CDS (II) 2020 General Knowledge · Question 65 of 120

Correct — A, Cirque. A cirque is glacial, not fluvial. It is the amphitheatre-shaped hollow gouged out at the head of a valley glacier by plucking and abrasion, steep-walled on three sides and open downslope; when the ice melts it often fills with a small lake called a tarn, and where three or more cirques cut back into the same peak they leave the knife-edged aretes and pyramidal horn that give high mountains their shape. The other three options are all made by running water. Fluvial means 'of a river', from the Latin fluvius, and that etymology alone separates the odd one out here.

• (b)Gorge — A gorge is a deep, narrow, steep-sided valley cut by a river through hard rock, where downward cutting far outpaces widening of the valley sides. The Indus, Brahmaputra and Bhagirathi all run through Himalayan gorges.
• (c)Braids — Braids form where a river carrying a heavy sediment load and a shallow, variable discharge splits into several shifting channels around temporary islands and bars — typical of the Kosi and other Himalayan rivers on the plains.
• (d)Canyon — A canyon is simply a very large gorge, usually cut in horizontally bedded rock in arid country, so the walls stand up as steps. The Grand Canyon of the Colorado is the standard example.

Landforms are classified by the agent that made them. Rivers, working through erosion, transport and deposition, produce the fluvial family — V-shaped valleys, gorges, canyons, waterfalls, potholes, meanders, oxbow lakes, braided channels, floodplains, natural levees and deltas. Glaciers produce cirques, aretes, horns, U-shaped valleys, hanging valleys, moraines, drumlins and eskers. Wind produces yardangs, zeugen, mushroom rocks, deflation hollows and barchans. Waves produce cliffs, wave-cut platforms, sea caves, arches and stacks.

The reliable way to answer any 'which is not a fluvial landform' item is to sort by agent rather than to hunt for the answer directly. Gorge, canyon and braids all involve water flowing in a channel; a cirque involves ice standing in a hollow. The name is the tell. A cirque is also called a corrie in Scotland and a cwm in Wales, and it is the birthplace of a valley glacier rather than anything a river could carve. One further check: a cirque is an erosional hollow that opens downhill, which is a shape running water simply does not produce.

• A cirque is an amphitheatre-shaped hollow cut by glacial erosion at the head of a valley glacier; a tarn is the lake left in it.
• Aretes are the sharp ridges between adjoining cirques, and a horn is the pyramidal peak where three or more cirques meet.
• Fluvial erosional landforms include V-shaped valleys, gorges, canyons, waterfalls and potholes; fluvial depositional landforms include floodplains, levees, braided bars and deltas.
• A braided channel forms where a heavy sediment load and shallow, variable discharge cause a river to split into shifting channels.
• Aeolian landforms such as yardangs and zeugen are cut by wind abrasion and deflation, and marine stacks by wave erosion.

• Reading 'cirque' as a river feature because it appears in a question about valleys.
• Assuming a canyon and a gorge are made by different agents; both are river-cut, and a canyon is essentially a large gorge.
• Mixing up a tarn, which sits in a cirque, with an oxbow lake, which is a cut-off river meander.

Nearly always as an odd-one-out that mixes one glacial, aeolian or marine feature into a fluvial list, or as a landform-to-agent match.

• practice — not a real PYQ

  The pyramidal peak formed where three or more cirques cut back into the same mountain is called

  • (a)an arete
  • (b)a horn
  • (c)a tarn
  • (d)a nunatak

  Answer(b) a horn — the arete is the ridge between two cirques and the tarn is the lake in the cirque floor.

• practice — not a real PYQ

  Which one of the following is a depositional landform of a river?

  • (a)Pothole
  • (b)Gorge
  • (c)Natural levee
  • (d)Waterfall

  Answer(c) Natural levee — a ridge of coarse sediment laid down along a channel bank in flood; the other three are erosional.