Karst terrain forms by dissolution of carbonate rocks, such as limestone or dolomite, or evaporites, such as gypsum or salt, and is characterized by features including sinkholes, disappearing streams, caves, and springs. Sinkholes (or sinks) are enclosed depressions that do not usually hold water; they often have a “throat” or opening at the bottom where they drain to the subsurface. When a stream flows into a sinkhole, it is known as a disappearing stream or losing stream. Water flowing into the ground can cause solution enlargement of natural fractures in the rock and eventually can grow into caves. The Ohio Revised Code defines a cave as “…a naturally occurring void, cavity, recess, or system of interconnecting passages beneath the surface of the earth or within a cliff or ledge…” (State of Ohio, 1989).
The many passageways formed in karst terrain allow for high connectivity between the land surface and the water table. These passageways permit water to bypass soil and rock layers that filter out contaminants. Consequently, when compounds such as fertilizers, pesticides, and waste enter sinkholes, they are rapidly transported to the water table and quickly pollute water wells, streams, and rivers. When water exits these solutional features, a spring is formed. Such springs enable release of these contaminants at the surface.
The different types of karst features may pose infrastructure complications; roads, utilities, houses, and other facilities built in karst areas are at risk of subsidence, collapse, or other damage. In order to provide a reference for future planning on both the local and regional scale, the Ohio Geological Survey has produced this map book identifying the known and suspected karst areas in the vicinity of Hillsboro, Ohio.