NDA (I) 2018 General Ability Test · Question 98 of 150

Correct — C, Stack. A stack is what erosion leaves behind, not what deposition builds up. Waves attacking a headland first cut a notch at its base, then hollow the notch into a cave; caves driven through from both sides meet to form an arch; and when the roof of the arch finally collapses, the seaward pillar is left standing alone in the water as a stack. Every stage of that sequence is the removal of rock. The other three options are all made of material that the sea has carried and dropped — a spit is a ridge of sand or shingle built out from the shore, a sand bar is a similar ridge lying offshore or across a bay mouth, and a tombolo is a bar that has grown far enough to tie an island to the mainland.

• (a)Tombolo — A tombolo is depositional. It is a neck of sand or shingle deposited in the sheltered water behind an island, joining that island to the coast; the island shelters the water from waves, which is why sediment settles there.
• (b)Sand bar — A sand bar is depositional by definition — a ridge of sediment laid down offshore or across the mouth of a bay. Where a bar seals off a bay, the water trapped behind it becomes a lagoon, as at Chilika and Pulicat.
• (d)Spit — A spit is depositional as well. Longshore drift carries sand along the coast and drops it where the shoreline turns, building a ridge that projects into the sea with one end still attached to the land, and its free end often curves inward into a hook.

Waves do two things to a coast, and every coastal landform belongs to one or the other. Where waves have room to gather force against resistant rock they erode, producing notches, caves, arches, stacks, stumps, cliffs and wave-cut platforms. Where the water loses energy — in the lee of an island, inside a bay, at a bend in the shoreline — the load it is carrying settles out, producing beaches, spits, hooks, bars, barrier islands, tombolos and lagoons. Longshore drift, the zig-zag movement of sediment along a shore driven by waves arriving at an angle, is the mechanism that supplies most of the material for the depositional group.

For an odd-one-out item of this kind, sort the options before comparing them: three of the four will belong to one family and one to the other. Here the useful test is to ask whether each feature is made of loose sediment or of solid rock left standing. Tombolo, sand bar and spit are all built of sand or shingle; a stack is bedrock. That single question settles the item without any need to recall the definitions in full. It is worth committing the erosional sequence to memory as an ordered story rather than as a list, because NDA has set both halves of it — notch, cave, arch, stack, stump — and once the story is in place the classification of any single term follows from where it sits in the sequence.

• A stack is an isolated pillar of rock left when a sea arch collapses — an erosional feature.
• The erosional sequence on a headland runs notch, cave, arch, stack and finally stump.
• A spit is a ridge of sediment built out from the shore by longshore drift, with a free end.
• A sand bar is a deposited ridge lying offshore or across a bay mouth; a bar sealing a bay creates a lagoon.
• A tombolo is a deposited bar that joins an island to the mainland.

• Assuming any coastal landform must be depositional because coasts are associated with beaches.
• Confusing a stack with a stump; the stump is the worn-down remnant of a stack, and both are erosional.
• Mixing up a spit, which is attached to the land at one end, with a bar, which lies detached.

NDA sets coastal landforms as odd-one-out items, as erosional-or-depositional classification codes, and as agent-to-feature match lists.

• practice — not a real PYQ

  A ridge of sand or shingle that joins an island to the mainland is called a

  • (a)spit
  • (b)tombolo
  • (c)stack
  • (d)notch

  Answer(b) tombolo — deposited in the sheltered water behind the island until it links up with the coast.

• practice — not a real PYQ

  Which one of the following is the correct sequence in which a headland is eroded?

  • (a)arch, cave, notch, stack
  • (b)notch, cave, arch, stack
  • (c)stack, arch, cave, notch
  • (d)cave, stack, notch, arch

  Answer(b) notch, cave, arch, stack — and the stack is eventually worn down to a stump.