Caves, arches, stacks and stumps are the landscape features of which one of the following?

Correct — B, Wave. Caves, arches, stacks and stumps are the classic sequence of erosional landforms cut by waves along a rocky coast, and they belong together because each is a stage of the same process. Waves attack the base of a cliff and open a notch, which deepens into a sea cave. Where caves are cut into both flanks of a headland and meet through it, the opening becomes an arch. When the roof of the arch collapses, an isolated pillar is left standing offshore — a stack. Continued undercutting reduces the stack to a low remnant washed over at high tide, a stump. The set is the standard illustration of coastal erosion in every physical geography syllabus.

• (a)River — Running water erodes downward and produces a different suite — gorges, waterfalls, potholes, meanders, oxbow lakes, and depositional forms such as flood plains and deltas. Rivers cut no arches or stacks.
• (c)Limestone — The tempting option, because limestone country does have caves. But karst landforms are the products of solution and carbonation working downward through soluble rock, giving swallow holes, sinkholes, uvalas, poljes, stalactites and stalagmites — never arches and stacks in this sequence.
• (d)Wind — Aeolian erosion works by abrasion near the ground and deflation, producing mushroom rocks, zeugen, yardangs, inselbergs and dunes such as barchans. Deserts do carry occasional natural arches, but the four-stage sequence named here is coastal.

Every geomorphic agent leaves a recognisable suite of landforms, and the examiner's method is to name three or four members of one suite and ask which agent made them. Waves produce cliffs, wave-cut notches and platforms, geos, blowholes, caves, arches, stacks and stumps on the erosional side, and beaches, spits, bars, tombolos and lagoons where material is deposited.

The reason arches and stacks are coastal rather than riverine is the direction of attack. Wave energy is concentrated in a narrow vertical band at sea level, so it undercuts rather than deepens, and a headland projecting into the sea is attacked from both sides at once. Refraction of waves around a headland concentrates energy on it while the bays between are sheltered, which is why erosion picks out promontories and leaves the sequence there. Well-known examples run from the Old Harry Rocks and the Twelve Apostles overseas to the sea arches of the Andaman coast and the rocky headlands of the Konkan.

• The wave erosion sequence runs notch, cave, arch, stack, stump.
• Wave refraction concentrates energy on headlands and disperses it in bays.
• River erosion gives gorges, waterfalls, potholes, meanders and oxbow lakes.
• Karst solution gives swallow holes, sinkholes, uvalas, poljes, stalactites and stalagmites.
• Wind erosion gives mushroom rocks, zeugen, yardangs and dunes.

• Choosing limestone because the word 'caves' appears in the list.
• Treating the four names as unrelated instead of as one sequence.
• Assuming natural arches must be coastal; they occur in deserts too, but not in this progression.

A name-the-agent item built from one landform suite. Look for the member of the list that only one agent can produce — here, the stack.

• practice — not a real PYQ

  Which one of the following is a depositional landform of coastal areas?

  • (a)Sea arch
  • (b)Stack
  • (c)Spit
  • (d)Wave-cut platform

  Answer(c) Spit — a ridge of sand and shingle built out from the coast by longshore drift.

• practice — not a real PYQ

  Swallow holes, uvalas and poljes are landforms associated with

  • (a)glacial erosion
  • (b)limestone solution
  • (c)wave action
  • (d)wind deflation

  Answer(b) limestone solution — the karst suite produced by carbonation.