The rate of erosion is greatest near the margins of glaciers, andis greater in temperate glaciers than in polar glaciers.

Cold-based glaciers, however, often have longer lifespans

Erosional processes

1. Abrasion2. Plucking and Quarrying3. Moving meltwater: abrasion and dissolution

The power of a glacier to move material is a function of its thickness and its speed

Abrasion

Glacier ice cannot abrade most rock due to softness (even cold glaciers).

Rock fragments act as abrasive elements

Ice is simply a power source and the matrix within whichrock abrades

Where do the rocks come from ? Free rocks, subglacial freezing and thawing, quarrying orvalley walls

Significant abrasion may only occur when large clasts or a large number of particles exist at the base

Quarrying and Bulldozing

Glaciers exert compressive forces on obstructing rock and tensileforces when parts of the glacier freeze to the bottom

Glaciers are capable of removing fractured segments of rock

Loose or fractured substrate can be bulldozed

Thrust-faulting can move basal material to the surface

Repeated advancing and retreating or changes in applied forceload and unload the substrate, causing bending and fracturing.This is exacerbated by freeze-thaw weathering.

Pressure melting point varies with snow accumulation, surfacemelting and crevassing (freeze-thaw zones change).

If glacier is frozen to surface and rock is fractured, it may be pluckedby the glacier above and incorporated into the ice.

Plucking Mechanism

A. Glacier frozen to bed where PMP below surface

B. Frozen bed may expand(eg. due to thinning)

C. Glacier advances, plucking some of the substrate frozen to the ice

D. After several cycles

Subglacial Meltwater Erosion

Large amount of water generated at base of temperate glaciers

Meltwater may flow through fractures, tunnels and thin sheets.

Subglacial lakes form under thick polar glaciers.Sudden release generates powerful subglacial floods.

Water flows abrade the substrate because they carry sediment.The water itself may dissolve carbonates.

Erosional features

Large-scale features

Roches moutonnéesCrag and tailDrumlinsFlutesCirqueSnow hollowsGlaciated valley features

Small-scale features

StriaeGroovesP-formsChannelsPotholes

Striae

Scratches produced by abrasion

Preserved best in fine-grained, brittle rock (eg. limestone, quartzite)

Form parallel to flow direction as rocks within the ice matrix abrade the underlying substrate

The form of striae provide a clue to the size, concentration and hardness of clasts

A. Multiple sets(deeper ones survive)

B. Wedge-shaped

C. Nailhead

D. Rat-tail

E. Polished Surface

Simple striae: Scratches of various length

Wedge-shaped Clasts abrade bedrock progressively deeplyand nailhead striae: until they are retracted back into the ice

(triangular or ellipsoidal)

Rat tail striae: Ridges formed downstream from an obstructiondue to abrasion

Polished surfaces Moving mass of silt or sand finely abradesor fine scratches: underlying substrate

Crescentic gouges: Semilunate scours, concave upstream formedafter a rock fragment is removed from betweenfractures

Crescentic marks: Presence of moving clast under pressure causes tensional stresses upstream and compressional forces downstream. Gouges or fractures form if bedrock strength exceeded.

Rat-tail

Crescentic Gouges

Grooves

Linear erosional features formed in solid bedrock: Less than 2m deep and about 50-100 m long.

Striae are visible inside.

Likely formation mechanism: Large boulders or bands of debris gouge the substrate. Followed by further abrasion by sediments inice or subglacial water

Multiple grooves, Sperry Glacier, Montana

Potholes

Potholes: Round (often deep) bedrock scours formed when small cavities are enlarged and deepened by rockclasts caught in turbulent vortices. The originalclast is often still in the (now dry) pothole.

Large-scale Erosional Features

Formed by glacial plucking, often accompanied by abrasion andflowing water.

Roche moutonnée

Streamlined forms with asmooth, gentle upslopeportion and a steep,jagged downslope portion.

Formed by both ice sheetsand valley glaciers

Formation of Roche Moutonnée

1. Pre-existing morphological irregularity of some sort (eg. smalloutcrop of relatively hard, especially igneous rock)

2. High stresses form upstream causing basal melting and the glacier slides

3. Embedded clasts abrade the bedrock upslope

4. Downslope, there is a pressure drop, so the pressure melting point rises. The glacier freezes to the base.

5. As glacier pulls away, tension causes quarrying or plucking offragmented rocks downslope.

Roche moutonnée, Yosemite national Park

Roche moutonnée, north of Ottawa, Ontario

Crag and tail

Consists of a resistant bedrock knob and a streamlined remnantof bedrock or sediments on the tail (lee side).

Crag and tail, Princess Mary Lake, Nunavut

Flutes

Sub-parallel grooves with ridges of variable size

They form in flat areas, parallel to the direction of glacier movement

Form on bedrock or sediment-covered terrain.

Mostly erosional, but also depositional as basal sediment is squeezed into fractures at the base of the glacier.

Fluted terrain,Peterborough,Ontario

Cirque

How are they formed ?

Small, thin glaciers near the snowline respond to rapidly changingclimatic conditions.

Rotational mass movements of the glacier carry ice and sedimentstoward the lip of the hollow

Erosion is efficient because of frequent freeze-thaw weathering

Sculpts mountains into steep arêtes (ridges) and horns (pyramidal mountains). The same process may sculpt nunataks

Nivation Hollows

Small niches cut into the sides of mountains through freeze-thaw cycles that break up local rocks and the movement of the resulting sediment downslope

Pyramidalform (horn) caused bycirque erosion(Matterhorn,Swiss Alps)

Nivation Hollows, Ellef Ringnes Island, Nunavut

Glaciated Valleys

Scoured by streams, then modified by glaciers

Traverse Shape: U-shape in cross section (glacial modification of V-shaped fluvial valley)

How does it change to a U-shape ?

1. Velocity of glacier higher at mid-sections of V-shapedvalley walls than at base or upper sections of wall

2. Velocity may reach zero at the base and upper sectionsof valley walls. A U-shape is most efficient for glacier flow.

Longitudinal Profile:

Generally, glaciers help to straighten and deepen valleys.

1. Erodability usually varies along longitudinal profile as a result of lithological and structural characteristics: eg. shale eroded preferentially to granite. This leads to steps.

Hanging valleys form where small glaciers meet larger onesdue to their weaker erosive capability

Glaciated valleys can be carved and then flooded during and/orafter ice retreat, resulting in fjords.

Sognefjord,Norway

DRUMLIN FIELD TERMINAL MORAINE

Glacial Transportation

Types of Glacial Drift

Supraglacial DriftSubglacial (Basal) DriftEnglacial Drift

Sediment added to a glacier by

(a) plucking and abrasion of the substrate(b) falling from side or head walls of valleys and nunataks(c) wind transportation of material onto glacier surface

Ice sheets get most of their sediment load from the surfaceValley glaciers get their sediment from both the bed and side

Sediments are transported(a) above the glacier (supraglacial drift)(b) within the glacier (englacial drift)(c) at the base (subglacial or basal drift)

Particles tend to concentrate in patches called moraines

(a) lateral moraines are derived from the valley walls(b) medial moraines form from the joining of lateral moraines(c) basal moraines form from the material eroded at the base(d) internal moraines form when sediments fall into crevasses,

where lateral moraines coalesce at the confluence of glaciers or when basal drift is thrust upward at the terminus (thrust-faulting)

Subglacial drift- composed of material derived from the local substrate (some

clasts may be added from other parts of the glacier or from previously-deposited glacial sediments)

- subglacial drift, where there is basal melting, forms a water-saturated moving carpet, facilitating basal sliding

- clasts abrade against bedrock and may also be crushed- fine powder or silt can also develop as a by-product of

abrasion

Supraglacial drift- Important in valley glaciers in which the confining walls provide

the material (largely angular particles)- In ice sheets, from nunataks, upward thrusting of basal material

and windblown sediment

Glacial DepositionTill: Material deposited directly by a glacier

Drumlin Shape:

Oval, streamlined, hills, shaped like inverted spoons or tear-drops (blunt, rounded heads and long, pointed tails along a straight axis). Lemniscate loop shape. Simple or composite

Generally 1-2 km long, 400 to 600 m wide and 15 to 30 m in height(“rock drumlins” can be larger)

Vary in size and shape, especially in different fields

Often occur in staggered pattern associated with small end moraines, and eskers

Glacial Landforms Formed by Glacial Sediments

Drumlin Composition:Composed of till, sometimesstratified

Drumlin Origin:

Erosional HypothesesDepositional HypothesesMeltwater Hypothesis

Erosional Hypotheses:

•Some drumlins have stratified material, always draped by till

•Pre-existing sediments get waterlogged and are reworked easily by advancing temperate glaciers

•Drumlins form around points where little or no deformation occurs (possibly associated with stress patterns within the glacier) and are eventually overlain with till

Depositional Hypotheses:

Theory 1:

Some sediment at the base of the glacier is granular dilatant material

This material must expand to move but this is not possible until a limiting stress level is reached

Once this strength overcome it becomes thixotropic and can flow and plaster around nuclei of more resistant, drier or coarser sediment

Streamlined deposits occur which can eventually become drumlins unless effective glacier power is too high (everything eroded)

Drumlins form within a restricted range of pressure (perhaps the pressures that are encountered behind end moraines)

Theory 2:

Under a glacier, sediments with pressured, interstitial waters move at different speeds in a “deformation carpet.”

Finer particles move faster in a pressurized slurry

Coarser materials are less easily moved, so static or slow moving obstructions develop and finer materials form streamlined mounds around these obstructions

Basal drift or previously-deposited material

Meltwater Hypothesis:

Drumlins form from catastrophic meltwater floods under temperate glaciers

During peak discharge, glaciers float: flood melts and abrades streamlined caverns in the glacier above.

The flow is reduced within these caverns and deposits material within them.

The glacier then melts, draping the mound in till.

Esker:A sinuous low ridge composed of sand and gravel formed by deposition from meltwater running through a channel beneath or within glacier ice.

Moraines:Accumulations of glacial sediment that form under moving parts of glaciers and under stagnant ice at glacier margins

1. Ground Moraines2. Terminal Moraines3. Recessional Moraine4. Interlobate Moraines5. Push Moraines6. Ice-thrust Ridges7. Lateral Moraines8. Prairie Mounds9. Moraine Plateaus10. Till Ridges

Ground Moraines

Basal lodgement till, often draped by ablation till

Deposited by rapidly retreating glaciers

Usually less than 3m in thickness

Corrugated surface with irregular ridges transverse to flow direction and fluting parallel to ice flow

Terminal Moraines

One or more subparallel ridges of accumulated glacial drift at the front of a glacier

Similar in shape to the glacier terminus

Formed because glacier terminus remains stationary while the rest of the glacier continues to carry sediment to the landform

Often have a hummocky topography (knobs and kettles)

Knobs and kettles are the result of differential ice melting and sediment release

Topographic map of hummocky topography of a terminal moraine

Two different explanations of end moraine formation have been identified:

Method 1

An actively-forming terminal glacier often has

(a) white ice zone(b) black ice zone (thin debris cover)(c) ice-cored zone with thick debris

Ice melts fastest in the black ice zone, creating a depression

Sediment slumps into the depression, eventually creating a thicker deposit, causing it to become an ice core stagnant zone (slower melt)

Eventually the glacier ice of the region is totally melted and new patterns of white ice, black ice and ice-cored zones develop, leaving a series of ridges (eg. Barnes’ Ice Cap, Baffin Island)

Method 2

Moraine ridges due to back-shifting of thrust zones behind the stagnant terminus of the glacier

Rate of glacier movement and melting rate are variable

More active periods cause more thrusting and more debris is brought to the terminus

During lower activity, the glacier thins by melting and debris-laden zones become stagnant

Eventually, the glacier becomes more active again and a new zone of higher accumulation occurs upslope from the earlier ridge

Thrust-moraine moraine landscape develops

Cross-sectional diagram of ground and end moraines.

Recessional MorainesMoraines formed in the same way as end moraines, during short-lived interruptions in glacier retreat (upslope from the main end moraine features)

Landforms left at the lower end of a valley, by a retreating glacier.Quarrying and abrasion is more severe higher in the valley, whiledrift thickens downvalley

Interlobate Moraines

Form when a large volume of sediment-laden meltwater is funneled between receding glacier lobes (eg. Oak Ridges Moraine,Ontario)

Up to 50m high and 10 to 100’s of kilometres long

Consist of stratified sand and gravel

Push Moraine

Glacier bulldozes and deforms glacial drift

Occurs at margin ofthe glacier

Usually less than 10min height

Ice-thrust Ridges

Deformed bedrock withfolds (often over basal till)and faults

Usually covered in ablationtill, especially in depressions

Fields of ridges up to 30m highSpaced 200 to 300 metresapart, traced for 100’s of km

Most common where bedrock is of varying strength

Eg. Milk River Ridge

Ice-thrust ridges in Saskatchewan. Ridges on horizon are cored by bedrock masses uplifted by ice pushing. Note also the spillway.

Ice-thrust moraine, Saskatchewan. Ridges in foreground are composed of deformed glacial sediments and uplifted sandstone bedrock. Depressions can be seen in the background.

Lateral Moraines

Ridges of till along edges of glaciated valleys

Debris is from the glacier and rocks fallen from the valley walls

Deposits are often reworked by meltwater streams (terracing)

Since they are ice-cored, there is differential melting leading to deformation as well as some slumping.

Stagnant-Ice Till Features

These features develop near the terminus of a glacierin zones where the glacier is stagnant.

Hummocky Disintegration MoraineSubtle knob and kettle topography, mantled by ablation till

Prairie MoundsSmall hills with doughnut-shaped top, and a central flat area covered by fine sediments and a round rim of diamicton (till)

Moraine PlateauFlat areas of hummocky moraines, often higher than surroundingknobs. Form due to topographic inversion of a supraglacial lake asthe glacier melts

Till RidgesSmall ridges (roughly 30m long) which cross and sometimes drape over one another. Form when crevasse fillings reach land as a glaciermelts or when water-saturated till is squeezed into basal cracks.