|Image: a collapsed cave or karst window at Rakov National Park, Slowenia.|
A collapsed cave is a typical karst feature, but at first it is very difficult to find and see. It is the last stage of cave destruction and so it is rather common. But spectacular specimens are really rare.
Caves form underground, but while the cave forms the surface above is continually lowered by limestone solution. While the cave becomes bigger, the rock falls from the ceiling make the cave "move" upwards. The covering limestone becomes thinner and thinner. Finally it collapses at several points producing collapse dolines.
When the cave is open the process speeds. Frost enters the cave and adds further destrucion, widens the cave at all entrances untill the collapsed roofs grow together. The result is a valley with steep walls, which follows exactly the former cave passage. There may be some natural bridges, the remains of the cave roof, until they finally collapse too. The merged dolines are at first called karst window.
A collapsed cave is a sort of polje, or probably its the link between a collapse doline and a polje. However, the term is generally used if most of the cave is collapsed, but the narrow gorge, some remaining speleothems and natural bridges still show much evidence of the former cave.
And last but not least: a natural bridge is not a collapsed cave, although this term is often used for them. The characteristic thing with the bridge is, that it has not yet collapsed. The common use of collapsed cave is the whole former cave, which may be divided into two different sections: the collapsed part is the karst window, and the not collapsed part is the natural bridge.
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