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Mammoth Cave is an example of a solution cave, which are formed by water slowly moving through carbonate rocks. The formation of Mammoth Cave began hundreds of millions of years ago, as water moved through the limestone of the region. As the water carved the passages of the cave, it worked its way further underground, resulting in several layers of passages. Mammoth cave is primarily a dry cave, without cave features such as stalagmites and stalactites actively forming in many of the explored passages. Some cave features are still present, however, especially in the Frozen Niagara portion of the cave. A sandstone and shale caprock has helped to preserve the cave, and is responsible for its current dry state. Caprocks are layers of harder or more impermeable rock types, usually sandstone or mafic rock types, which are overlaying softer, more permeable rock. Because the sandstone is impermeable to water, rainwater is unable to leak into most of the cave and further wear down the limestone. To learn more about how caves are formed and some of the formations present in Mammoth Cave, explore the other pages in this section.