Glacial erosion
 Abrasion
 Pure ice not hard enough to abrade underlying rock
 Glaciers use rock debris as “cutting tools”
 Produces “rock flour,” abundant in meltwater
 Worn and fresh debris transferred by meltwater
 Evidence: striations, grooves, polished stones
Glacial erosion
 Plucking/Quarrying
 Pressure melting/refreezing pulls blocks loose
from underlying bedrock
 most effective in fractured bedrock
 uncertain as to how much fracturing is caused
prior, during, and after glaciation
Glacial erosion
• Plucking/Quarrying
Melting point (C)
-2.000
-1.500
-1.000
-0.500
0.000
0
1000
melting
ICE
freezing
4000
WATER
5000
6000
7000
8000
9000
Pressure/depth melting curve for glacial ice
(based on measurements in Antarctic ice sheet)
Depth (ft)
3000
pressure
2000
Glacial deposition
 General term: glacial drift
 Note that depositional materials (i.e.,
sediments) are different from landforms.
Glacial deposition
 General term: glacial drift
 Ice deposits: till



poorly sorted, unstratified
clay matrix surrounding polished clasts
dense
 Meltwater deposits: outwash (glaciofluvial,
glaciolacustrine)


Better sorting/stratification, coarser material
Braided channels
Glacial deposition
 General term: glacial drift
 Glaciolacustrine deposits
 formed where glaciers terminate in a lake
 poorly sorted/unstratified like till, but less compact
w/random fabrics
 varves
 Glaciomarine deposits
 formed where glaciers terminate in a lake
 poorly sorted/unstratified like till, but less compact
w/random fabrics
 In situ marine fossils
 Glacioeolian deposits
 loess
Glacial deposition
 Ice deposits: till


lodgement till
 deposited at base of glacier
 plastered, smeared and sheared against rock surface
 compact, oriented clasts
ablation till
 deposited on land surface as glacier melts away
 (“supraglacial”)
 coarser than lodgement till
 often deposited in water-saturated state
Ice deposits: till
Glacial deposition
 Meltwater deposits: outwash



Meltwater discharge was highly variable
 Resulting deposits show abrupt changes in grain size
and sedimentary structure compared to conventional
fluvial deposits.
Coarse-grained (clast-spported), imbricated clasts
 may become finer downstream
 eventually grades into normal fluvial deposits
Many outwash deposits have been reworked by postglacial fluvial systems.
Meltwater deposits:
outwash
Braided Streams on Glacial Outwash Plain, Berg Lake Trail, Canada?
Geusebroek Photography: North America's Beautiful Landscape and Pristine Places
Meltwater deposits:
outwash
Meltwater deposits:
outwash
Modern glaciers near Terrace 5 at Xidatan, Qinghai (Tibet),
4400 m above sea level. Terrace 5 is dated to about
12,000 years ago using cosmogenic Be-Al dating.
Archaeological materials on Terrace 4 (West of the pictured
location) are dated to about 6000-7000 years ago and include
obsidian artifacts traced to even higher elevation sites on the Plateau
(Tibet Paleolithic Project, UCLA, 2007)
Meltwater deposits:
outwash
Meltwater deposits:
outwash
Carroll Glacier in Queen Inlet in the west arm
of Glacier Bay has thinned and stagnated
since 1906. It has gone through the transition
from a calving, tidewater glacier in open water
in 1906 (upper), to a grounded, debris-covered
glacier in 2004 (lower). Queen Inlet has been
transformed from a 170 metre (560 feet) deep
fiord (upper), to a glacial outwash plain that is
well above sea level (lower). About 1.3 cu.km
(0.3 cubic mile) of sediment has filled the
upper section of the inlet. Since 1750, 2,500
cu.km (600 cubic miles) of ice has melted from
Glacier Bay.
Photos courtesy of USGS
Upper: C.W Wright 1906
Lower: B.F. Molnia 2004
Iowa is almost entirely covered by loose sediments left behind by the continental glaciers of the Pleistocene ice
ages. So like many other Midwestern and northern states, Iowa needs two maps, one for the buried bedrock and
one for the surface deposits. Only in the Paleozoic Plateau, in the Driftless Area, is bedrock widespread—but there
you can visit the classic strata of the Mississippian sequence along the Mississippi River with their associated
fossiliferous beds and cave-bearing karst terrain.
The latest advance of the ice, known as the Wisconsinan event, left behind the hummocky moraine country of the
Des Moines Lobe. The rest of the state is covered with deposits from earlier episodes. The southern region is
deeply dissected by later erosion, while the northern areas on either side of the Des Moines Lobe were leveled by
permafrost conditions during the late ice age. Wind erosion of the dusty glacial drift left widespread, thick beds of
loess—pure silt, valued for farmland—all around the state, particularly in the Loess Hills along the Missouri River
valley. The modern Missouri and Mississippi rivers have built fairly narrow alluvial plains in the years since the
glaciers left. (Geology.about.com)
Glaciolacustrine deposits:
varved sediments
Glacioeolian deposits:loess
Terminal Pleistocene and Holocene loess exposure at Heimahe, Qinghai Lake (Tibet), 3100 m above
sea level. Human occupation surfaces with fire pits, cobbles, stone tools and bone are found at the base
of the exposure and are dated to about 12,913 years ago. Note the buried paleosol at approximately 2m
below the surface. The Tibet Paleolithic Project is studying climate change and hunter-gatherer adaptations
on Plateau envioronments during the last glacial cycle. Scale bar at right is in 0.5m units.
(Tibet Paleolithic Project, UCLA, 2007)
Glaciolacustrine
deposits
John Olsen examines an 18,000 year old ice-wedge cast
in a Pleistocene beach at Lenghu, Qinghai.
(Tibet Paleolithic Project, UCLA, 2007)
This is North Dakota's surface blanket—it's bedrock in the west and glacial sand and gravel in the east.
The Williston basin in the west is exposed rocks, the brownish tones signifying rocks dating from Tertiary
times (younger than 65 million years). An arc of older, Cretaceous rocks to its south includes the famous
Hell Creek Formation, which has yielded so many memorable dinosaur fossils.
The rest of North Dakota's surface material was laid down by the Pleistocene ice-age glaciers in the last 2
million years. The two shades of green represent till, mixed sediment of all sizes created and laid down by
the grinding and shoving action of the ice. The blue and yellow represent sediment laid down by lakes and
rivers, respectively. (Geology.about.com)
The surface of Maine is largely covered not with bedrock,
but with sand and gravel and clay from the Ice Ages of
Pleistocene time, starting about 1.6 million years ago.
Enormous continental glaciers slowly bore down from the
Canadian Shield several times, each time covering the
region thousands of meters deep in ice. The weight of the
ice depressed the crust by as much as 150 meters. And
because so much water was frozen into ice, the sea level
was much lower than today.
Glaciers scrape up huge amounts of sediment as they
move, carrying it toward their toes. There the sediment is
piled up into long heaps called moraines. And when
glaciers melt, the sediment they're carrying drops straight
down in a mixed form called till. That's what covers the
great majority of Maine, shown in light green. The
moraines are largely gone because the sea has covered
them. In fact, the meltwater filled the sea faster than the
depressed land could rebound, and as a result a large
fraction of Maine was under the ocean just 10,000 years
ago. That's where the dark blue glaciomarine deposits
come from.
Another notable glacial legacy in Maine is an abundance
of eskers. These are narrow, winding mounds of sand that
were built underneath the glaciers where rivers of
meltwater flowed. Some are as level as roadbeds, and
many have been quarried for their sand. In the low,
swampy land of much of Maine's interior, eskers can be an
important ecological niche. Geology.about.com