The Mammoth Cave region of Kentucky was once submerged under a shallow sea. During the Mississippian Period, 360–325 million years ago, sediments were deposited at the bottom of the sea, and eventually solidified into rock layers of limestone, shale, and sandstone.

Karst is the term for a landscape formed when a layer or layers of soluble bedrock dissolves away. Limestone's primary mineral, calcium carbonate, is slightly soluble in rainwater and groundwater because of their natural acidity. As rainwater seeps through limestone, the water dissolves tiny amounts of calcium carbonate until it becomes saturated with the mineral.

When groundwater flows through soluble rock to a lower level outlet instead of pooling in the rock, caves and sinkholes develop. The flow of water carries the dissolved minerals away and unsaturated water entering from above dissolves additional mineral matter. Abrasion from the flowing water also assists in enlarging the cavity.

When erosion moves surface materials through a dissolved opening into an underground passage or when the upper surface of an underground cavity collapses or caves in, sinkholes develop. Either way, the presence of sinkholes in an area indicates soluble rock below with active or potentially active waterways. A sinkhole may be dry when first observed, but sinkholes become active waterways whenever it rains.

Karst areas typically have few ground waterways because streams flow into abundant sinkholes and crevices, disappearing from view. Streams flow underground, enlarging crevices and caverns along the way, and sometimes reappearing above ground at a lower elevation. Because the water flows freely through underground passages instead of filtering through rock layers, karst areas are vulnerable to water pollution.

To learn more about how karst systems, check out Caves and Karst.

To learn more about how caves form, check out How Caves Form.

For more information the impact of erosion, check out Water Erosion and Breaking it Down - Weathering and Erosion.

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