The dissolution of calcite from limestone by slightly acidic ground-water results in the gradual widening of cracks and joints that may ultimately develop into a series of openings, or caves. Most caves develop below the water table. After the caves are elevated above the water table or when the water table drops, the water drains out and the caves become filled with air.
The groundwater that percolates through the cracks in the cave contains calcium and bicarbonate from the dissolution of limestone. As the water drips from the cave's ceiling, CO2 gas is released and a small amount of calcite crystallizes where the drop is attached to the ceiling. More CO2 is lost from the water when the drop hits the floor, causing more calcite to precipitate. By this process, stalactites and stalagmites form. Stalactites look like icicles that hang from cave ceilings; stalagmites are cone-shaped masses that build up on cave floors underneath dripping stalactites. A column results when stalactites and stalagmites grow long enough to join into one structure. A more general term for a deposit of calcite precipitated by dripping water is dripstone, and as a group, the varieties of dripstone found in caverns are called speleothems. Ribbony, sheetlike calcite deposits that are deposited by a thin film of water running over cave surfaces are called flowstone.
Caves become less structurally stable as greater amounts of limestone are eroded away. When a portion of a cave system collapses, it may create a sinkhole, or basinlike depression, at the surface. Sinkholes, which can develop suddenly and be large enough to “swallow” buildings and homes, are prevalent in states such as Florida, Missouri, Indiana, and Kentucky, which are underlain by abundant limestone. Karst topography is an irregular land surface dotted with numerous sinkholes and depressions related to underlying cave systems.
Groundwater that has a high concentration of silica is the primary agent in forming petrified wood. The groundwater soaks through the buried wood and precipitates silica in the porous organic structure, preserving the finest details. When silica or calcite layers precipitate from groundwater in a spherical cavity (usually in limestone), the often intricately layered mass that results is called a geode. A concretion is a mass of silica or calcite that precipitates around an organic nucleus, such as a leaf or fossil, in sedimentary rocks. Usually geodes and concretions are more resistant to weathering than the enclosing rock and stand out in weathered rock faces.
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