cored, volcanic-rich tongues of Ross Sea drift in the mouths of Garwood (right), Marshall (center),
and Miers (left) Valleys in the Royal Society foothills along the western coast of McMurdo Sound. Sea ice and the floating McMurdo
Ice Shelf are in the foreground. View is inland, with the Royal Society Range in the background. The Ross Sea drift tongues were deposited by grounded ice lobes and by floating lake-ice conveyors flowing inland from a grounded ice sheet in McMurdo Sound. The
volcanic clasts in Ross Sea drift were derived from off-shore volcanic islands. See text for details. See Fig. 12 for location of this photograph.
79), 3630 + 90 14Cyr BP (QL-1223),and3610 ± 40
14C yr BP (QL-1132).Applyinga correctionfactor
of 1300 14Cyr for the marinereservoireffect as in
Table2 impliesthatthe debrisbandis quiteyoung.
However,datesof shellsin thelong debrisbandthat
trendsnorthwardfromBlack Islandto the edge of
the shelf show a systematic aging trend. Shells
fromnearthenortherntipof BlackIslandweredat190
ed to 1260 + 30 14C yr BP (QL-1128) (Table2);
when the reservoirerroris considered,this result
indicates that shells are now being incorporated
intothebandneartheshore.Shell samplesprogressivelyfarthernorthaffordedcorrectedagesof 3330
± 80 14C yr BP (QL-1127), 4370 ± 100 14C yr BP
(QL-84),5210 + 50 14CyrBP (QL-1126),and5300
+ 60 14Cyr BP (QL-166)(Stuiveret al. 1981; KelGeografiska Annaler · 82 A (2000) · 2-3
RECONSTRUCTION OF GROUNDED ICE SHEET
Fig. 7. Aerial view of the upper
limit of Ross Sea drift on the western coast of McMurdo Sound near
the mouth of Miers Valley. See
Fig. 12 for location of this photograph.
logg et al. 1990) (Figs 3 and 12) (Table 2). These
results suggest that the debris band is inherent to
the present flow regime of the McMurdo Ice Shelf,
because the shells were frozen into the shelf near
Black Island and then carried northward by the
shelf. The debris in the band was either frozen into
the shelf concurrently with the shells or was incorporated into the shelf from ice-cored Ross Sea drift
on the northern shore of Black Island.
Several similar short debris bands occur elsewhere on the McMurdo Ice Shelf. In particular,the
easternmost of the Dailey Islands exhibits a horseshoe-shaped band whose head wraps around the
southern portion of the island and whose tails extend northward from the island to the shelf edge.
One shell sample from the debris band adjacent to
the island afforded an uncorrected age of 1370 ± 50
14Cyr BP (QL-77) (Fig. 3), suggesting that the shells
are currently being incorporated into the moraine
band much as they are along the northeasternshore
of Black Island (Stuiver et al. 1981). Hence, this
band probably originated near easternmost Dailey
Island and then moved northward in the ice shelf.
Nearby, another of the Dailey Islands has a marginal debris band with shells that date to 1340 ± 30 14C
yr BP(QL-1225) (Figs 3 and 12) (Stuiver etal. 1981;
Kellogg et al. 1990). Some of the minor debris
bands and dirty-ice stripes that extend northward
from Brown Peninsula and Black Island could also
be local in origin, frozen into the shelf nearshore.
There are debris bands of at least two origins on
the McMurdo Ice Shelf. As discussed above, one
Geografiska Annaler · 82 A (2000) · 2-3
Fig. 8. Aerial view of Ross Sea drift on the flank of Hjorth Hill
alongside of the mouth of Taylor Valley. Sea ice floating in New
Harboris at the bottom of the photograph. See Fig. 12 for location
of this photograph.
191
G.H. DENTON AND D.R. MARCHANT
type is inherentto the modem northward-flowing
ice shelf. The othertype occursin a protectedenclave on the southernreaches of the shelf near
Brown Peninsulaand Mount Discovery (Fig. 3).
This second type is probablya relict of a former
westward-flowinggroundedice sheet that filled
McMurdoSoundat the LGM(Kellogget al. 1977,
1990; Stuiver et al. 1981).
Previously,we discusseddebrisbandsanderratic stripson Ross Sea drifton Black Island,Brown
Peninsula,andMountDiscovery.Thegeneraltrend
of these debris bands is consistent with former
westwardflow in this area of the groundedRoss
Sea ice sheetreconstructedbelow.Generally,these
debrisbandsterminateabruptlyat the tidal crack
thatseparatesRoss Seaice-coreddrifton landfrom
the McMurdoIce Shelf. However,severaldebris
bands pass directly from land onto the ice shelf
without offset along the northeasternshore of
MountDiscoveryand the easternshoreof Brown
Peninsula.Somebandsdrapeovertheice-shelfsurface andextendto 190 m in elevationon the southernflankof MountDiscovery(Fig. 3), andthedrift
coveris continuousfromthe shelf surfacewell up
onto the easternslope of BrownPeninsula.
This southernremnantportionof the McMurdo
Ice Shelf is stagnant,or nearlyso (Kellogg et al.
1990). The overallpatternof theseremnantdebris
bandsimplies folding due to compressiveflow of
formergroundedice againstthe obstacleof Brown
Peninsula.Dissipationof this groundedice sheet
strandedice-coreddebrisbands in the mouthsof
Miers and GarwoodValleys along the western
coast of McMurdoSound,on the northernflankof
Mount Discovery, in the saddle between Mount
Discovery and Brown Peninsula,on the eastern
flankof BrownPeninsula,andon Black Island,as
well as theprominentremnantbandson thesurface
of the southernMcMurdoIce Shelf.The corrected
datesof shellsat theice-shelf surfaceshowthatthe
ice- shelfgroundinglinerecededbeneaththenorthernportionof these remnantdebrisbandsby 6450
14CyrBP(QL-1443)(Kelloggetal. 1990).Corrected datesof surfaceshell materialnearthe presentday southerntidal crackare as old as 3600 14Cyr
BP(QL-4034),4310 14Cyr BP(QL-1454),and3130
tical loweringof the surfaceof the McMurdoIce
Shelfto withina few metersof its presentelevation.
Thedatesare4140 ± 60 14Cyr BP(QL-85)fromthe
long morainethat leads northfrom Black Island
(Fig. 3) and 3370 ± 80 14Cyr BP (QL-97)on icecoreddrifton the easternmostDailey Islandadjacentto theice shelf(Fig.3). Hence,thesedatessuggest thatthe ice shelf formedpriorto 4140 14Cyr
BP, whichis consistentwithage estimatesbasedon
radiocarbondatesof shells.
Overall,the availabledatasuggestthatthe McMurdoIce Shelfhasbeenin existencesinceatleast
6450 '4C yr BP (QL-1443) and that it probably
formedin partby thinningof a groundedice sheet
(reconstructed
below) in McMurdoSound.As discussed below, the chronologyand distributionof
raised beaches and emergedmarinedeposits togethersuggestthatuntilrecentlythe ice shelf covered much of McMurdoSound. Today,the only
glacier that feeds into the westernMcMurdoIce
Shelf is the KoettlitzGlacier,althoughthe Ferrar
Glacier,the Blue Glacier,the ErebusIce Tongue,
andotherglaciersfromRoss Islandprobablyalso
fed the formerextendedMcMurdoIce Shelf.
Scott Coast: Koettlitz Glacier to Cape Bernacchi
Eastern foothills of Royal Society Range. The
mountainsof the Royal Society Rangeare among
thehighestinAntarctica(Fig.2). Situatedabout20
km inlandfromthe coast, MountListerat 4025 m
elevationis thehighestpeakin therange.TheBlue
andKoettlitzGlaciersdrainice fromthe slopes of
the Royal Society Rangearoundthe easternfoothills andemptyintoMcMurdoSoundandthe McMurdoIce Shelf, respectively.A series of closely
spacedcrevasseson the lowerpartof the Koettlitz
Glacier,about2 km northof HealdIsland,marks
the positionwherethe ice comes afloat.
Theeasternfoothillsof theRoyalSocietyRange
containfour smallvalleys (8-12 km long and 1-3
km wide) thatopen to McMurdoSoundand/orthe
McMurdoIce Shelf (Fig. 2). In contrastto thehigh
peaksof theRoyalSocietyRange,thefoothillsand
small valleys are largely ice free. From northto
south,themainvalleysin theseeasternfoothillsare
14C yr BP (QL-167) (Kellogg et al. 1990; Stuiver et
theSalmon,Garwood,Marshall,andMiersValleys
al. 1981) (Fig. 3; Table 2).
(Fig. 12). Thesevalleysareseparatedby east-west
Two radiocarbonsamples were collected from trendingbedrockridgesup to 800 m in elevation.
algalpeatmatsstrandedin place on ice-coreddrift The exposed bedrock is Precambrianmarble,
of theMcMurdoIce Shelf (Stuiveret al. 1981)(Ta- gneiss, andschistof theRoss System.Smallalpine
ble 2). The peat mats markthe formerpositionof glaciersoccurwithinandat the headsof thesevalsmallkettlelakesandaffordminimumagesforver- leys. The largestalpineglacieris the Hobbs Gla192
Geografiska Annaler · 82 A (2000) · 2-3
RECONSTRUCTION OF GROUNDED ICE SHEET
Fig. 9. Glacial ice core in Ross Sea
drift at mouth of Garwood Valley.
See Fig. 12 for location of this photograph.
cier, which flows seaward to within 1 km of the
coast near Cape Chocolate.
A nearly continuous drift sheet, commonly ice
cored (Fig. 9), mantles the eastern slopes and valley
mouths of the coastal foothills. The drift is thickest
near the coast (>2.0 m). It wraps around bedrock
coastal headlands as ground moraine and extends inland on intervening valley floors as glaciolacustrine
sediment (Fig. 6) (see also Hendy et al. 2000). On
coastal headlands, the upper limit of the drift sheet
is a sharpmoraine ridge, as much as 4 m in relief, that
is a conspicuous physical feature (Figs 7, 8, 10c-g).
This moraine ridge slopes inland at the valley
mouths. Elevations along the upper edge of the drift
sheet on the coastal headlands are generally uniform
(250-260 m) from Hobbs Glacier in the north to
Howchin Glacier in the south. The drift limit alongside the southernmargin of the Blue Glacier rises to
about320 m elevation.Adjacent to WalcottBay west
of Koettlitz Glacier, the drift limit rises to 315 m elevation. At Heald Island, just south of the presentday Koettlitz Glacier grounding line, the drift limit
slopes from about 310 m in the southwest to 290 m
elevation in the northeast.Overall, the driftis matrixsupportedand contains far-traveledvolcanic clasts,
as well as cobbles and boulders of granite, dolerite,
and sandstone. Matrix sands are slightly oxidized to
15 cm depth. Kenyte erraticsare common in the drift
between Blue Glacier and Miers Valley; but are absent in the drift south of Miers Valley. The numerous
kenyte erratics in the eastern foothills of the Royal
Society Range indicate westward flow of grounded
glacier ice across McMurdo Sound from the kenyte
Geografiska Annaler · 82 A (2000) · 2-3
bedrock outcrops on Ross Island at Cape Royds,
Cape Bare, Cape Evans, and Turks Head.
The drift that mantles coastal headlands along
the eastern foothills of the Royal Society Range is
in contact with Hobbs Glacier. Several folds in drift
distal to Hobbs Glacier reflect recent advance of the
Hobbs terminus. This suggests that Hobbs Glacier
is now at its maximum extent since deposition of
the widespread coastal drift blanket (Stuiver et al.
1981).
In contrast to the situation along the coastal forelands, where Ross Sea ground moraine is bounded
by a well-developed moraine ridge, the valley
floors are mantled by a unique facies of glaciolacustrine drift. This drift was deposited by lake-ice
conveyors that floated on proglacial lakes dammed
by lobes of the grounded Ross Sea ice sheet that deposited the ground moraine on the coastal headlands. The floating ice cover on these proglacial
lakes rafted material inland from the valley mouth
glacier lobes, distributing it as moat ridges,
mounds, and stratified sediments with datable algal
mats and carbonate layers. Clayton-Greene (1986)
and Clayton-Greene et al. (1988) described such
glaciolacustrine deposits in Miers Valley, and
Dagel (1984) in Garwood Valley. Only one such
lake-ice conveyor system remains today on Trough
Lake (Fig. 10m, n), and is the basis for Hendy et
al.'s (2000) paper on lake-ice conveyors.
Ferrar Glacier valley. The Ferrar Glacier valley is
an east-west trending trough that extends 65 km inland from McMurdo Sound and forms the major
193
G.H. DENTON AND D.R. MARCHANT
Fig. 10a. Aerial view of the McMurdo Ice Shelf in southern McMurdo Sound.
Fig. 1Ob.Aerial view of upper limit
of Ross Sea drift on Brown Peninsula.
Fig. 10c. Aerial view of Ross Sea
drift on the western coast of McMurdo Sound near the mouth of
Miers Valley in the eastern foothills of the Royal Society Range.
The drift is largely ice cored, and a
moraine ridge usually marks its
outer limit.
194
Geografiska Annaler * 82 A (2000) · 2-3
RECONSTRUCTION OF GROUNDED ICE SHEET
Fig. 10d. Aerial view of Ross Sea
drift on the western coast of McMurdo Sound near the mouth of
Miers Valley along the eastern
foothills of the Royal Society
Range. The drift is largely ice
cored, and a moraine ridge usually
makes its outer limit.
Fig. 10e. Aerial view of the upper
limit of Ross Sea drift on the western coast of McMurdo Sound just
south of Miers Valley.
Fig. 10f. Aerial view of Ross Sea
drift on the flank of Hjorth Hill
alongside easternmost Taylor Valley. The drift extends continuously
from New Harbor (bottom) to the
Hjorth Hill moraine at its upper
limit. Raised beach ridges do not
occur in New Harbor.
Geografiska Annaler · 82 A (2000) · 2-3
195
G.H. DENTON AND D.R. MARCHANT
Fig. 10g. Aerial view of the upper
limit of Ross Sea drift, marked by
the Hjorth Hill moraine. In the
background is Marble Point, which
features raised beaches (Hall and
Denton 2000b).
Fig. Oh.Perched relict delta on the
wall of Taylor Valley adjacent to
the terminus of Rhone Glacier, a
local alpine glacier that flows
down the north wall of Taylor Valley. The delta in the foreground
was deposited in Glacial Lake
Washburn about 16,470 + 250 14C
yr BP (QL-1046), the date of algae
enclosed within the delta (Stuivei
et al. 1981). The Rhone Glacier is
advancing over the upper ends of
the deltas and therefore now occupies its maximum position since
the LGM. See Stuiver et al. (1981)
for details.
Fig. 10i. Aerial view of the floor of
eastern Taylor Valley showing the
surface morphology (sinuous
longitudinal ridges, cross-valley
ridges, and mounds) of the glaciolacustrine facies of Ross Sea drift
deposited from a lake-ice conveyor
on Glacial Lake Washburn. See
Hall et al. (2000) for details about
glaciolacustrine sediments in eastern Taylor Valley. See Hendy et al.
(2000) for description of Antarctic
lake-ice conveyor systems.
196
Geografiska Annaler · 82 A (2000) · 2-3
RECONSTRUCTION OF GROUNDED ICE SHEET
Fig. 10j. Aerial view of the floor of
easternTaylor Valley showing surface morphology (sinuous longitudinal ridges, cross-valley ridges,
and mounts) of the glaciolacustrine
facies of Ross Sea drift deposited
from a lake-ice conveyor on Glacial Lake Washbur. See Hall et al.
(2000) for details about eastern
Taylor Valley. See Hendy et al.
(2000) for description of Antarctic
lake-ice conveyor systems. See
Fig. 10m and n for an example of a
modern ice-conveyor system.
between the KukriHills and the Asgard Range (Fig.
topographic break between the Royal Society
block of the TAM to the south and the Dry Valleys
2). The eastward-flowing Taylor Glacier, which
block to the north. Ferrar Glacier drains ice both drains Taylor Dome at the edge of the East Antarcfrom Taylor Dome and from mountain glaciers tic Ice Sheet, terminates about two-thirds of the
along the western flank of the Royal Society Range. way down the valley at 57 m elevation. Details on
Along its lower reaches, Ferrar Glacier floats on a
long arm of the sea that penetrates far into the valley. A thin (<1.0 m), little-weathered, and matrixsupported drift sheet mantles the south wall of Ferrar Glacier valley. The upper drift limit is highest
near the coast, where it reaches an elevation of 400
m, and lowest in the central valley near Overflow
Glacier, where it reaches an elevation of 200 m,
only about 50 m above the present surface of Ferrar
Glacier. The drift lacks an outer moraine ridge, and
instead thins to feather edges on weathered bedrock
and/or colluvium.
We found only three kenyte clasts in FerrarGlacier valley, even though drift exposed along the
McMurdo Sound coast to the north and south (eastern Royal Society foothills and Taylor Valley, see
below) contains numerous kenyte erratics. Hence,
glacier ice with kenyte clasts was blocked from
flowing westward into FerrarGlacier valley. However, the Strand Moraine, situated offshore at the
outer margin of Bowers Piedmont Glacier about 5
km south of FerrarGlacier, contains a high percentage of kenyte clasts. The implication is that Ferrar
Glacier blocked the mouth of FerrarGlacier valley
when westward-flowing grounded ice filled McMurdo Sound.
Taylor Valley. Taylor Valley extends east-west
through the TAM and opens to McMurdo Sound
Geografiska Annaler
.82 A (2000) · 2-3
Fig. 10k. Raised beaches at Kolich Point on the Scott Coast. See
Hall and Denton (2000b) for details.
197
G.H. DENTON AND D.R. MARCHANT
Fig. 101. Raised beaches at Spike
Cape on the Scott Coast. See Hall
and Denton (2000b) for details.
Fig. 10m. Aerial view of Trough
Lake, Pyramid Valley. Koettlitz
Glacier is in foreground. This is the
only remaining example of the
lake-ice conveyor system (Hendy
et al. 2000) that was so widespread
at the LGM (Clayton-Greene et al.
1988; Hall et al. 2000).
glacial deposits in Taylor Valley are given in Hall et
al. (2000); Hall and Denton (2000a), and Higgins
et al. (2000a, b). A little-weathered, kenyte-rich
drift sheet mantles the valley floor at the mouth of
TaylorValley and extends upward without break to
a prominent moraine ridge at 350 m elevation on
the flank of Hjorth Hill (Fig. lOf, g). The slope of
the Hjorth Hill moraine, along with the areal extent
of the drift, indicate deposition by a thick lobe of
westward-flowing ice that was grounded in New
Harbor in the mouth of Taylor Valley.
Contemporaneous deposition of glaciolacustrine sediment from a lake-ice conveyor on Glacial
Lake Washburn,dammed by the ice lobe grounded
in the valley mouth, occurred far inland on the valley floor; 189 perched lacustrine deltas in Taylor
198
Valley, many radiocarbon-dated (Hall and Denton
2000a), document the existence of this former proglacial lake (Fig. lOh). These glaciolacustrine sediments feature sorted silty sand (locally sandy silt)
with dropstones ranging from gravel (most common) to boulders, many striated. Stratification is
common. The sediments also include well-sorted
sand that is generally horizontally stratified and
contains rarethin layers of lacustrine blue-green algae and/or disseminated scallop shell fragments
(Adamussium colbecki (Smith)) and sponge spicules. A third common unit consists of stratified,
interbedded silt, sand, and gravel capped by cobbles and boulders. Fossil algae are present, as well
as broken pieces of barnacles, sponge spicules, corals and shells. This type of glaciolacustrine deposit
Geografiska Annaler · 82 A (2000)
22-3
RECONSTRUCTION
OF GROUNDEDICESHEET
is unique to the McMurdo Sound region. It is described in detail in Hendy et al. (2000) and Hall et
al. (2000).
These sedimentary units make up a suite of distinctive morphologic features on the valley floor
and walls (Fig. 1Oi,j). Included in this suite are lateral ridges and benches, cross-valley ridges, sinuous longitudinal ridges, and mounds (Hall et al.
2000). All are interpreted as having been formed by
sediment transportedinto Taylor Valley first by the
grounded ice lobe in New Harbor and then by a
lake-ice conveyor floating on Glacial Lake Washburn (Hall et al. 2000).
Scott Coast: Cape Bernacchi to Cape Roberts
The Wilson Piedmont Glacier terminates in the
western Ross Sea along most of this sector of the
Scott Coast, which is characterized by fast sea ice
that breaks out only rarely. Several ice-free capes,
islands, and points mark this otherwise ice-bound
shore. Thin, discontinuous Wilson drift occurs on
such ice-free areas (Hall and Denton 2000b). On
Cape Bernacchi, Wilson drift extends from a few
meters above sea level onto the slopes of Hjorth
Hill, where the upper drift edge rises from 450 m elevation on the eastern slope to 490 m on the northern slope. Wilson drift also extends from the Wilson Piedmont Glacier south to Hjorth Hill, where it
merges with the kenyte-rich Ross Sea drift in eastern Taylor Valley. Here segments of the Hjorth Hill
moraine delineate the upper limit of both drifts. An
interlobate moraine extends northeast for about 2
km from the HjorthHill moraine, marking the junc-
Fig. 1On.Aerialview of TroughLake,PyramidValley.Thisis the
onlyremainingexampleof the lake-iceconveyorsystem(Hendy
et al. 2000) thatwas so widespreadat theLGM(Clayton-Greene
et al. 1988; Hall et al. 2000).
Fig. 10o. Typical granite erratic at
Cape Royds. See Fig. 12 for loca-
in Fig. 10.
tionsof all photographs
Geografiska Annaler · 82 A (2000) - 2-3
199
G.H. DENTON AND D.R. MARCHANT
tion of the two drifts. On Marble, Gneiss, and
Kolich Points, as well as on Spike Cape, Wilson
drift extends discontinuously from the uppermost
limit of Holocene raised beaches (16.5-21 m elevation) to the present-day Wilson Piedmont Glacier. On Cape Dunlop, patches of Wilson drift occur between the coastal cliffs and the Wilson Piedmont Glacier. In situ Wilson drift is not preserved
on Cape Roberts and Dunlop Island, because icefree areas in both localities are below the marine
limit (Hall and Denton 2000b).
Wilson drift is a loose, coarse-grained, sandy till
containing numerous ventifacts and stained clasts.
Glacially molded and striated stones are rare, except in isolated locations. Wilson drift contains almost exclusively local lithologies, including
gneiss, granite, schist, marble, and dolerite. Kenyte
and sandstone erratics are absent, in sharp contrast
to the situation in the adjacent and contiguous
kenyte-rich drift deposited on Cape Beracchi and
in eastern Taylor Valley. Overall, Wilson drift
shows little weathering. Unlike the coeval kenyterich drift in eastern TaylorValley, Wilson drift does
not contain reworked shells, corals, or other marine
organisms (Hall and Denton 2000b).
Numerous striations occur on small patches of
glacially molded and plucked bedrock surfaces
with crescentic gouges that project through Wilson
drift at Cape Dunlop, Spike Cape, South Stream,
Marble Point, and near Cape Bernacchi. These features indicate deposition of Wilson drift by seaward-flowing ice of the Wilson Piedmont Glacier
that covered this portion of the Scott Coast at the
LGM (Hall and Denton 2000b).
Ross Sea drift
We can now group together all mapped drift units
that extend without break from at or near sea level
to a well-defined upper limit on coastal areas of
McMurdo Sound. We make this correlation not
only because of the similar distribution of drift at
each locality with regard to present-day sea level,
but also because of common physical characteristics (Table 1). Just as Stuiver et al (1981) before us,
we call all these units Ross Sea drift. Table 1 summarizes the physical characteristics of Ross Sea
drift throughout the McMurdo Sound region.
In McMurdo Sound, Ross Sea drift occurs at
northern Minna Bluff, northern Mount Discovery,
Brown Peninsula, northern Black Island, northern
White Island, and Ross Island (Cape Bird, Cape
Royds/Cape Barne, Cape Evans, and Cape Cro200
zier). At Brown Peninsula and Mount Discovery,
Ross Sea drift with erratic stripes extends seaward
across the tidal crack and makes up the swirl-like
debris bands on the southern McMurdo Ice Shelf.
Ross Sea drift also extends northward along the
west coast of McMurdo Sound from the southern
Royal Society Range to the vicinity of Hjorth Hill
at the northern side of lowermost Taylor Valley.
Here Ross Sea drift merges with coeval Wilson drift
(Hall and Denton 2000b). On the eastern flank of
Hjorth Hill the two drifts even share a common upper moraine ridge. As discussed above, Wilson drift
extends along the Scott Coast from Cape Bernacchi
to Cape Roberts. We also include as Ross Sea drift
a coeval and unique glaciolacustrine facies that was
deposited on the floors of valleys that open to McMurdo Sound along the TAM front (ClaytonGreene et al. 1988; Hall et al. 2000; Hendy et al.
2000).
Reconstructionof grounded ice sheet in
McMurdo Sound
The upper limit and areal distribution of Ross Sea
drift deposited by grounded ice define the configuration of the ice sheet that at its maximum extent
filled McMurdo Sound (Fig. 11). We argue that this
ice sheet must have been grounded on the floor of
the sound. First, the upper limit of Ross Sea drift
rises eastward from a low of 250 m elevation in the
coastal foothills of the Royal Society Range to a
high of about 710 m elevation at Cape Crozier at the
eastern tip of Ross Island. Waterdepths required to
float ice up to these elevations range from 1830 m
to 2160 m. Because maximum water depths in McMurdo Sound are 800-1000 m, with typical depths
of 400-600 m, the ice sheet must have been
grounded. Second, the ice sheet must have rested
on the sea floor to maintain the surface ice slopes
recorded by the drift limits. Third, reworked marine
shells and corals within Ross Sea drift are consistent with ice-sheet grounding on the floor of McMurdo Sound. Finally, Ross Sea ice lobes dammed
deep proglacial lakes in Taylor, Marshall, and
Miers Valleys alongside western McMurdo Sound.
In all three cases the moraine at the upper limit of
Ross Sea drift on the adjacent headlands slopes
downward into the valley mouth. These features all
require that the Ross Sea ice lobes must have been
grounded on the sea floor off the valley mouths.
Figure 11 shows the surface contours of the
grounded ice sheet in McMurdo Sound based on
the elevation of the upper limit of Ross Sea drift on
Geografiska Annaler · 82 A (2000) - 2-3
RECONSTRUCTION OF GROIUNDEDICE SHEET
coastal ice-free terrain adjacent to McMurdo
Sound. The elevation data for the upper limit of
Ross Sea drift are given in Table 2. They are not corrected either for isostatic uplift following deglaciation or for any possible tectonic uplift. Raised
beaches along the Scott Coast north of Cape Bermacchi (Hall and Denton 2000b) and at Cape Bird
on Ross Island (Dochat et al. 2000) show relatively
minor isostatic uplift after deglaciation. It is also
doubtful that significant tectonic uplift has occurred recently, as shown by 39Ar/40Ardates of volcanic units near present-day sea level. For example,
the ages of subaerial lava flows near the coastline in
the southern Royal Society foothills place a limit of
65 m for surface uplift since late Miocene time
(Sugden et al. 1999). 40Ar/39Ar analyses of
anorthoclase crystals show that low-elevation
kenyte bedrock at Cape Royds was emplaced subaerially at 73 ± 5 kyr; similar low-elevation kenyte
bedrock at Cape Barne dates to 88 + 3 to 91 + 2 kyr
(Esser et al. 1994). The implication is that uplift of
at least this portion of Ross Island did not affect the
elevation of Ross Sea drift. In fact, submergence
beneath the growing volcanic load is more likely
than uplift.
The flowlines in Fig. 11 are drawn perpendicular
to the ice-surface contours reconstructed from the
data in Table 2. These flowlines are consistent with
ice-flow trends inferred from striations on Scallop
Hill on Black Island, with the surface slope of the
upper Ross Sea drift limit, and with the trend of erratic stripes and debris bands on the flanks of
Mount Discovery, Brown Peninsula, and Black Island, as well as near Hobbs Glacier and in Miers
and Garwood Valleys. These flowlines imply that a
grounded ice sheet flowed landward from the Ross
Sea into McMurdo Sound to terminate along the
TAM front, plugging the mouths of Taylor Valley
and of ice-free valleys in the eastern foothills of the
Royal Society Range. This landward ice flow into
McMurdo Sound was split by Ross Island into two
major segments. We now describe, first, the flowlines that passed north of Ross Island and, second,
those that entered the sound south of Ross Island.
Ross Sea drift with TAM erratics shows that
grounded ice impinged on Cape Crozier to an elevation of 710 m. Ice grounded on the floor of McMurdo Sound reached to more than 590 m elevation on Cape Bird. The former ice-flow direction at
Cape Bird could not have been directly eastward
from the TAM, because the upper limit of Ross Sea
drift is considerably lower along the western coast
of McMurdo Sound than it is at Cape Bird. MoreGeografiska Annaler · 82 A (2000) · 2-3
over, the flow direction could not have been northward through McMurdo Sound along the western
coast of Ross Island from Cape Bare to Cape Bird.
Such northward flow would have deposited on
Cape Bird numerous erratics of kenyte derived
from outcrops farther south on western Ross Island. The few kenyte erratics that do occur at low
elevations at Cape Bird (five in total) are probably
retransported clasts originally carried into the
sound by kenyte-rich icebergs (Dochat et al. 2000).
Thus we conclude that Ross Sea drift on Cape Bird
must have been deposited by the same grounded ice
body that impinged on Cape Crozier and then
flowed westward around northern Ross Island.
South of Cape Bird, the areally scoured bedrock
and subglacial meltwater channels on Cape Royds/
Cape Bare, both associated with TAM erratics and
Ross Sea drift, point to north-to-south ice flow. The
implication is that grounded ice that passed around
northern Ross Island and impinged on Cape Bird
then flowed southward across Cape Royds/Cape
Barne. The lack of a bounding upper moraine suggests that local glaciers from the eastern flank of
Mount Erebus merged with the grounded ice sheet.
Farthersouth, Cape Evans exhibits only a few TAM
erratics and none of the subglacial channels as near
Cape Barne. Far-traveled erratics foreign to Ross
Island do not occur south of Cape Evans, for example at Turks Head or Hut Point Peninsula. The reduction in the overall numbers of granite, dolerite,
and sandstone erratics from Cape Bird south to
Cape Evans probably reflects the progressive input
of local glacial ice from the adjacent slopes of
Mount Erebus. Thus we conclude that the southward-flowing ice that crossed Cape Royds/Cape
Barne also deposited the TAM erratics at Cape
Evans. Somewhere between Cape Evans and Turks
Head, this flowline was deflected westward across
the sound. The probable reason for this deflection
is an opposing westward flow of grounded ice
around southern Ross Island. Near Ross Island this
southern flowline was conspicuously free of TAM
erratics. A possible reason is that the strongest glacier flow from volcanoes of Ross Island is southward into Windless Bight. As shown in Fig. 11, this
local inflow would have formed the clean western
margin of a Ross Sea sheet that flowed over Hut
Point Peninsula.
By this reconstruction, grounded Ross Sea ice
flowing into McMurdo Sound was split around
Ross Island. The split flow merged between Cape
Evans and Turks Head, and was then deflected
westward across the sound. An important point is
201
G.H.DENTONAND D.R. MARCHANT
Fig. 11. Flowlinesandsurfacecontoursof the groundedice sheetthatdepositedRoss Sea driftin
theMcMurdoSoundregionfromDentonandHughes(2000).Theflowlinesarederivedfromthe
distributionof TAMerratics,the orientationof subglacialchannels,the kenyteerratictrain,striationtrendson bedrock,surfaceerraticstripes,andthe slopeof theRoss Sea driftlimit.Contours
arebasedon the elevationsof the Ross Sea driftlimit.
202
Geografiska Annaler · 82 A (2000) · 2-3
C)
CD
(D
13
:>
_F
G.H. DENTON AND D.R. MARCHANT
that ice in these merged flowlines eroded kenyte erratics from the heavily scoured bedrock between
Cape Royds and Turks Head. In addition, some
kenyte erratics may have been entrained by glacier
flow off of the western slope of Mount Erebus onto
the Ross Sea grounded ice sheet. These combined
flowlines from western Ross Island spread numerous kenyte erratics in Ross Sea drift between Hjorth
Hill at the mouth of Taylor Valley in the north, and
the Royal Society coastal forelands near Miers Valley in the south. The conspicuous exception is Ferrar Glacier valley. A likely reason is that local expansion of Ferrar Glacier blocked this part of the
coast from incursion by landward-flowing grounded ice from the Ross Sea. A similar situation occurs
with regard to the Wilson Piedmont Glacier along
the Scott Coast (Hall and Denton 2000b). Here the
seaward margin of Wilson Piedmont Glacier advanced eastward across the coast at the LGM. This
expanded piedmont glacier deflected westwardflowing grounded ice in northernMcMurdo Sound
as shown in Fig. 11. Thus kenyte-bearing grounded
ice never reached the coastline north of Cape Bernacchi.
The reason that piedmont glaciers along the
Scott Coast advanced at the LGM is probably that
they had extensive marine portions and hence behaved in much the same fashion as grounded ice in
McMurdo Sound. For example, substantial segments of Wilson Piedmont Glacier are grounded
below sea level (Calkin 1974), and much of the ablation is by iceberg calving into the Bay of Sails.
Thus these piedmont glaciers can react as marine
ice sheets. This is in sharp contrast to the behavior
of independent terrestrial alpine glaciers in the
TAM, which receded behind their present-day margins at the LGM. These alpine glaciers have since
expanded during the Holocene, until most now occupy their maximum positions since the LGM (Stuiver et al. 1981; Denton et al. 1989; Hall and Denton 2000a). For example, Taylor, Rhone, and Hughes Glaciers in Taylor Valley are either advancing
over, or are adjacent to, deltas of Glacial Lake
Washburnthat are dated to the LGM (Stuiver et al.
1981). Canada and Hobbs Glaciers are expanding
into Ross Sea drift deposited during the LGM.
Cross-cutting moraine patterns show that Walcott
Glacier in the eastern foothills of the Royal Society
Range fluctuated in an out-of-phase fashion with
grounded Ross Sea ice (Denton et al. 1971). Overall, independent alpine glaciers in the McMurdo
Sound region were retracted during the LGM, and
have since expanded to their maximum Holocene
204
positions. We agree with Dort (1970) and Stuiver et
al. (1981) that the most likely cause of alpine glacier recession at the LGM was the removal of a major precipitation source when the Ross Sea was
filled with grounded ice. Presumably the out-ofphase behavior with regard to grounded ice in the
Ross Sea is due to reduced LGM precipitation in
comparison with the Holocene situation.
Figure 11 implies that radially outflowing ice
from Ross Island fed the grounded Ross Sea ice
sheet in McMurdo Sound at the LGM. The lack of
upper bounding moraines at Cape Bird and Cape
Royds/Barne indicates that this grounded ice must
have dammed, and thus thickened, the lower segments of ice flowing outward from the flanks of the
high volcanoes of Ross Island so that they fed the
grounded ice sheet in McMurdo Sound. Because of
the damming effect from grounded ice, this outflow
occurred even in the face of decreased accumulation at the LGM. In this fashion, volcanic erratics
(including kenyte) could have been transportedinto
this grounded ice body by outflow from Ross Island. This situation on Ross Island is akin to that
along the TAM front, where the grounded ice sheet
in the Ross Embayment dammed East Antarctic
outlet glaciers. Again, the result was that the
dammed East Antarctic outlet glaciers fed the
grounded ice sheet and carried erratics from the
TAM into the grounded ice body in the Ross Sea.
As depicted in figs 3 and 4 of Denton and Hughes
(2000), most of the ice from the outlet glaciers in
the Ross Sea ice sheet passed westward of Ross Island to the LGM grounding line on the outer continental shelf. But a minor amount of grounded ice
leaked into the McMurdo Sound oasis, where it left
diagnostic TAM erratics on volcanic islands and
peninsulas and, in turn, carriedkenyte erratics from
Ross Island westward across the sound.
The southern flowlines, which entered McMurdo Sound between the tip of Minna Bluff and Ross
Island, did not incorporate kenyte erratics, and
hence can be differentiated easily from the northern
flowlines. Ice from these southern flowlines was
funneled first westward and then southwestward,
where it terminated against the eastern foothills of
the Royal Society Range. The southern flow into
the sound appears to have been weaker than the
northern flow, as indicated by the deflection of
southern by northern flowlines. One reason is that
the southern flow encountered the physical obstacles of White and Black Island, as well as Brown
Peninsula. Another is that the southern flowlines lie
largely within an extensive wind-induced ablation
Geografiska Annaler · 82 A (2000) · 2-3
RECONSTRUCTION OF GROUNDED ICE SHEET
zone, as shown by the distinct upper moraine limits
of Ross Sea drift on northern Minna Bluff, Black
Island, northern Mount Discovery, Brown Peninsula, and the Royal Society foothills.
In constructing Fig. 11, we immediately removed from consideration the possible expansion
of alpine and outlet glaciers from the Dry Valleys
and the Royal Society Range as the source for granite, dolerite, sandstone, gneiss, and marble erratics
in Ross Sea drift, because data given above indicate
that grounded ice flowed westward across McMurdo Sound. The evidence includes westward-sloping moraines that wrap around high-elevation cinder cones at the upper limit of Ross Sea drift on
Mount Discovery and Brown Peninsula, westward
transportof kenyte erratics from Cape Royds, Cape
Barne, and Turks Head to the southern Scott Coast,
glacially molded and striated bedrock at Scallop
Hill that indicates ice flow directions at about
N15°W, and finally the overall westward dip of
Ross Sea drift from Cape Crozier (mapped limit of
710 m elevation) to the southern Scott Coast
(mapped limit between 250 and 260 m elevation in
Miers and Marshall Valleys).
The geometric relationship between Ross Sea
drift and alpine moraines also indicates that local
alpine glaciers did not contribute significantly to
the grounded ice sheet in McMurdo Sound. For example, the geometric relation of Ross Sea drift to
alpine moraines implies that the Canada, Commonwealth, Hobbs, and Walcott Glaciers now seem to
be at maximum frontal positions since the LGM
(Stuiver et al. 1981; Hall and Denton 2000a,b). Finally, data presented in Stuiver et al. (1981) concerning lacustrine deltas and strandlines show that
alpine glaciers in Taylor Valley, as well as Taylor
Glacier itself, were less extensive than now at the
time when the Ross Sea ice sheet dammed Glacial
Lake Washburn in Taylor Valley. Several exceptions to this pattern are shown by the surface slope
of Ross Sea drift, suggesting that Koettlitz and Blue
Glaciers were probably dammed by, and hence
were contiguous with, the grounded ice sheet in
McMurdo Sound. Also, we have already mentioned that Bowers and Wilson Piedmont Glaciers,
along with lower Ferrar Glacier, were contiguous
with grounded ice in McMurdo Sound.
Debenham (1921) postulated that an expansion
of Koettlitz Glacier could account for the distribution of granite erratics on Ross Island. This scenario is not in accord with the flow directions inferred
from kenyte erratics or with measured elevations of
Ross Sea drift, both of which indicate significantly
Geografiska Annaler · 82 A (2000)
.2-3
higher ice levels on Ross Island (at least 590 m elevation at Cape Bird) than on the Royal Society
foothills near Koettlitz Glacier (where the highest
moraine is at 320 m elevation). Rather, we agree
with Scott (1905) and David and Priestley (1914),
who suggested that outlet glaciers passing through
the TAM and merging with the grounded Ross Sea
ice sheet south of McMurdo Sound provided the
source for far-traveled erratics on Ross Island.
Such an origin for the sandstone, dolerite, marble,
and granite erratics is in accord with observed upper limits of Ross Sea drift. South of Ross Island
these upper limits decrease from a minimum of 637
m elevation at eastern Minna Bluff, to 561 m at
White Island, to 522 m at Black Island, to 519 m at
Mount Discovery, to 365 m at Brown Peninsula,
and finally to 350 m on the southern Royal Society
foothills, showing ice inflow westward into the
sound. As mentioned above, this westward flow involved only a minor amount of ice that peeled off
into McMurdo Sound from the main body of
grounded ice flowing northward to the continental
shelf edge. This makes it likely that most of the ice
entering McMurdo Sound originated from outlet
glaciers that flowed into the Ross Sea Ice Sheet just
south of Minna Bluff.
Raised beaches
An explanation is required for the striking observation that raised beaches (Fig. 10k, 1)of very moderate elevation occur at only two localities within
the limits of the thick marine-based ice sheet complex that filled McMurdo Sound at the LGM. One
locality is along the Scott Coast north of TaylorValley (Hall and Denton 1999, 2000b). The second is
at Cape Bird (Dochat et al. 2000). The Scott Coast
raised beaches lie within the area covered by the expanded Wilson Piedmont Glacier when it coalesced
with the grounded Ross Sea ice sheet. Those at
Cape Bird were covered with the Ross Sea ice sheet
itself as it flowed north around Ross Island into McMurdo Sound.
Colhoun et al. (1992) suggested that the lowness
of the Scott Coast raised beaches, together with the
downward slope of the marine limit from north to
south, indicate that only limited grounded ice, centered near Mackay Glacier, occupied the western
Ross Sea at the LGM. This conclusion is not in accord with the configuration of the grounded Ross
Sea ice sheet in Fig. 11. Nor does the newly determined rebound pattern of the southern Scott Coast,
based on radiocarbon chronology, show an uplift
205
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207
G.H. DENTON AND DR. MARCHANT
center near Mackay Glacier (Hall and Denton
1999, 2000b). Instead, the amount of uplift since
any given time is similar along the Scott Coast between Mackay Glacier and Cape Beracchi, instead of decreasing southward from Mackay Glacier as required by the model of Colhoun et al.
(1992). For example, the age (4500 14Cyr BP) of the
12-13 m raised beach at Dunlop Island (marine
limit >21 m) is similar to the ages of the 12-13 m
raised beaches at Marble Point (marine limit = 21
m) and at South Stream (marine limit = 14 m). This
indicates an equal amount of uplift all along this
portion of the Scott Coast since 4500 4C yr BP
(Hall and Denton 1999).
As suggested by Stuiver et al. (1981), a simple
explanation for the north-to-south tilt of the marine
limit along the northern Scott Coast is that recession of the ice-shelf calving front was delayed until
well after retreat of the ice-shelf grounding line. If
so, then the age and elevation of the marine limit
may not accurately reflect the date of deglaciation
of grounded ice, nor the true amount of relative sealevel change, because a long-standing ice shelf can
preclude formation of raised beaches. This postulated Holocene ice shelf would simply represent a
northward extension of the present-day McMurdo
Ice Shelf. The ice-shelf front stretched seaward
from the northern Scott Coast after grounded ice
had cleared McMurdo Sound. The southward-retreating ice-shelf front cleared Marble and Gneiss
Points by 5700 14Cyr BP and Cape Beracchi by
4300 14C yr BP (Hall and Denton 2000b). On the
eastern side of the sound the retreating shelf edge
cleared Cape Bird by 3585 14C yr BP (Dochat et al.
2000). The lack of raised beaches south of Capes
Bernacchi and Bird indicate recent ice-shelf recession. The present-day McMurdo Ice Shelf is hence
a remnant of a former large ice shelf that covered
all of McMurdo Sound.
Chronology of Ross Sea drift
- Maximum ages for Ross Sea drift in the McMurdo Sound region come from Cape Bird, Cape
Royds/ Barne, and eastern TaylorValley. Marine
shell fragments reworked into Cape Bird drift
(correlated with Ross Sea drift by Dochat et al.
2000) are as young as 26,860 '4C yr BP (Dochat
et al. 2000). Anorthoclase crystals from kenyte
bedrock that has been subglacially scoured and
dissected by the Ross Sea grounded ice sheet
yielded 40Ar/39Ardates of 88 ± 3 kyr to 91 ± 2
kyr at Cape Barne, 73 + 5 kyr at Cape Royds, and
208
32 ± 6 kyr at Cape Evans on Ross Island (Esser
et al. 1994). Resting in the subglacial channel
that is younger than 88 + 3 kyr at Cape Barne is
a hummock of ice-cored Ross Sea drift with glacially transportedshells that date to >45,700 14C
yr BP (Table 2). A mollusc shell from this deposit
was U/Th dated to 120,000 yr ago. Elsewhere on
Cape Barne/Cape Royds, glacially transported
shell deposits yielded ages of >47,700 14C yr BP
and 37,700 4C yr BP (Table 1, Fig. 4c). In eastern
Taylor Valley, marine shell fragments reworked
into Ross Sea drift are as young as 29,367 14C yr
BP (Hall and Denton 2000a).
The glaciolacustrine facies of Ross Sea drift has
been dated in eastern Taylor Valley and Miers
Valley (Clayton-Greene 1986; Clayton-Greene
et al. 1988; Hall and Denton 2000a). These glaciolacustrine sediments were deposited by lakeice conveyors attached to Ross Sea ice lobes and
floating on proglacial Lakes Washburn and
Trowbridge, respectively. The deposits contain
algae that lived on the lake floor or carbonate
plates that precipitated onto the lake floor. The
potential reservoir correction for these dates is
unknown, but is probably variable and potentially large (Hall and Denton 2000a; Doran et al.
1999). The major problem is that these lakes
may have been density stratified and hence may
not have overturned. In nearby valleys, ages of
some samples of algae and coprecipitated carbonate measured by both the radiocarbonand U/
Th methods were similar, but ages of other samples were offset by as much as 3000 years (Hall
personal communication, 1999). All but one of
the 79 algae samples from lake-ice conveyor deposits in Taylor Valley date between 8700 and
19,680 14C yr BP (Hall and Denton 2000a). Radiocarbon-dated carbonate plates in Miers Valley yielded ages between 23,000 and 10,000 14C
yr BP (Clayton-Greene et al. 1988).
Radiocarbon dates of algae in deltas that were
deposited in the margins of Glacial Lake Washburnin TaylorValley do not need a reservoir correction (Doran et al. 1999; Hall and Denton
2000a). Most of these dates record a dam of
grounded Ross Sea ice in eastern Taylor Valley,
for otherwise the lake could not have risen above
the level of the valley floor thresholds. Thus the
existence of Glacial Lake Washburn must have
coincided with deposition of Ross Sea drift at
the valley mouth. A total of 119 dates of deltas
indicate that grounded ice in the mouth of Taylor
Valley dammed Glacial Lake Washburn beGeografiska Annaler
.82 A (2000) · 2-3
RECONSTRUCTION
OFGROUNDEDICESHEET
tween 8340 and 23,800 14C yr BP (Hall and Denton 2000a).
- Radiocarbon dates associated with moraines deposited at or near the maximum of grounded
Ross Sea ice occur at Hjorth Hill (Hall and Denton 2000a), in the mouth of eastern Taylor Valley, and along the eastern foothills of the Royal
Society Range (Table 2). The Hjorth Hill moraine near the mouth of Taylor Valley dates to
12,785-14,602 14C yr BP (Hall and Denton
2000a). A moraine embankment makes up the
valley-mouth threshold and marks the maximum extent of grounded Ross Sea ice in eastern
Taylor Valley at the LGM. Ross Sea ice was still
grounded at this threshold shortly after 12,00012,500 14C yr BP (Hall and Denton 2000a). Farther south, grounded Ross Sea ice still stood at
the LGM moraine near the present-day Blue
Glacier at 12,300 14C yr BP (Table 2; Stuiver et
al. 1981). This is consistent with two dates from
nearby Marshall Valley, which indicate that the
ice was still at or close to its maximum at
12,530-13,300 14C yr BP (Table 2; Dagel 1984).
In eastern Miers Valley, an algal mat beneath the
gypsum moraine (Clayton-Greene et al. 1988),
thought to mark a former grounding line, yielded several ages close to 19,400 14C yr BP (Table
2).
-Radiocarbon dates of Ross Sea drift slightly lower in elevation than, but only a few hundred meters distant from, the Hjorth Hill moraine are
about 10,800 14C yr BP in age (Hall and Denton
2000a). Ice-proximal deltas at 200-215 m elevation on Hjorth Hill are as young as 10,445 14C
yr BP(Hall and Denton 2000a). The youngest lacustrine delta alongside McMurdo Sound in
eastern Taylor Valley (deposited in a valleymouth lake dammed by grounded Ross Sea ice)
is 8340 14C yr BP (Hall and Denton 2000a). Deltas on Ross Sea drift on the southern wall of Ferrar Glacier valley date to 9860 to 10,000 14C yr
BP and must have formed when ice began to recede from the valley. Near Hobbs Glacier a minimum age for Ross Sea drift at 134 m elevation
is 9490 14C yr BP and at 100 m elevation is 6100
14C yr BP(Table 2). In the floor of Garwood Valley near the coast, a minimum age for Ross Sea
drift at 24 m elevation is 6190 4C yr BP(Table 2).
-A minimum age for Wilson drift comes from marine shells in raised beaches along the Scott
Coast north of Cape Bernacchi. Uplift curves
based on these shells show that the marine limit
etched into Wilson drift dates to about 6500 14C
Geografiska Annaler * 82 A (2000) · 2-3
yr BP(Hall and Denton 2000b). A minimum age
for Ross Sea drift in eastern TaylorValley comes
from the oldest shells in emerged marine deposits, dated to 5370 14Cyr BP (Stuiver et al. 1981;
Hall and Denton 2000a). A minimum age for
Ross Sea drift in the southern McMurdo Sound
region comes from barnacles dated to 6450 14C
yr BPon the surface of the McMurdo Ice Shelf
(Kellogg et al. 1990) (Table 2, Fig. 12).
Conclusions
- A thick grounded ice sheet filled McMurdo
Sound at the LGM. This grounded ice body was
not the result of regional expansion of alpine and
outlet glaciers from the adjacent Royal Society
and Dry Valleys blocks of the TAM. Rather, the
reconstructed flowlines show that the major ice
flux was landward from the Ross Sea into McMurdo Sound. Such a flow configuration requires an extensive grounded ice sheet in the
Ross Embayment. This Ross Sea ice sheet
dammed and thickened TAM outlet glaciers
both south and north of McMurdo Sound. At the
LGM most ice flow was seaward past McMurdo
Sound to near the outer edge of the continental
shelf (Shipp et al. 1999). But a minor amount of
ice peeled off the main sheet and flowed westward to feed a grounded ice body in McMurdo
Sound.
- Grounding of the Ross Sea ice sheet in McMurdo Sound occurred after about 26,865 14Cyr BP.
The grounded ice sheet remained robust, at or
close to its LGM configuration, from 23,800 14C
yr BP to 12,700 14C yr BP. Rapid lowering of the
surface of the grounded ice sheet occurred only
after 10,800 14C yr BP. The last remnants of
grounded ice near the mouth of Taylor Valley
post-date 8340 14C yr BP. The oldest marine
shells preserved in the mouth of Taylor Valley
indicate deglaciation by 5370 14Cyr BP. This is
consistent with the oldest dates of 5300-6450
14Cyr BP for shells from the McMurdo Ice Shelf
and of 6190 14Cyr BP for an algal mat near sea
level in eastern Garwood Valley. In accord with
this chronology, rapid emergence of the Scott
Coast north of Taylor Valley suggests unloading
of grounded ice just prior to 6500 14C yr BP (Hall
and Denton 1999).
- Within the limits of the coalesced ice sheet
grounded in McMurdo Sound at the LGM,
raised beaches of very modest elevation occur
only along the Scott Coast north of TaylorValley
209
G.H. DENTON AND D.R. MARCHANT
and at Cape Bird on Ross Island. Raised beaches
are strikingly absent along most shorelines of
McMurdo Sound. The apparentreason is that an
extensive version of the McMurdo Ice Shelf lingered in the sound well after recession of the icesheet grounding line, hence precluding the formation of raised beaches. In this situation the elevation of the marine limit at any particular locality records only the rebound following retreat
of the ice-shelf front, rather than the total rebound after recession of the ice-sheet grounding
line.
Acknowledgements
This research was supported by the Office of Polar
Programs of the National Science Foundation. Colleagues too numerous to mention have assisted in
this work over the past three decades. We thank M.
Stuiver at the University of Washington Quaternary
Isotope Laboratory and Warren Beck at the NSFArizona AMS Facility, for making the radiocarbon
measurements. Two reviewers greatly improved
the manuscript. J. Splettstoesser edited the manuscript. R. Kelly drafted the figures.
George H. Denton, Department of Geological Sciences and Institutefor Quaternary Studies, Bryan
Global Sciences Center, University of Maine, Orono, Maine 04469-5790, USA.
David R. Marchant, Department of Earth Sciences,
Boston University, 675 Commonwealth Avenue,
Boston, Massachusetts 02215, USA.
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