GL15 Landforms of glcaial erosion

Question 1

What are corries?

Answer 1

• Corries are armchair-shaped hollows found on upland hills or mountainsides that have a steep back wall, an over-deepened basin, and a lip at the front.

Question 2

Describe the formation of a Corrie?

Answer 2

• Corries are armchair-shaped hollows found on upland hills or mountainsides. They have a steep back wall, an over-deepened basin and often have a lid at the front, which may be solid rock or made of morainic deposits
• Corrie formation is the result of several interacting processes. Development starts with nivation of a small hollow on a hillside in which snow collects and accumulates year on year. Over time these hollows enlarge and contain more snow, which eventually compresses into glacier ice.
• At a critical depth, the ice acquires a rotational movement under its own weight. This enlarges the hollow further. Meanwhile. the rotational movement causes plucking of the back wall, making it increasingly steep. The debris derived from plucking and weathering above the hollow falls into the bergschrund or crevasse which abuts the back wall.
• This rock debris helps to abrade the hollow and causes it to deepen. Once the hollow has deepened, the thinner ice at the front is unable to erode so rapidly and so a higher lip is left. The lip may also consist of moraine deposited by the ice as it moves out of the corrie.
• In the post-glacial landscape the corrie may become filled with water forming a small circular lake or tarn.

Question 3

What are arêtes and pyramidal peaks?

Answer 3

• An arête is a narrow, steep-sided ridge found between two corries.
• Arêtes form from glacial erosion, with the steepening of slopes and the retreat of corries that are back to back or alongside each other.
• When three or more corries develop around a hill or mountain top and their back walls retreat, the remaining mass will be itself steepened to form a pyramidal peak.

Question 4

Describe the formation of a trough.

Answer 4

• Glaciers flow down pre-existing river valleys under gravity. As they move, they erode the sides and floor of the valley, causing the shape to become deeper, wider, and straighter. The mass of ice has far more erosive power than the river that originally cut the valley.
• Although they are usually described as being U-shaped, they seldom are. Rather, they are parabolic, partly due to the weathering and mass movement of the upper part of the valley sides that goes on both during the glacial period and in the subsequent periglacial period as the glacier retreats.
• The resultant scree slopes that accumulate at the base of the valley sides lessen the slope angle. There are often variations in the long profile of glacial troughs. When compressing flow occurs, the valley is over-deepened to form rock basins and rock steps. This process may be particularly evident where there are alternating bands of rock of different resistances on the valley floor, with the weaker rocks being eroded more rapidly to form the basins.

Question 5

What are roche moutonnées and striations?

Answer 5

Roches mountonnées develop their distinctive morphology due to the pattern of stress on a bedrock surface beneath a sliding glacier.

On the stoss side of bedrock bumps, normal stresses are relatively high and particles embedded in the ice are moved across the underlying surface where they carry out abrasion, which is the process of debris being carried by the glacier scraping against valley sides/floors. The evidence of such abrasion is the common occurrence of striations (i.e. scores and scratches on bedrock) on the sloping upper surface and flank of Roches moutonnees

On the lee side of bedrock bumps, normal stresses are lower which means that the meltwater refreezes, and carries with it rocks plucked from the lee side.

Through an understanding of how roches mountonnées are formed, glaciologists are able to make inferences about the nature of past glacier systems where such landforms are found.
Roche moutonnées can indicate the direction the ice moved through an area.

Question 6

Describe the formation of an Ellipsoidal Basin

Answer 6

• Ellipsoidal basins are major erosional landforms created by ice sheets.
• Ellipsoidal basins are formed by the erosive power of ice sheets. As the ice sheets move, they erode the bedrock through the processes of plucking and abrasion.
• The weight of the ice sheets also leads to isostatic lowering of the surface landscape. The erosive action of the ice sheets produces a series of ellipsoidal basins in the landscape, with the larger master basin holding water and the smaller basins containing the Great Lakes.
• These basins have an ellipsoidal shape due to the irregular distribution of stress and strain under the ice sheets. The weight of the ice sheet causes the earth’s crust to deform and sink, and the ice sheet adjusts its shape accordingly.