Cave, chamber beneath the surface of the earth or in the side of a hill, cliff, or mountain. Caves vary in size and shape, and many have large openings to the surface.
Formation of Caves
Naturally formed caves evolve in various ways, mainly as a result of the solvent action of water and compounds in it. Known technically as caves of solution, such chambers are most common in limestone formations, particularly in regions that have ample rainfall. The surface water in such regions contains carbon dioxide and humid acids derived from the organic constituents of the soil. Attacking the soluble limestone, this acidic water dissolves and carries the limestone away in solution. Over long periods of time, such action results in the formation of subterranean chambers. The depth of such chambers depends on the depth of the water table (see Water). If after several unusually wet years the water table is rising, old cave chambers become flooded and new ones begin forming at higher levels. Likewise, during a long dry spell, chambers will begin forming at lower levels, closer to the declining water table. Over thousands of years, fluctuations such as these produce multi-level cave systems, as in Mammoth Cave National Park in Kentucky, where a subterranean stream flows through the lowest level. Underground rivers erode and transport sediments and rock fragments in a manner analogous to that of surface streams. If such action has been predominant, the cave is said to have been formed by mechanical abrasion.
Other types of caves include the sea cave, which is formed by wave action against seaside cliffs; lava caves, which form under lava flows; and ice caves, which form in glaciers and icebergs. River action forms still another type of cave, commonly with a very large opening that gives it the appearance of a natural amphitheater. A river entrenched in a steep-walled canyon most actively erodes that portion of the canyon wall against which the current is strongest, as at a bend or in a meander. By erosion, solution, and quarrying, the river excavates a large quantity of rock, forming a large undercut area in the side of the canyon. With the passage of time the riverbed is lowered, and eventually the cave is left high in the side of the canyon. Such rock shelters were used extensively in what is now the southwestern United States by the prehistoric Native Americans known as Cliff Dwellers, who built their homes within them (see Cliff Dweller).
Finally, aeolian, or wind, action is partly responsible for the formation of small caves that are confined mostly to desert or semidesert regions. The action of windblown sand is one of several forces involved in the formation of these grottoes and caves in rock ledges and cliffs.
The presence of caves in limestone regions may be detected by means of clues provided by the topography of the land. In such a region the roofs of large caverns may collapse and leave depressions and troughs at the surface of the ground. Natural bridges, another phenomenon of cave regions, may remain after the collapse of a tunnel bearing an underground stream. The Natural Bridge in Virginia is a classic example of this type of formation. In the phenomenon known as disappearing streams, which is a common feature in areas underlain by caves, whole watercourses may vanish down sinks, or sinkholes, leading to the underground caverns. The sinks are indicative of caves below. Because of the capture of the surface waters by the subterranean drainage system, some cave regions have a rather dry, dusty, poorly vegetated appearance. Such regions are said to have a karst topography, a name derived from a famous cave region along the Adriatic Sea in Italy and Slovenia. Steep-walled sinks called cenotes, found in Yucatán, Mexico, constituted a chief source of water for the Mayan peoples.
Caves range in size from small hillside openings to vast interconnected subterranean systems of many chambers and galleries. Some cave systems extend for miles underground and may have many outlets.
Natural air conditioning occurs in large caverns if the temperature varies only a few degrees yearly, and the caves are more or less constantly ventilated with fresh air. These conditions are, in part, the result of complex meteorological phenomena, mainly variations in barometric pressure.
Caves formed by abrasion commonly consist of myriads of winding tunnels and former underground waterways that show many features analogous to those of surface streams, such as deposits of sand and gravel. Abrasion-formed caves normally lack the weird formations found in caves of solution.
In caves of solution, the dissolved lime carbonate is often precipitated in such a fashion as to form grotesque deposits. The best-known structures are the stalactites, which hang like icicles from the roofs of caves, and the stalagmites, which extend upward from the cavern floors (see Stalactite and Stalagmite). If the two growths meet and join, a pillar forms, helping to support the roof. Less well-known forms of carbonate deposition include flowstone and dripstone. Depending on dissolved mineral impurities brought into the cave by the groundwaters, the formations vary in color from alabaster white to hues of dusky red and brown. The dripstone formations may be exceedingly thin and translucent. Among rare formations is the helictite, a twisted, flowerlike variety of stalactite. Many cave formations are rather delicate and easily broken, and some of the best examples have been damaged or removed by unscrupulous cave explorers and visitors to public caves.
Many formations in commercial caves have been given fanciful names, such as "Rock of Ages" and "Temple of the Sun" in Carlsbad Caverns and "Martha Washington's Statue" and "Fatman's Misery" in Mammoth Cave. Frequently recurring names include "Japanese Temple," "Frozen Waterfall," "King's Bed Chamber," and "Great Hall."
A practice in many large caves, particularly those administered by the U.S. National Park Service, is to illuminate the more spectacular formations for the benefit of sightseers. Many public caverns have miles of lighted trails, with stairways and adequate safety guards near areas considered dangerous. In some caves visitors can take all-day hiking tours.
Through evolutionary processes some plants and animals have become specially adapted to living in caves. As a rule, these organisms are confined to the area near the entrance, but some species penetrate to the darkest reaches of the cave. Properly, cave life may be divided into those forms living exclusively in caves and those that live in caves part of the time but forage in the open. The cave cricket and some cave fish are examples of animals modified for continuous life in caves. Although such species are blind, as is usual among animal forms completely adapted to cave life, their organs of touch are highly developed. Animals that live completely in caves commonly have as their diet the edible matter, such as microorganisms and decaying organic material, carried into the caves by streams.
Bats exemplify animals that utilize caves for resting and hibernating purposes but forage in the open for their food. Rich deposits of guano, or bat droppings, have accumulated over the centuries in caves where bats congregate. Such detritus deposits support a variety of insects and simpler organisms. The guano is sometimes marketed as fertilizer. The number of bats inhabiting a large cave may be astoundingly large. For example, hundreds of thousands of the animals can be observed in the evening flight of bats from Carlsbad Caverns.
Because lack of sunlight precludes green-plant growth, fungi make up the only plant life that can grow in caves. Groundwaters containing dissolved organic substances frequently provide nutrients for the fungi.
In past ages people often took shelter in caves, notably in western Europe, the Mediterranean regions, China, southern Africa, and Chile. These early cave inhabitants popularly have been called cavemen, but the term is misleading, for it implies that a race of people at one time dwelt exclusively in caves. Actually, during the Ice Age, people, like other animals, sought refuge in caves from time to time. Many artifacts of Paleolithic and Neolithic peoples have been found in refuse heaps near the entrances of caves. Primitive paintings have been found on the walls of some caves, notably in France and Spain. Modern critics acclaim the artistic beauty of these paintings, attributed to the Cro-Magnon, a race of the late Paleolithic period. See Cave Dwellers.
The science of cave study is termed speleology. A subdivision of geology, speleology has furthered knowledge in mineralogy, hydrodynamics, archaeology, biology, and many other formal disciplines. Speleologists use many special contrivances and methods in exploring caves. One technique is the use of dye stains to reveal the outlets of complicated underground-stream systems. Use of special shoes, safety helmets, flexible ladders and cables, and dependable lamps enables present-day speleologists to explore the recesses of large caves much more thoroughly than was formerly possible. Cave explorers occasionally stay underground for days, mapping and studying an extensive area.