Glacial Erosion

Question 1

What are the three dominant processes of erosion:

Answer 1

Abrasion
Percussion/ plucking
Meltwater

Question 2

Where does erosion occur?

Answer 2

In the soft bed compartment, erosion is going to occur at the ice/sediment interface.

There is also going to be erosion within the till and at the base of the deforming layer.

Hindmarsh (1996)

Question 3

How does abrasion occur?

Hindmarsh (1996) talks about..

Answer 3

This is either from the clasts held in the ice or the till sliding over the bedrock.

Hindmarsh talks about the scale of the abrasion- he suggests that 1 mm clasts make striations, finder material, less than 1 mm causes polishing.

Question 4

How does rotation occur?

What is marbleising?

Answer 4

As the clasts rotate they rub each other and so they’re causing abrasion not between the bedrock and underlying bedrock but within the till itself.

Marbelising- high density of rigid clasts rotating against a rigid substrate and each other.

Question 5

What is quarrying (plucking/ fracture)

Answer 5

Low frequency/ high magnitude

Little explosion hitting the bed hard- fragments of rock is chipped/ frozen out and carried along in the glacier.

Question 6

There are different types of striations and they are most common…

Answer 6

beneath warm-based ice, so can be used when reconstructing former ice sheets.

Question 7

Medium scale glacial erosion landforms

Answer 7

Whalebacks/ streamlined hill

Roche moutonnee

Crag and tail

Rock drumlins

Question 8

What is a roche moutonnee?

Answer 8

When a more resistant rock persists in the passage of glaciers, the upstream side is smoothened by abrasion and it’s downstream side is roughened and steepened by plucking called Roche moutonnee.

Question 9

What is a drumlin?

Answer 9

Deposition of glacial till, sand and gravels in various proportions in different layers around a rock or glacial till leads to formation of a upside down boat alike landform with little steeper slope at upstream and gentle slope along downstream, known as drumlins.

Question 10

Difference between “Drumlins” and “Roche moutonnee” on the basis of:
i. Type of landform:

Answer 10

“Drumlins” are glacial depositional landforms, whereas “Roche moutonnee” are glacial erosional
landform.

Question 11

Difference between “Drumlins” and “Roche moutonnee” on the basis of:
ii. Type of surface:

Answer 11

“Drumlins” have smooth surface at both upstream and downstream side, whereas “Roche moutonnee” have smooth surface at upstream side and rough surface at downstream side.

Question 12

Difference between “Drumlins” and “Roche moutonnee” on the basis of:
iii. Place of origin:

Answer 12

“Drumlins” are formed in outwash plains at the foothills of the mountainous area, whereas “Roche moutonnee” are formed at a high altitude compared to drumlins.

Question 13

Difference between “Drumlins” and “Roche moutonnee” on the basis of:
iv. Rock type:

Answer 13

“Drumlins” are made up of rock strata which is less resistant to erosion than “Roche moutonnee” which is made up of high resistant rock.

Question 14

What is a crag and tail?

Answer 14

Crag and tail tends to be larger than a roche moutonnée. Crag and tail is the opposite of the roche moutonnee as the ice hits the steep resistant rock outcrop first. This protects the lee (far) side of the obstacle from erosion.

Question 15

Large scale glacial erosion

Answer 15

Cirques
Glacial troughs
Aretes
Pyramidal peaks

Question 16

Cirques

Answer 16

Where glaciers start their lives, the hollows in the hillside start by that slow process of abrasion and percussion.
Everytime temperature reduced in the quaternary glaciers grew in the cirque and eroded them out.
When the glacier reaches the valley it will erode v shaped valleys.

Question 17

Large scale erosion

Answer 17

Formed mostly by abrasion and quarrying over multiple glaciations.
Deforming bed may be important, with enhanced abrasion, quarrying and marbleisation- possibly associated with rigid bed ice streams.

Question 18

What is the buzzsaw theory?

Answer 18

Mountain heights controlled by:
Uplift, crustal strength and erosion
(all the mountain ehights of the world have been controlled by glacial erosion.

Landscapes concentration elevations around the ELA (lowest level of intense erosion).

Question 19

Links with climate

Answer 19

Mountains are produced by tectonic processes (e.g. rise of the Himalayas), and jet stream position/ create monsoon.

Need continent over the pole/s

Raymo and Ruddiman (1992) suggested that increased chemical weathering associated with glaciation lead to more CO2 being taken out of the atmosphere and lead to a cooling in the climate.

Question 20

Cosmogenic Exposure dating

Answer 20

Good technique to estimate how much erosion has occured
When anything sits out in the atmosphere it is bombarded by cosmic waves.
If something gets buried it’s not exposed to these cosmic rays anymore.
The bed of the glacier is shielded from the cosmic rays, so clasts eroded from the bed will only be exposed to these once the ice has gone.
10Be is most commonly measured.

Question 21

Ice thickness

Answer 21

Measure exposure ages above and below the trimline (boulders above and below ice).

Question 22

Cohen et al, 2006, Gurnell and Clark, 1987 argue that plucking is dominant because…

Answer 22

Subglacial streams transport eroded sediment and half of this is large and so in order to get the particles that would have to plucked

Question 23

What are straie?

Answer 23

Glacial striations are scratches or gouges cut into bedrock by glacial abrasion.

Question 24

Where are straie most common?

Answer 24

Beneath warm based ice, so can be used when reconstructing former ice sheets.

Question 25

What is streamlining?

Answer 25

Abrasion, rotation and quarrying associated with a deformable bed.

Question 26

What are crag and tails?

Answer 26

Elongated streamlined hills

• Eroded stoss side
• Lee side depositor or preservation of existing rock

Question 27

What landform doesn’t agree with the buzzsaw theory?

Answer 27

Volcanoes (formed recently).

Cold based glaciers

Question 28

What does Zoet et al (2012) suggest speeds up erosion rate?

Answer 28

Stick slip motion (both abrasion and plucking).

Question 29

What are whalebacks?

Answer 29

A symmetrical rounded bedrock feature (in contrast to the braided stoss and jagged lee form of roche moutenee)

Question 30

Why, with roche moutnees, is the stoss side braided and the lee side jagged?

Answer 30

Stoss side- due to abrasion, ice polishes the smooth surface.

Lee side due to plucking.