Question 5 of 7: Short Answer (Paragraphs) and Fill in the Blank

Question 5: Short Answer (Paragraphs) and Fill in the Blank (20 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.

1) Glacial landforms

a) Erosional landforms

Alpine (valley) glaciers erode bedrock by plucking (freeze-thaw and ice adhesion pull blocks loose from jointed bedrock at the glacier bed and headwall) and abrasion (rock debris embedded in the ice grinds and scours the bed as the glacier moves), producing a family of diagnostic erosional landforms:

• Cirque — a bowl-shaped, steep-walled amphitheatre carved at the head of a valley glacier by plucking and rotational sliding (nivation) at the ice/bedrock interface; when several cirques erode back-to-back around a peak they leave a sharp, pyramidal summit (a horn, e.g. the Matterhorn).
• Arete — a narrow, knife-edged ridge left between two adjacent cirques or parallel glacial valleys as headward erosion thins the rock divide from both sides.
• U-shaped (glacial trough) valley — a valley originally cut by a river (V-shaped) is widened and deepened by a glacier flowing through it into a broad, steep-sided, flat-floored U-profile, in contrast to the narrower V-shape of stream erosion.
• Hanging valley — a tributary glacial valley left perched well above the floor of the deeper main trunk valley, because the larger trunk glacier eroded its bed faster/deeper than the smaller tributary glacier; streams from hanging valleys often form waterfalls where they rejoin the main valley.
• Roche moutonnee — an asymmetric bedrock knob, smoothly abraded and striated on its up-glacier (stoss) side and steep, plucked/jagged on its down-glacier (lee) side, recording both erosional processes and the direction of ice flow.

b) Till (depositional) landforms

Where a glacier deposits the unsorted rock debris it carries (till) directly from the ice, rather than via meltwater, several characteristic landforms result:

• Terminal (end) moraine — a ridge of unsorted till bulldozed up and dumped at the farthest point the glacier snout reached, marking the maximum extent of the ice advance.
• Lateral moraine — a ridge of till accumulated along the sides of a valley glacier, from debris that fell onto the ice margin from adjacent valley walls and was carried along the glacier's edges.
• Ground moraine — a widespread, relatively thin and gently undulating sheet of till plastered across the landscape beneath a retreating glacier as it melts back, without forming a distinct ridge.
• Drumlin — a smooth, elongated, streamlined hill of till, steep on the up-ice (stoss) side and gently tapered on the down-ice (lee) side, shaped by ice actively flowing over and moulding previously-deposited till; drumlin long axes record the local direction of ice flow.
• Kettle — a depression, often filled by a pond or lake, formed where a large block of stagnant ice buried in outwash/till melts out after the glacier retreats, leaving a hole in the till/outwash surface (technically a periglacial collapse feature associated with till/outwash deposition).

2) Fill in the blanks – Figure Q5-1, Features from a Major Weathering Process

The figure title (a "major weathering process" that sculpts alpine peaks) together with the given label "Tarn" (a small lake occupying a glacially-carved cirque floor) identifies this as a classic post-glacial alpine landform panorama. Reading the four unlabelled callouts as the standard set of features shown alongside a tarn in this type of textbook diagram: