Subglacial Erosion Exam Flashcards
What are the three dominant processes of erosion:
Abrasion
Percussion/ plucking
Meltwater
Where does erosion occur?
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.
How does abrasion occur?
Hindmarsh (1996) talks about..
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.
How does rotation occur?
What is marbleising?
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.
What is quarrying (plucking/ fracture)
Low frequency/ high magnitude
Little explosion hitting the bed hard- fragments of rock is chipped/ frozen out and carried along in the glacier.
Medium scale glacial erosion landforms
Whalebacks/ streamlined hill
Roche moutonnee
Crag and tail
Rock drumlins
What is a roche moutonnee?
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.
What is a drumlin?
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.
Difference between “Drumlins” and “Roche moutonnee” on the basis of:
i. Type of landform:
“Drumlins” are glacial depositional landforms, whereas “Roche moutonnee” are glacial erosional
landform.
Difference between “Drumlins” and “Roche moutonnee” on the basis of:
ii. Type of surface:
“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.
Difference between “Drumlins” and “Roche moutonnee” on the basis of:
iii. Place of origin:
“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.
Difference between “Drumlins” and “Roche moutonnee” on the basis of:
iv. Rock type:
“Drumlins” are made up of rock strata which is less resistant to erosion than “Roche moutonnee” which is made up of high resistant rock.
What is a crag and tail?
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.
Large scale glacial erosion
Cirques
Glacial troughs
Aretes
Pyramidal peaks
Cirques
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.
