Geology of Mammoth Cave

Abstract

Mammoth Cave owes its origin to the rock called limestone, which readily dissolves in water. Underground water passing through thick limestone layers toward the deep valley of the Green River has formed and enlarged the cave over the past several million years. The rocks are 330–340 million years old, but the cave is much younger. A cap of resistant sandstone overlying the limestone has protected the main parts of the cave from surface erosion, thus preserving many clues to past events. Passage types reflect the nature of the water that formed them. Below, at, and slightly above the local Green River level, all openings in the ground are filled with water. Above this zone, underground water drains downward by gravity along the steepest available cracks in the rock. Where water follows the narrow fissures between rock layers, it forms tall canyon-like passages. Well-shaped vertical shafts form where water is able to descend straight downward along fractures that cut across the layers. Below the water table, underground water follows the most efficient paths of flow, along the widest openings, to form tube-shaped passages. Former positions of the water table can be determined from the change in passage shape, which provides a history of groundwater flow in the region.