Sinkholes are common in Florida, and they are the result of the solution and erosion of limestone formations in the soil by surface and ground water. The limestone generally lies beneath a covering of sediments and deposits (called overburden) that were left throughout Florida at various altitudes during periods of high sea level. The thickness and composition of this covering are among the factors that determine the type and severity of sinkhole that occurs. There are three major kinds of sinkholes.
Solution sinkholes form when the limestone is at or very near the soil surface. As percolating surface water carries away dissolved limestone, a gradually expanding bowl-shaped depression with sloping sides results. If the surface runoff also carries particles of clay and sand into the sinkhole, the bottom of the depression may become impermeable enough to retain some water so that a lake or marsh is created.
Cover-subsidence sinkholes occur in areas where the underlying limestone is covered by unconsolidated material (such as sand) that surface water can permeate easily, and where sand migrates downward grain by grain to replace dissolved limestone. If the sand cover is 50 to 100 feet thick, the resulting subsidence sinkholes are typically only a few feet wide and deep. An even thicker sand covering can, over time, result in a broad and extensive subsidence of the land surface rather than individual sinkholes. If the overburden is dense plastic clay, which is less water-permeable than sand, the downward movement of groundwater and the development of cavities in the underlying limestone may be slowed.
Cover-collapse sinkholes are the most dramatic and potentially catastrophic. This type of sinkhole is most likely to occur in areas where limestone lies near the ground surface and is also subjected to accelerated dissolving action from the water table. The sinkhole forms abruptly when such a large cavity has been created in underlying limestone, and the overburden covering the cavity collapses from its own weight. Collapse sinkholes are typically round in shape with steep or overhanging walls in which limestone is exposed. Such a cave-in can be quite large and often causes abrupt changes in the local landscape. Over time, the action of surface drainage, erosion, and sedimentation smoothes the contours of collapse sinkholes until they resemble the other types.