Destructive Action of Groundwater - While percolating through the rocks the groundwater has in some places a destructive, in some places a constructive effect on the rocks through which it is passing. It comes in contact with some minerals that are soluble and takes them in solution, and this solution acts as a solvent for others. The material dissolved is carried away by the streams to the ocean, leaving cavities in the rock from which the material was carried.
Caves - Carbon dioxide, derived partly from the air and partly from the soil, dissolves limestone when it is brought in contact with this rock by the percolating groundwater which likewise acts as the agent to carry away the material after it is dissolved. In this way such caves as Mammoth Cave in Kentucky, Luray in Virginia, Wyandotte and Marengo in Indiana and Howe's Cave in New York are formed.
Nearly every bed of limestone has numerous caverns large or small. In the upland areas of central Kentucky it is estimated that there are not less than 10,000 miles of limestone caverns. Limestone caves vary in length from a few feet to many miles, in depth below the surface from a few feet to several hundred feet.
Life in Caves - As might well be imagined caves frequently form retreats or hiding places for different animals, the most common being the bats, which fly abroad in summer nights but spend the days and the winter season in the caves where they sometimes cluster in great masses hanging frofti the roof of the cavern. Beetles, lizards, mice, wolves, bears, and foxes are some of the other animals that find a home in the caves. Blind fish are sometimes found in them.
Savage man in ages past found shelter there. Relics of his handiwork, his implements and his carving on the walls, have been found in different caves often associated with bones, and pictures made by him of now extinct animals and even the bones of primeval man himself have been found.
Corradingf Action of Cave Streams - Besides the dissolving action of the groundwaters on the rocks there is in places a corrading or wearing action on the bottoms and sides of caves similar to the work of surface streams. In some places several caves occur one above the other. The upper ones are dry while the lowest one frequently has a stream in part of the cave. Most of the corrading work done in the cave is not done by the permanent stream but by the temporary streams that pour in through openings in the roof during the rainy seasons.
Lost River in Indiana flows through limestone caverns for about 10 miles of its course, but in flood season when there is more water than can find escape through the underground channel the surplus flows in the surface channel until the flood subsides when it disappears into the cave. Presumably there is considerable corrasion in such a cave.
Sink Holes - In nearly every limestone region where there are caves there are numerous basin-like or
funnel-shaped depressions, called sink holes or swallowholes. These are often shaped like a funnel, the large opening serving to catch the rainfall and to lead it into the narrow opening at the bottom, corresponding to the stem of the funnel. Through the sink hole the surface water drains into larger caverns.
A limestone surface much diversified by the action of the groundwater dissolving the rock along cracks and jointplanes, thus leaving many deep irregular fissures, is called by the Germans the Karsten. There is no English word for this phenorTDcnon although it occurs in many places in New York and elsewhere in the United States.
Natural Bridges - Natural bridges are formed sometimes by the breaking down of part of the roof over one of these subterranean streams. The portion of the roof that remains spanning the now open chasm is called a natural bridge. Natural bridges are sometimes formed in other ways.
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