Formation of distinctive landforms predominantly influenced by erosion

Question 1

Formation of corries (cwms)

Answer 1

Corries are armchair shaped hollows found on upland hills or mountain sides, and have a steep back wall, an over-deepened basin and often have a lip 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, and over time these hollows enlarge and contains more snow, which eventually compresses into glacier ice
- at a critical depth, the ice acquires a rotational movement under its own weight, which enlarges the hollow further
- 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 crevasse or bergschrund 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 unstable to erode so rapidly and so a higher lip is left

Question 2

Formation or aretes and pyramidal peaks

Answer 2

An arete is a narrow, steep-sided ridge found between two corries; the ridge is often so narrow that it is described as a knife-edge
- aretes from glacial erosion, with the steepening of slopes and the retreat of corries that are back to back or alongside each other

Pyramidal peaks –> when three or more corries develop around a hill or mountain top and their back walls retreat, the remaining mass will be itself steepened into a pyramidal peak; weathering of the peak may further sharpen its shape

Question 3

Formation of a trough

Answer 3

Glaciers flow down pre-existing river valleys under gravity, and as they move they erode the sides and floor of the valley, causing the shape to become deeper, wider and straighter
- the mass of the ice has far more erosive power than the river that originally cut the valley

They are parabolic, partly due to 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 side less the slope angle

Question 4

The formation of a roche moutonnee

Answer 4

Projections of resistant rock are sometimes found on the floor of glacial troughs - as advancing ice passes over them, there is localised pressure melting on the up-valley side
- this area is smoothed and streamlined by abrasion and often has striations, which are scratches or grooves made by debris embedded on the base of the glacier

On the down-valley side pressure is reduced and meltwater re-freezes, resulting in plucking and steepening