About 200 to 240 million years ago (in the Carboniferous Period, at the end of the Paleozoic Era), the eastern Tennessee area was covered with a shallow sea, the sediments of which eventually formed limestone rock. About 200 million years ago, this area was uplifted, forming Lookout Mountain and nearby ridges. Slightly acidic groundwater running through cracks in the limestone slowly dissolved the relatively soluble rock, causing narrow cracks to widen into passages. This process, called chemical weathering, caused various caves to form inside Lookout Mountain.