Glasser and Bennet 2004 (6)

Question 1

What is glacial erosion?

Answer 1

The removal and transport of bedrock and/or sediment by glacial quarrying, glacial abrasion and glacial meltwater.

Question 2

How are landforms of glacial quarrying (such as roches moutonnées, rock basins and zones of areal scouring) created?

Answer 2

When cavities form between an ice sheet and its bed.

This means that they are indicative of low effective basal pressures and high sliding velocities that are necessary for ice bed separation.

Question 3

How can abrasion be achieved?

Answer 3

Bodies of subglacial sediment sliding over bedrock or by individual clasts contained within ice.

Question 4

When is abrasion favoured?

Answer 4

In situations where effective basal pressures are greater than 1 MPa and where there are low sliding velocities

Question 5

What is palaeoglaciology?

Answer 5

The reconstruction of ancient ice sheets and the spatial analysis of glacial landforms is an essential tool in this process.

Question 6

Understanding glacial erosional processes and landforms and how they relate to former ice sheets is important for a number of reasons.

Answer 6

Over the timescales of a glacial cycle, glacial erosion is capable of modifying the bed of an ice sheet and therefore altering large-scale ice dynamics and mass balance.

Landforms of glacial erosion are used as palaeo-environmental indicators.

Question 7

What is quarrying/ plucking?

Answer 7

The removal of large blocks from the bed.

Question 8

What is abrasion?

Answer 8

The surface wear created by the passage of isolated clasts in ice or the sliding of sediment over the bed (Hindmarsh, 1996a).

Question 9

What two processes are involved in quarrying/plucking?

Answer 9

(1) the fracturing or crushing of bedrock beneath the glacier; and (2) the entrainment of this fractured or crushed rock

Question 10

When does the fracturing of bedrock take place?

Answer 10

When a glacier flowing over bedrock creates pressure differences in the underlying rock, causing stress fields that may be sufficient to induce rock fracture.

Question 11

Evacuation of rock fragments along joints in the bed is possible where…

Answer 11

Localized basal freezing occurs.

Question 12

Where is quarrying/ plucking favoured?

Answer 12

Beneath thin, fast-flowing ice (Hallet, 1996)

Question 13

What are the dominant glaciological conditions required for quarrying?

Answer 13

Low effective basal pressures (0.1–1 MPa) and high sliding velocities because these conditions favour extensive ice/bed separation (subglacial cavity formation) and also concentrate stresses at points, such as the corners of bedrock ledges, where ice is in contact with the bed (Iverson, 1991a; Hallet, 1996).

Question 14

How can abrasion be achieved?

Answer 14

By bodies of subglacial sediment sliding over bedrock (Cuffey and Alley, 1996; Hindmarsh, 1996a) or by individual clasts contained within ice (Hindmarsh, 1996b).

Question 15

The effectiveness of meltwater as an agent of erosion depends on:

Answer 15

(1) the susceptibility of the bedrock involved, in particular the presence of structural weaknesses or its susceptibility to chemical attack; (2) the discharge regime, in particular the water velocity and the level of turbulent flow; and (3) the quantity of sediment in transport.

Question 16

What are microscope landforms of glacial erosion?

Answer 16

Those below 1 m in size.

They are of low relief amplitude and are often found superimposed on larger landforms.

Question 17

There are four types of micro-scale landforms, what are they?

Answer 17

Striae, micro-crag and tails, bedrock gouges and cracks, and p-forms and micro-channel network

Question 18

What are Straie?

Answer 18

Striae are the lines or scratches on a rock surface produced by the process of glacial abrasion as debris or clasts entrained in basal ice are dragged over rock.

Question 19

Striae can be placed in one of three morphological categories:

Answer 19

Type 1 striae become progressively wider and deeper down glacier until they end abruptly, often as deep steep walled gouges.

Type 2 striae start and terminate as faint, thin traces. They steadily broaden and deepen until they reach a maximum width and depth near their centre point.

Type 3 striae begin abruptly as deep gouges and then become progressively narrower and shallower down-glacier. They form where a striator point contacts and indents the bed. Clast rotation with little dis- placement along the bed produces a low ploughing angle, so that a gradual reduction in indentation depth occurs as sliding proceeds (Iverson, 1991b).

Question 20

What are micro crag and tails?

Answer 20

Micro crag and tails are small tails of rock that are preferentially protected from glacial abrasion in the lee of resistant grains or mineral crystals on the surface of a rock.

Question 21

What are bedrock gouges and cracks?

Answer 21

This heading comprises a family of small cracks, gouges and indentations created on bedrock surfaces

Question 22

Gouges and cracks can provide information regarding…

Answer 22

The direction of forward dip of the fracture provided a good indication of ice movement direction (Harris, 1943).

Question 23

The term ‘p-form’ was introduced by Dahl (1965) and is applied to…

Answer 23

a variety of forms including smooth sinuous depressions and grooves sculpted on glaciated bedrock surfaces.

The name given to these forms reflects the fact that their form was originally attributed to the plastic flow of ice around bedrock obstacles.

Question 24

What are meso-scale landforms?

Answer 24

Those between 1 m and 1 km in size and comprise a family of landforms including streamlined bedrock features, stoss and lee forms, rock grooves and rock basins, and meltwater channels

Question 25

At a meso-scale the most common effect of glacial erosion is…

Answer 25

to streamline bedrock protrusions to produce positive and upstanding landforms (Hindmarsh, 1999).

Question 26

What are stoss and lee forms?

Answer 26

Stoss and lee features possess both abraded and quarried surfaces.

These landforms characteristically have a pronounced asymmetric profile with an abraded slope on the up-ice side (stoss) and a steeper rougher quarried slope on the down-ice side (lee).

Question 27

Streamlined bedrock features (‘whalebacks’) morphology

Answer 27

Streamlined bedrock eminences with abraded surfaces on all sides

Question 28

Stoss and lee forms (‘roches moutonnées’)

morphology

Answer 28

Upstanding bedrock eminence with both abraded and quarried faces
Detailed morphology controlled by preglacial weathering characteristics and patterns of bedrock jointing.

Question 29

Rock grooves and rock basins morphology

Answer 29

Smooth-walled sculpted depressions and channels cut in bedrock

Question 30

Streamlined bedrock features (‘whalebacks’) glacial significance

Answer 30

Warm-based ice carrying a basal debris load
High effective normal pressures (>1 MPa) inferred from intimate ice/bedrock contact and cavity suppression
Thick ice
Low sliding velocity with little available basal meltwater

Question 31

Stoss and lee forms (‘roches moutonnées’) glacial significance

Answer 31

Warm-based ice carrying a basal debris load
Low effective normal pressures (0.1–1 MPa) inferred from the presence of basal cavities
Quarried faces indicate abundant basal meltwater with regular fluctuations in basal water pressure
Rapid sliding velocity
Some evidence that roches moutonnées form under thin ice, e.g., possibly during ice sheet build-up and decay
May indicate direction and orientation of ice flow

Question 32

Rock grooves and rock basins glacial significance

Answer 32

Warm-based ice carrying a basal debris load
Quarried landforms indicate low effective normal pressures (0.1–1 MPa) inferred from the presence of basal cavities
with abundant basal meltwater and regular fluctuations in basal water pressure
Quarried landforms indicate rapid sliding velocity and thin ice They may also indicate direction and orientation of ice flow Landforms occurring in association with striae indicate high effective normal pressures (>1 MPa) inferred from intimate ice/bedrock contact and cavity suppression

Question 33

Subglacial meltwater channels morphology

Answer 33

Steep sided channels cut into bedrock or till Channel orientation may be discordant with the local topography
Channels may have an irregular convex-up long profile

Question 34

Ice-marginal meltwater channels morphology

Answer 34

Either a complete channel cross-section or a channel floor and one wall wherever the other wall was formed by ice (half channel) Channels start and end abruptly
Often associated with other ice-marginal depositional landforms

Question 35

Subglacial meltwater channels glacial significance

Answer 35

Warm-based ice carrying a basal debris load
Channel systems can be used to calculate former hydraulic potential gradient and therefore to infer the regional pattern of subglacial drainage, and to estimate ice surface slope and ice thickness
Calculations of palaeovelocity and palaeodischarge possible from measurements of channel shape, channel width and size of material transported by meltwater flow

Question 36

Ice-marginal meltwater channels glacial significance

Answer 36

Release of large quantities of supra-, en- or subglacial meltwater Channels indicate the location of the former ice margin
and patterns of ice recession
Gradient of channel long profile may indicate that of the ice margin
Calculations of palaeovelocity and palaeodischarge possible from measurements of channel shape, channel width and size of material transported

Question 37

What are rock basins?

Answer 37

Rock basins are individual depressions carved in bedrock. They are often found in association with roches moutonnées and may fill with water on deglaciation.

Question 38

What are glacial meltwater channels?

Answer 38

Glacial meltwater channels span a large spatial scale from small-scale channels cut in soft-sediment, through to large meltwater channel system.

Question 39

What are subglacial meltwater channels?

Answer 39

Characteristics of subglacial meltwater channels include convex-up or undulating long profiles, lack of significant drainage basin and abrupt inception and termination.

Question 40

What are ice-marginal channels?

Answer 40

Ice-marginal channels run parallel to the direction of the ice margin (Rodhe, 1988). They are most common in the ablation area of glaciers where rates of surface ablation and meltwater production are highest.

Question 41

What are glacial troughs?

Answer 41

Glacial troughs are deep linear features carved into bedrock rep- resenting the effects of glacial erosion where ice flow is confined by topography and channelled along a trough or valley.

Question 42

What are cirques?

Answer 42

Cirques are bedrock hollows that open down-slope and are bounded up- slope by a cliff or steep slope known as a headwall (Evans and Cox, 1974).

Question 43

What are tunnel valleys?

Answer 43

Tunnel valleys are large, elongate, overdeepened depressions cut into bedrock or glacigenic sediment.