|Image: shallow bowl-shaped solutional doline in the area called Schwäbische Alb (Swabian Jura), Germany.|
A doline, sink or sinkhole is a closed depression draining underground in karst areas. It can be cylindrical, conical, bowl- or dish-shaped. The diametre ranges from a few to many hundreds of meters.
The name doline comes from dolina, the Slovenian word meaning valley. So this was originally a colloquial Slovenian word which was used by the geologists to describe a geologic feature.
There are two different mechanisms for the forming of dolines:
The solution produces large amouts of clay (depending on the pureness of the limestone). This clay is water resistant and sometimes plugs the drainage, so little lakes of rain water can sometimes be found in dolines, a rare thing in waterless karst areas.
|Image: Cylindrical doline, the entrance to Dunmore Cave, Ireland. Image by Mathias Duckeck.|
If the impact of this collapse reaches the surface, if the overlying layers are too thin. The ceiling collapses and a doline is formed. The doline is often a natural entrance to the cave.
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