NDA (II) 2016 General Ability Test · Question 138 of 150

Correct — A, 4 1 2 3. Ground water goes with lapies (4), the grooved and fluted limestone surfaces left where slightly acidic water dissolves the rock along its joints. Running water goes with the plunge pool (1), the deep basin scoured out at the foot of a waterfall by the hammering of falling water and the stones it carries. A glacier goes with horns (2), the sharp pyramidal peaks that result when three or more cirques cut back into the same mountain from different sides. Wind goes with playas (3), the flat, often salt-crusted floors of desert basins. The quickest way in is the two unambiguous ones — a plunge pool is unmistakably a river feature and a horn is unmistakably glacial — and only one code assigns B to 1 and C to 2 together.

• (b)3 2 1 4 — Assigns the plunge pool to the glacier and the horn to running water, which inverts the two easiest pairs in the whole question. It also sends lapies to the wind, though lapies is a solution feature that requires water and soluble rock.
• (c)3 1 2 4 — Gets running water and glacier right but exchanges ground water and wind, giving playas to ground water and lapies to wind. Playas are basin floors in arid country, and dissolution of limestone is not something wind can do.
• (d)4 2 1 3 — Correctly pairs ground water with lapies and wind with playas, then swaps the middle two so that running water gets horns and the glacier gets the plunge pool. Horns are formed by the backward erosion of cirques, which only ice can do.

Each geomorphic agent — running water, ground water, glaciers, wind, waves — works by its own mechanism and therefore leaves its own signature landforms. Running water erodes mechanically by hydraulic action and abrasion. Ground water works chemically, dissolving carbonate rock to give the karst family of landforms. Glaciers pluck and abrade rock beneath and beside the ice. Wind sorts and moves loose material where vegetation is sparse, and in deserts the interior basins collect the fine sediment and the salts that runoff brings in.

The odd word in the list is lapies, which is why the question uses it. Lapies, also written lapiés or karren, are the grooved, ridged, sharp-edged limestone surfaces you get where rainwater made mildly acidic by dissolved carbon dioxide eats into the rock along its joints — the same process that further down produces sinkholes, caverns, stalactites and stalagmites. Once you know lapies belongs to the karst family, the whole code follows, because no other agent in the list can dissolve rock. Note also the difference between a horn and a cirque: the cirque is the armchair-shaped hollow the ice carves, and the horn is what is left standing when several cirques meet.

• Lapies are grooved and fluted limestone surfaces produced by solution, and belong to the karst landforms made by ground water.
• A plunge pool is the deep basin scoured at the base of a waterfall by falling water and the debris it carries.
• A horn is a sharp pyramidal peak left where three or more cirques erode headward into the same mountain mass; the Matterhorn is the type example.
• Playas are the flat floors of enclosed desert basins, often coated with salt after the shallow water that collects there evaporates.
• Stalactites, stalagmites and pillars are the depositional counterparts of lapies, formed when the dissolved carbonate is redeposited inside caves.

• Not recognising lapies and guessing it into the wind column; it is a solution feature and only water can make it.
• Mixing up cirque and horn — the first is the hollow, the second is the peak left between several hollows.
• Assuming everything in a desert is made by wind; playas are shaped as much by the water that occasionally collects in the basin as by the wind that fills it.

Landform-to-agent matching is one of NDA's most reliable geography formats, and it is nearly always winnable by locking down the two features you are certain of and letting the codes eliminate themselves.

• practice — not a real PYQ

  A sharp pyramidal peak formed where three or more cirques erode headward into the same mountain is called

  • (a)an arête
  • (b)a horn
  • (c)a nunatak
  • (d)a drumlin

  Answer(b) a horn — the arête is the knife-edge ridge between two cirques, while the horn is the peak left where several of them meet.

• practice — not a real PYQ

  Sinkholes, caverns and lapies are characteristic landforms of regions underlain by

  • (a)granite
  • (b)basalt
  • (c)limestone
  • (d)sandstone

  Answer(c) limestone — these are karst features produced by the solution of carbonate rock in mildly acidic ground water.