Question 9 of 10: Question 5, Option 2: Glacial Erosional and Till Landforms

Question 5, Option 2: Glacial Erosional and Till Landforms (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Erosional landforms.

Cirque. A bowl-shaped, steep-walled amphitheatre carved at the head of an alpine glacier by combined plucking (glacier freezes onto joint blocks and rips them free as it moves) and abrasion (rock-flour-laden ice scours the floor); marks the glacier's zone of nourishment/erosion.

Arête. A sharp, narrow, knife-edged ridge formed when two cirques erode headward from opposite sides of a mountain ridge, progressively thinning the rock wall between them until only a jagged crest remains; where three or more cirques converge on one peak, the result is a pyramidal horn (e.g. the Matterhorn).

U-shaped (glacial trough) valley. A valley glacier, being far wider and stiffer than a river, erodes (plucks + abrades) the full width and depth of its valley rather than just a channel, planing the formerly V-shaped fluvial valley into a broad, steep-sided, flat-floored U-profile; tributary valleys left "hanging" above the main trough (their smaller glaciers eroded less deeply) produce hanging valleys and waterfalls once the ice retreats.

Roche moutonnée (a fourth example). An asymmetric bedrock knob abraded smooth on its up-ice (stoss) side and plucked into a steep, irregular face on its down-ice (lee) side, recording ice flow direction directly in its shape (see Question 2.5 above).

(b) Till landforms.

Ground moraine. A blanket of unsorted, unstratified till (clay to boulders, no bedding) draped evenly over the landscape as the ice sheet's base simply melted out and released its entrained debris load in place, without concentrating it into a ridge.

Terminal (end) moraine. An arcuate ridge of till bulldozed up and dumped at the ice margin where it stood still (equilibrium between forward ice flow and melting) for long enough to accumulate a distinct ridge; marks the maximum (or a still-stand) position of the ice front. A lateral moraine is the equivalent ridge built along the SIDE of a valley glacier.

Drumlin. A smooth, elongate, streamlined hill of till, blunt and steep on its up-ice (stoss) end and tapering gently on its down-ice (lee) end, moulded by ice flowing over and reshaping previously deposited till (or plastering fresh till around a rock core); drumlins occur in "swarms" and their long axes record the local ice-flow direction.