Mammoth Cave National Park - The Powerful Work of Water

Echo River, a waterway that flows through the lowermost depths of Mammoth Cave, offers a key to understanding the origins of this subterranean wonderland. For water is the subtle sculptor that carved -- and is still carving -- Mammoth's seemingly endless labyrinth of chambers and passageways. Rain falling through the air and then seeping through decaying vegetation on the ground picks up enough carbon dioxide to become a mild solution of carbonic acid -- and given enough time, this acidic water is capable of dissolving solid limestone and opening passageways in the rock.

Such was the origin of Echo River's channel and, indeed, of every other passageway in the cave network. Eons ago, what is now the river's course was probably marked by the minutest of fractures in the limestone that makes up the Mammoth Cave Plateau. Bit by bit, seeping water gradually dissolved the rock. Now the calm, greenish river flows along a channel that averages 20 feet in width and in some places is as much as 25 feet deep.

The story of the cave began about 300 million years ago, when the region was flooded by a shallow inland sea. For millions of years ,the limy shells and skeletons of sea creatures, many of them microscopic in size, settled to the floor of the sea and accumulated into layers of limestone hundreds of feet thick. Mud and sand later washed in and were transformed into layers of shale and sandstone atop the limestone.

When the land began to rise and the sea retreated, about 280 million years ago, countless cracks and fissures developed in the limestone, and erosion began to attack the caprock. On the plains southeast of the park -- the source of most of the water that carved the cave -- the protective layers of shale and sandstone have disappeared entirely. Here the land is pockmarked with sinkholes, huge circular pits that were eroded into the limestone where surface water plunges underground. (Sinkholes can also be formed or enlarged by the collapse of cave ceilings.)

Flowing through fissures in the limestone, the water gradually enlarged them into mighty passageways where underground rivers rushed toward their outlets, the springs along the Green River. Then, as the Green River deepened its valley, upper passages were left high and dry and the water began carving new channels at lower levels in the rock. At the lowest levels -- along Echo River some 360 feet below ground level, for instance -- the process still goes on. A living cave, Mammoth continues to grow.