Thirty-seven million years ago, erosion had already exposed the ancient crystalline core of the Black Hills, whose long blue profile to the west looked much as it does now. About that time, streams flowing from the west began depositing sediments in this area, most of them wind blown volcanic ash that had settled into the watershed feeding the streams, though rivers sometimes also carried gravel and stream-worn rocks from the Black Hills themselves. The ash weathered into clay and turned into rock. Erosion did not begin carving gullies into the Badlands layers and shaping the present landscape until quite recently, perhaps only half a million years ago.