This region of the Great Plains exposes an immense geologic record of the earth's activity. Ninety million years ago, the area was a vast inland sea. Seventy five million years ago, uplifts of the Rocky Mountains and Black Hills caused the sea to drain, changing the ocean floor into a subtropical jungle, later an Africa-like savannah, and eventually the semi-arid grasslands of today. At Toadstool Geologic Park, the visible record begins at 34 million years ago, when heavy clouds of ash from volcanoes in the Great Basin of Utah and Nevada blanketed the land. An ancient river carved the valley as the landscape became a semi-arid plain. Layered rocks and clays reveal this natural history, exposed by the river that once flowed through the area. Seasonal floods and volcanic ash reshaped the land and added new sediments to the floodplain, and continuing wind and water erosion eventually exposed the layers seen today. Toadstools are sandstone blocks resting on small pedestals and resembling mushrooms because softer clay at the bottom eroded more readily than the more resistant sandstone on top; many have toppled over and many are still being formed. Distinctive bands in the rocks and clay show geologic processes from millions of years ago, with the bottom layers deposited earliest and therefore oldest.