Glacial Geomorphology
• How do glaciers and meltwater shape landforms?
• Most processes covered by ice, so
glacial terrain exposed by melting
http://upload.wikimedia.org/wikipedia/commons/5/5c/Perito_Moreno_Gla
cier_Patagonia_Argentina_Luca_Galuzzi_2005.JPG
Alpine Glaciers
Valley glaciers get most of sediment loads from
mass wasting and intense weathering of adjacent slopes
Rock debris of lateral and medial moraines stays high up
Tongas National Forest, Alaska
Continental Glaciers
• Continental ice sheets cannot gain much
sediment from mass wasting - little is exposed
• Polar ice sheets are mostly devoid of sediment
except windblown dust from lower latitudes,
except near their bases
Processes and Bedforms
Huge Glacial Erratic
End Moraine
• Glaciers have no competence limit - all
grain sizes "from clay to giant boulders"
are carried regardless of flow rate
Glaciotectonism
Ice pushed ridges of [wet sediment] become stacked as imbricate thrust sheets
Crushing and Fracture
• The weight of kilometers of ice is
insufficient to crush most rock
• However rocks held firmly in basal ice are
often hard enough to cut the underlying
bedrock
Chattermarks
http://chattermarks.ncascades.org/chattermarks/
Plucking and Abrasion
• Lee-side rock
fragments get
incorporated
into regelation
ice and are
carried away.
They become
sharp tools that
can abrade the
upglacier (stoss
Gr. "push") side
The tools concentrate the force of the glacier's weight on a small area
Fluvial Erosion Beneath Glaciers
• Water flowing in subglacial channels can
erode the bedrock or sediment beneath
the ice.
• Erosion of subglacial sediment …is
claimed to be the source of almost all of
the outwash ….
Erosional and Residual Landforms
• Glacial troughs in former stream valleys
• Converted to U-shape by plucking of
lateral channel walls ("ice-abraded
shoulders"). Fast: 10,000 years
• Different lithologies exposed in valley
result in paternoster lakes
Trough (Proglacial) Lakes
• A rock sill and/or end moraine blocks lower
end of trough
Hanging Troughs (Hanging Valleys)
Another type of waterfall
U-Shaped Valley in Tracy Wilderness, Southeastern Alaska
Seawater Flooded U-Shaped Valleys: Fiords =Fjords
When glaciers melt, sea-level rises
Horn
Cirques
Aircraft Engine
Frost Wedging
Accumulation
Arête
Ablation
ice-rock plug
w/ imbricate thrusting
Equilibrium Line
Col
Accumulation
Crevasses
Icefall
Skipped 381 left - 382 left
lateral moraines
Tarn
Icefalls and Ogives
• Ogives - Arcuate bands or waves, with
their apices pointing down-glacier, that
develop in an icefall. Alternating light and
dark bands are called band ogives. Each
pair of bands, or one wave and trough, is
believed to represent a year’s movement
through the icefall.
Ogives on Svinafellsjökull, southern Iceland. Photo J. Alean, 2001.
Glacial or Periglacial Erosion of Finger Lakes
Glacial Erosion – Roche Moutonée
Glacial Erosion – Roche Moutonée
Yosemite NP, Calif
Huge Glacial Erratic
Glacial Deposits: Drift
1. Unsorted and Unstratified Drift: Till dropped from ice melting
Stationary & Retreat: End Moraine at Terminus
2. Sorted and Stratified Drift:
Outwash
3. Ice-contact stratified drift
http://www.geomorphology.org.uk/pages/education/alevel/coldenvirons/Lesson%2015.htm
Till – dropped by melting ice unsorted unstratified
Boulders to fine silts
Ice Contact Stratified Drift ICSD
• Ice-contact stratified drift - drift modified by
meltwater during or after deposition
• Formed close to the ice
• Large range of sizes, chaotic sedimentary
structures. slump features. faults, till
inclusions
http://www.dcnr.state.pa.us/topogeo/photogallery/icecontact.aspx
http://www.geol.umd.edu/%7Ejmerck/nature/landscapes/images/outwash21962.s.jpg
Stratified Drift - Outwash
Outwash fines are source of Loess: fines lift by wind
and re-deposited elsewhere
• Alluvium deposited by meltwater not in
close proximity to melting ice.
• Better sorted than ICSD
Skipped Prest Classification p384-385 right
Landforms of Till: Drumlins
• Elliptical Hills, blunt on the upglacier side
• Form under the ice
• Many theories to explain their origins
e.g. former drift moved by glacial return:
shaped by subglacial meltwater floods
An Origin of Drumlins Theory
Glacier retreats, leaving behind
a terminal moraine. Later it
advances again, and meltwater
reshapes the moraine into a
drumlin.
Landforms of ICSD: Kames and Kettles
• Commonly found together in the subglacial ice contact zone.
• Kettles are lakes form in ablation zone
when isolated chunks of melting ice are
buried in glacial drift deposited from
meltwater.
• A stratification of the drift forms a Kame;
during higher melting, larger particles are
suspended and deposited near the ice; at
lower discharge, more fines are deposited.
Moulin Kames
Moulin
Kame
Moulin Kames
Kame Terrace
• Interaction between valley wall and glacier
wall.
• Example: Blackfoot River Valley
Kame terrace
Melted Ice sheet yields
“ground moraine”
Kame and kettle topography
Landforms of ICSD-Eskers
• Deposits in subglacial ice tunnels
Outwash: Clark Fork of the Yellowstone
Isostacy
Lowered Sea-level - Landbridge
Outburst Floods (Jökulhlaup)
• Sudden drainage
of a glacial lake
Missoula/Spokane floods
-Breaching of moraine
dams or floating of ice
dams.
-Typically cyclic
-Massive geomorphic
work
We studied these giant ripples earlier in our consideration of Froude numbers
Pluvial Lakes excess rain near
continental glaciers
• Closed basins
– Bonneville
– Lahontan
Lake Bonneville Salt Flats
Proglacial Lakes
Lakes dammed by moraine or ridge in front of melting glacier
Proglacial Lake at Sheridan Glacier, near Cordova, Alaska
Next time: New Jersey Glaciation