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Difficulty: MediumLandform Evolution by Running Water and Underground Water

In a limestone region, a surface stream suddenly disappears underground through an opening formed by carbonation and solution, leaving the river valley downstream dry. Which karst landform is formed at the exact point where the stream sinks underground?

  1. Swallow holeAnswer
  2. B
    Uvala
  3. C
    Stalagmite
  4. D
    Polje

Answer

A swallow hole (also known as a ponor or sinkhole shaft) is the feature formed where a surface stream disappears subterraneanly into carbonated limestone bedrock.
In limestone regions, surface water containing carbonic acid dissolves vertical joints in the bedrock. Over time, these joints widen into funnel-shaped openings or vertical shafts known as swallow holes (or ponors). When a surface stream reaches a swallow hole, it plunges underground into a subterranean drainage system, leaving the valley lower downstream completely dry.

Step-by-Step Solution

1
Analyze the geomorphic process described in the stem
Groundwater carbonation dissolves joints in limestone bedrock, allowing surface runoff to carve vertical passages underground.
Rainwater containing dissolved carbon dioxide (H2CO3\text{H}_2\text{CO}_3) acts on calcium carbonate (CaCO3\text{CaCO}_3) to dissolve limestone bedrock along vertical joints.
2
Identify the specific landform created at the point of river disappearance
The entrance or shaft through which a river flows into subterranean cavern systems is known as a swallow hole or ponor.
As the swallow hole enlarges, the entire river is diverted underground, leaving the dry valley downstream without water flow.

Key Concept

Karst Drainage Features and Stream Disappearance
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Topics

Landform Evolution by Running Water and Underground Water

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