GROUND WATER is another AGENT of erosion. Ground water is the water flowing through the rocks and soil underground. It is part of the hydrologic cycle. As the ground water moves through the rocks and soil, it interacts with it. In some circumstances, it erodes the subsurface materials sufficiently to effect the overlying surface. The process by which it erodes the subsurface material is SOLUTION. An area that has evidence of solution by ground water beneath it is said to have KARST TOPOGRAPHY. Note that the word karst is an adjective to describe the topography, not a noun.
In an area of karst topography, the ground water has been flowing through the bedrock, concentrated particularly along bedding planes and joins or faults within the rock. Rocks especially susceptible to the development of karst topography are those containing a high amount of the mineral calcite, such as LIMESTONE. Through time, chemical weathering along some of these pathways of ground water has dissolved and removed rock material. These voids are called CAVERNS. Some of these caverns may remain filled with ground water, while others closer to the surface may be filled with air. As ground water makes its way from the surface downward, by the pull of gravity, it may come out on the roof of a cavern. This drop might evaporate and leave the dissolved materials contained within it behind. With time, these deposits accumulate to form STALACTITES. (Helpful mnemonic: The word stalactite has letters sticking up from the main line, so they are on the roof, or the stalactite must "hold tight to stay up there.") Other water droplets may drip off of the roof or roof deposit and drop to the floor of the cavern to evaporate before soaking into the cavern floor. Upon evaporating, again, the deposited materials will be left behind as a deposit. This deposit built up from the floor of the cavern is a STALAGMITE. (Helpful mnemonic: The word stalagmite has the letter stick down, so it is on the bottom or floor.) If a stalactite above and a stalagmite below grow large enough to merge, they form a COLUMN. As the caverns grow larger and as erosion occurs at the surface, eventually the cavern roof may become weakened and collapse. A depression formed at the surface as a result of the collapse of the roof of a cavern is a SINKHOLE. Sometimes steams flow into sinkholes leaving their water to go into the caverns and become part of the ground water. These streams are referred to as DISAPPEARING STREAMS. Ground water emerging at the surface creates a SPRING. In areas of prolonged erosion of karst topography, many layers of caverns may be exposed through time. Hills of limestone or other soluble bedrock may rise above the valleys, the old cavern floors, as HAYSTACKS. Areas particularly noted for haystacks include Puerto Rico and a region of southern China. The term karst topography is derived from a region of it in former Yugoslavia.
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