Collapsed Cave

Karst Window

A collapsed cave or karst window at Rakov National Park, Slovenia.

A collapsed cave is a typical karst feature, but for beginners it is very difficult to find and see. It is the last stage of cave destruction and so it is rather common. Spectacular specimens are actually rare.

Caves form underground, but while the cave forms the surface above is continually lowered by limestone solution on the the rock surface below the soil. While the cave becomes bigger, the rock falls from the ceiling make the cave slowly "move" upwards. The covering limestone becomes thinner and thinner. Finally it collapses at one, then several places, which produces cave entrances or collapse dolines.

When the cave is open the process speeds. Frost enters the cave and increases the weathering substantially, widens the cave at all entrances until the openings grow together. The merged dolines are at first called karst window or Karstfenster. There may be some natural bridges, the remains of the cave roof, until they finally collapse too. The final result is a valley with steep walls, which follows exactly the former cave passage.

A collapsed cave is a sort of polje, or probably its the link between a collapse doline and a small 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 finally we want to rectify a common misuse of the term collapsed cave. 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, so it is actually the uncollapsed cave. The common use of collapsed cave is for 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.