The Apuseni Mountains are characterized by a remarkable karst landscape, both for its extension and for its richness and variety of forms.
The main karst feature is the development of a particular underground hydrological network which has created a huge and amazing variety of relief micro forms, both above and underground.
By its definition, the karst is considered an interactive and complex system incorporating distinct relief, life forms, energy fluxes, water, gases, types of soils and substrata.
The cliffs, mountains meadows and surface karst phenomena are landscape units of major interest. These are represented by dolines, short blind valleys (where the water infiltrates the limestone rocks through sink holes or swallow holes), karst plateaus, dry valleys, karren, polje (closed and flat hydrographic depressions with underground drainage), gorges, defilés etc.
A particularity of Bihor Mountains is the presence of the forested karst, one of the few regions of this kind in our country.
The forest influences the development of underground caves but, at the same time, it sustains a remarkable biodiversity on the surface.
The existence of this kind of landscape is one of the premises for the existence of ice caves: Scarisoara Ice Cave, Bortig Pothole, Focul Viu Ice Cave, Vârtop Ice Cave, Barsa Ice Cave, all situated in completely forested areas.
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Dolina = a natural depression or hole in the surface topography caused by the removal of soil or bedrock, often both, by water. Sinkholes may vary in size from less than a meter to several hundred meters both in diameter and depth, and vary in form from soil-lined bowls to bedrock-edged chasms.
Polje = a large, flat-floored depression within karst limestone, whose long axis develops in parallel with major structural trends and can become several miles (tens of kilometers) long. Drainage may be either by surface watercourses (as an open polje) or by swallow holes (as a closed polje) or ponors. Usually, the ponors cannot transmit entire flood flows, so many poljes become wet-season lakes.
Ponor = a natural surface opening, which may be found in landscapes, where the geology and the geomorphology is characterized by some kind of karst.The term ponor has become the international geological term for larger karst-induced surface water inlets.
While a sinkhole is a depression (doline) of surface topography with a pit or cavity directly underneath, a ponor is kind of a portal, where a surface stream or lake flows either partially or completely underground into a karst groundwater system. Steady water erosion may have formed or enlarged the portal in (mainly limestone) rock, in a conglomerate or in looser materials.
Speleology (also spelled spelæology or spelaeology) = the scientific study of caves and other karst features, their make-up, structure, physical properties, history, life forms, and the processes by which they form and change over time. The term is also sometimes applied to the recreational activity of exploring caves.
Stalagmite = a type of speleothem that rises from the floor of a limestone cave due to the dripping of mineralized solutions and the deposition of calcium carbonate.
Stalactite = the corresponding formation, for a stalagmite, on the ceiling of a cave. When these formations grow together, the result is known as a column.