Which one of the following karst landforms is the largest in size ?

Correct — A, Polje. Karst depressions form a size ladder, and the polje sits at the top of it. A polje is a large flat-floored basin, often several kilometres long and sometimes tens of kilometres, enclosed by steep limestone walls, with a floor of alluvium that can be farmed and a stream that crosses part of it before vanishing into a swallow hole. Below it comes the uvala, formed where neighbouring dolines run together, measured in hundreds of metres. Below that come the individual sinkhole and the swallow hole, each measured in metres or tens of metres.

• (b)Uvala — The next rung down. An uvala is a compound hollow made where several dolines coalesce, so it is bigger than any one of them and smaller than a polje, and its floor is uneven rather than flat.
• (c)Swallow Hole — One of the smallest karst features. It is the opening at which a surface stream disappears underground, and it may be no more than a few metres across.
• (d)Sink Hole — A closed surface depression, funnel-shaped or cylindrical, formed by solution or by collapse. Individually they are the building blocks of the larger forms, not the largest form themselves.

Karst is the landscape of soluble rock, chiefly limestone. Rainwater takes carbon dioxide from the air and the soil, becomes weakly acidic, and dissolves calcium carbonate along joints and bedding planes. Because the rock is jointed and permeable, drainage goes underground rather than staying on the surface, and the characteristic result is a country with caves, dry valleys and closed hollows but very few surface rivers.

The word karst comes from the limestone country at the head of the Adriatic, and the vocabulary of the topic is largely Slavic for the same reason — polje means field, and doline means valley. The polje is the one form in the list with real economic weight, because its flat alluvial floor makes it the only good farmland in an otherwise barren limestone upland, and villages in the Dinaric karst sit on the edges of poljes for that reason. In India, karst features are found in the Meghalaya plateau, in the Vindhyan and Cuddapah limestones and around Borra in Andhra Pradesh, though full poljes on the Dinaric scale are not part of the Indian landscape.

• Karst develops on soluble rock, chiefly limestone, through solution by weakly acidic rainwater.
• The size order of karst depressions runs sinkhole and swallow hole, then doline, then uvala, then polje.
• A polje has a flat alluvial floor, steep enclosing walls and a stream that sinks underground at a ponor.
• Stalactites, stalagmites and pillars are the depositional forms produced inside caves by the redeposition of dissolved carbonate.
• The term karst comes from the limestone region at the head of the Adriatic, which is why the vocabulary is Slavic.

• Treating a swallow hole and a sinkhole as the same thing; one is an inlet for a stream, the other a closed depression.
• Assuming an uvala is the largest form because the word is the least familiar.
• Confusing the erosional depressions of the surface with the depositional forms inside caves.

Ordering items give four terms from one process and ask for an extreme. Learn the family as a sequence, since that answers both the largest and the smallest version of the question.

• practice — not a real PYQ

  The point at which a surface stream disappears underground in a limestone region is called a

  • (a)polje
  • (b)swallow hole
  • (c)stalagmite
  • (d)terra rossa

  Answer(b) swallow hole — also called a ponor where it drains the floor of a polje.

• practice — not a real PYQ

  Karst topography develops most typically on which one of the following rocks?

  • (a)Granite
  • (b)Basalt
  • (c)Limestone
  • (d)Quartzite

  Answer(c) Limestone — it is soluble in weakly acidic water, which the other three are not.