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Cave, also called cavern, natural opening in the earth large enough for human exploration. Such a cavity is formed in many types of rock and by many processes. The largest and most common caves are those formed by chemical reaction between circulating groundwater and bedrock composed of limestone or dolomite. These caves, called solution caves, typically constitute a component of what is known as karst terrain. Named after the Karst region of the western Balkan Peninsula extending from Slovenia to Montenegro, karst terrain in general is characterized by a rough and jumbled landscape of bare bedrock ledges, deranged surface drainage, and sinkholes, as well as caves. It should be noted, however, that there is considerable variation among karst areas. Some may have dramatic surface landforms but few caves. By contrast, others may have extensive cave development with little surface expression; for example, the Guadalupe Mountains of New Mexico, the site of Carlsbad Caverns and various other caves, have very few surface karst features.
Karst landscapes are formed by the removal of bedrock (composed in most cases of limestone, dolomite, gypsum, or salt, but in some cases of such normally insoluble rocks as quartzite and granite) in solution through underground routes rather than through surface weathering and surface streams. As a result, much karst drainage is internal. Rainfall flows into closed depressions and down their drains. Further dissolution in the subsurface forms continuous conduits that serve as integrated drains for the rapid movement of underground water. The outlets for the water-carrying conduits often are springs of majestic size. Caves are fragments of such conduit systems, and some of them provide access to active streams. These caves may be completely water-filled; others are dry passages left behind by streams that cut to lower levels. Surface streams flowing from areas underlain by insoluble rock often sink when they reach the border of a karst region. These sinking streams form tributaries of the underground drainage system.
Not all caves are part of karst landscapes. A substantial number of relatively small caves, called volcanic caves, are formed in lava and by the mechanical movement of bedrock. Other caves are formed in glaciers by the melting of ice. Still others are created by the erosive action of water and wind or from the debris of erosive processes; these are sea caves, eolian caves, rock shelters, and talus caves.
These are long tunnels formed near the snouts of glaciers between the glacial ice and the underlying bedrock. Meltwater from the surface of a glacier drains downward through crevasses, which are enlarged to form shafts leading to the base of the glacier. Because the inlet water is slightly above the melting point of ice, it gradually melts the ice as it seeps along the base of the glacier.
Glacier caves may reach lengths of several kilometres. Mature caves of this sort are tubular conduits, often with intricately sculptured walls. Some of them have a branching pattern. The floors of glacier caves usually consist of rock. Most glacier caves can be explored only when the surface is frozen; at other times they are filled with water.
Sea caves, eolian caves, rock shelters, and talus caves
Sea caves are formed by wave action on fractures or other weaknesses in the bedrock of sea cliffs along coastlines. They may be mere crevices in the cliff or roomy chambers. Some can be entered only by boat at low tide, while others, occurring along beaches, can be walked into. A sea cave may have an opening to the surface at its rear that provides access from the top of the cliff. In some cases, the ceiling entrance serves as a blowhole from which water spouts during times of high tide or rough seas. Sea caves rarely are more than a few hundred metres long.
Eolian caves are chambers scoured by wind action. They are common in desert areas where they are formed in massive sandstone cliffs. Wind sweeping around such a cavity erodes the walls, floor, and ceiling, resulting in a bottle-shaped chamber usually of greater diameter than the entrance. Eolian caves are rarely longer than a few tens of metres.
Rock shelters are produced by bedrock erosion in insoluble rocks. A common setting is where a resistant rock such as a sandstone overlies shale or some other relatively weak rock. Surface weathering or stream action wears away the shale, cutting it back into the hillside. The sandstone is left behind as a roof to the rock shelter. Rock shelters are minor features as caves, but many are important archaeological or historical sites.
Talus caves are openings formed between boulders piled up on mountain slopes. Most of them are very small both in length and in cross section. Some boulder piles, however, do have explorable interconnected “passages” of considerable length. Some of the largest talus caves occur among granite blocks in New York and New England, where integrated systems of passages between boulders have been mapped to lengths of several kilometres.