An irregular limestone region with sinkholes, underground streams, and caverns. Karsts owe their existence to the removal of bedrock in solution and to the development of underground drainage without the development of surface stream valleys. Karst is characterized by the formation and growth of cavities resulting from chemical weathering and erosion in regions of carbonate and evaporite rocks. Karsts show much variation and are usually described in terms of a dominant landform. Most important are fluviokarst, doline karst, cone and tower karst, and pavement karst. Approximately 15 percent of the Earth's land surface is karst. The most extensive karst area of the United States occurs in the limestones of Mississippian age (about 325,000,000-345,000,000 years old) of the Interior Low Plateaus. Karst also occurs in the limestones of Ordovician age (about 430,000,000-500,000,000 years old) in Kentucky and Tennessee.