Glaciers and Glacial Landscapes
A glacier is an agent of erosion: Removal and transport of sediments
Glacier: mass of snow and ice that moves under its own weight.
Glacial ice is deformed snow.
What causes glaciers to form?
Climate: Accumulation > melting for a very long time period when Summer
temperatures do not melt all snow.
Glaciers form at: High altitudes (alpine glaciers) and High latitudes (continental glaciers)
Glaciers grow by Accumulation and Diminish by Ablation:
Ablation includes:
Melting (ice to water)
Sublimation (ice to water vapor)
Calving
Advancing glacier is growing and a Retreating glacier is getting smaller, but still moving forward!
Glaciers move by:
Deformation: Ice under confining pressure is deformed, moves by plastic flow in response
to overlying weight
Sliding : Partial melting at base provides lubrication; mass of ice slides
Crevasses: Large cracks in ice; Evidence of movement
Ice Fall : area of many crevasses
Glaciers are constantly moving forward, even if they are retreating, as in this diagram of a
retreating glacier.
The material moved forward by the glacier is deposited as the ice melts.
Where does this material come from? Sediment, rocks, boulders that have been
“bulldozed” by the moving ice, and incorporated into the ice.
Mini-example in Duluth in spring: Sand and gravel forming dirty ice as the ice melts.
Snow piles in Duluth are black with the sediment that remains behind as snow melts
Glacial deposition
Glacial drift : term for all glacial deposits
Till : deposited directly by ice
Unsorted, non-stratified
Stratified drift (outwash) : deposited by meltwater from the ice
Sorted and stratified
Glacial erosion
Abrasion
Polishing
Plucking
Roche moutonnée: Streamlined bedrock knob created by abrasion and plucking,( e.g.
Half Dome, Yosemite)
Alpine Glaciers: High altitude; Low temperatures, high precipitation
Accumulation in low areas (cirques and valleys)
Glacial valley: U-shaped (Pre-glacial stream valley : V-shaped)
Cirque: Bowl-shaped depression at top of mountain where ice accumulates; Source area
for alpine glaciers; valley glaciers flow from cirques
Cirque glaciers
Hanging valleys: Main glacier carves deeply; leaves tributary valleys “high and dry”; often
contain waterfalls
Truncated spurs: steep bluffs on sides of glacial troughs between tributary (hanging)
valleys
Arête : Ridge between 2 cirques
Horn : Peak carved by 3 or more cirques
Moraine : Accumulation of rock debris deposited directly by ice
Composed of till
Lateral, Medial, Terminal moraines
Continental Glaciers (ice sheets) : High latitudes; More extensive in area; 300-5000 m thick
“Bulldozer” effect
Net Erosion : near source region; Net Deposition: near terminus
Ice-scoured plain :Vast flat to rolling plain; stripped of soil; many lakes and swamps
(Canadian shield)
Glacial grooves: Streamlined depressions in rock ; Parallel to ice movement
Glacial striations: Scratches on rock surfaces; Result of abrasion
Moraines: composed of till
Irregular belt of knobby hills, depressions, lakes, wetlands
“Knob and Kettle Topography” (Example: Kettle Moraine, Lab )
Ground moraine (till plain): blanket of till over previous surface
Terminal moraine : wide belt of till marking furthest extent of glacier
Recessional moraines: represent stages of retreat; lobe-shaped
Kettle : Circular depression in moraine (or outwash)
Process of kettle formation: At edge of ice sheet, debris is in rubbly heap;
ice boulders are buried in till; as ice melts, till and outwash subside, leaving
round depression.
Kettle lakes: water-filled kettle
Drumlin: Streamlined hill of plastered till; Occur in swarms
Esker : Long, winding, narrow ridge; Formed from subglacial meltwater stream
Outwash plain : Tremendous amount of meltwater carrying sediment from glacier
Plain similar to till plain, but sorted, stratified, usually sandy deposit
Brainerd pitted outwash plain
e r r a t ic
gouges
Ice Ages
Earth is approx. 4.7 billion yrs old
Evidence is that for MOST of that time, no glaciers of any significance.
In past 1 billion yrs., evidence of 5 “brief” periods (couple million years) of continental
glaciation : Ice Ages
Those 5 Ice Ages:
First four: hundreds of millions of years ago
Fifth: began 2.5 – 3 million yrs. ago
Late Cenozoic Ice Age (Pleistocene Ice Age)
During Ice Age:
Fluctuations in ice extent:
Glacial Periods:
 ice sheets expand, grow, thicken,
 Alpine glaciers coalesce
 Sea level drops
 Cooling of global climate
Interglacial Periods:
 Intervening periods of warmth
 Ice sheets retreat
 Alpine glaciers disappear or shrink
We are in an Interglacial Period of an Ice Age now.
Major Ice sheets today: Greenland, Antarctica
Alpine glaciers today are relatively small, compared to what they were during glacial
period.
4 most recent glacial periods: Nebraskan , Kansan, Illinoian, Wisconsin