Karst fenster is a geomorphic feature formed from the dissolution of carbonate bedrock

Bedrock

In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...

 where a spring emerges then the discharge abruptly disappears into a sinkhole

Sinkhole

A sinkhole, also known as a sink, shake hole, swallow hole, swallet, doline or cenote, is a natural depression or hole in the Earth's surface caused by karst processes — the chemical dissolution of carbonate rocks or suffosion processes for example in sandstone...

. The word fenster is German for 'window', as these features are windows into the karst

KARST

Kilometer-square Area Radio Synthesis Telescope is a Chinese telescope project to which FAST is a forerunner. KARST is a set of large spherical reflectors on karst landforms, which are bowlshaped limestone sinkholes named after the Kras region in Slovenia and Northern Italy. It will consist of...

 landscape.

Karst fenster, also known as a karst window, is an unroofed portion of a cavern, revealing part of a subterranean river

Subterranean river

A subterranean river is a river that runs wholly or partly beneath the ground surface – one where the riverbed does not represent the surface of the Earth ....

 (Easterbrook). Underground a complex system of caves, known as karst topography

Karst topography

Karst topography is a geologic formation shaped by the dissolution of a layer or layers of soluble bedrock, usually carbonate rock such as limestone or dolomite, but has also been documented for weathering resistant rocks like quartzite given the right conditions.Due to subterranean drainage, there...

; evolve from the effects of water erosion on carbonate rocks such as limestone, dolomite

Dolomite

Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....

 or gypsum

Gypsum

Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...

. A karst fenster is born from a caving in of portions of the roof of subterranean stream, thus making some of the underground stream visible from the surface (Ritter). Theories in the creation of karst topography and karst fensters involve vadose water above the water table and deep circulating phreatic

Phreatic

The term phreatic is used in Earth sciences to refer to matters relating to ground water below the water table . The term 'phreatic surface' indicates the location where the pore water pressure is under atmospheric conditions...

water (water in the zone of saturation) eroding away subsurface rock. Karst fensters may also form because of weathering from above.