Cave

I

Introduction

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.

II

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 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. If after several unusually wet years the water table rises, old cave chambers become flooded and new ones begin forming at higher levels. Likewise, during a long dry spell, chambers 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, United States, where a subterranean stream flows through the lowest level. Underground rivers and streams 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 amphitheatre. A river entrenched in a steep-walled canyon most actively erodes that part 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 south-western United States by the prehistoric Native Americans known as cliff dwellers, who built their homes within.

Finally, aeolian, or wind, action is partly responsible for the formation of small caves that are confined mostly to desert or semi-desert 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.

III

Cave Detection

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. Such collapsed caves exist at Wingates Pass, Derbyshire, and at Burrington Combe in the Mendip Hills. 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 Maya peoples.

IV

Interesting Features

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 calcium 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 upwards from the cavern floors. 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 colour 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, flower-like 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”. Frequently recurring names include “Japanese Temple”, “Frozen Waterfall”, “King's Bed Chamber”, and “Great Hall”.

A practice in many large caves is to illuminate the more spectacular formations for the benefit of sightseers. Many public caverns have kilometres of lighted trails, with stairways and adequate safety guards near areas considered to be dangerous. In some caves visitors can take all-day hiking tours.