Karst window

A karst window, also known as a karst fenster, is a geomorphic feature found in karst landscapes where an underground river is visible from the surface within a sinkhole.[1][2] In this feature, a spring emerges, then the discharge abruptly disappears into a sinkhole. The word fenster is German for 'window', as these features are windows into the karst landscape.

The term is used to denote an unroofed portion of a cavern which reveals part of a subterranean river.[3] A complex system of caves, known as karst topography, evolves from the effects of water erosion on carbonate rocks such as limestone, dolomite or gypsum. "A karst fenster is caused by a caving in of portions of the roof of a subterranean stream, thus making some of the underground stream visible from the surface".[4] Theories in the creation of karst topography and karst fensters involve vadose water above the water table, and deep-circulating phreatic water (water in the zone of saturation) eroding away subsurface rock. Karst fensters may also form because of weathering from above.

An example of a karst window or fenster, recognized by the Kentucky Geological Survey, is Short Creek in Pulaski County, County, where a small river emerges and disappears in a space of less than 100 meters in length.[5] An example of this formation on public parkland is Cedar Sink in Mammoth Cave National Park, Kentucky, where small watercourses emerge and disappear at the bottom of a large sinkhole.

Illustration of development of karst window by roof rock collapse[6]