At Medicine Rocks, a vast river flowed through the region 61.5 million years ago, depositing layers of underwater sandbars along its deep channel. Over time, other rivers buried the sandbars with mud, and after the rivers dried up, mineral-rich ground water saturated the buried grains of sand and cemented them into sandstone. Millions of years later, erosion exposed the sandstone at the prairie surface, and wind sculpted the soft rock into mottled pillars, arches, and buttresses. The formations show cross bedding, where thin layers of sandstone lie at angles within thicker beds because of the ancient river's movement. As the sandstone gradually erodes, it reveals hollows and dark halos left where mineral matter formed around trapped tree limbs and trunks that later disintegrated, as well as pits, caves, and holes carved by swirling winds. The tan color comes from the iron oxide limonite, while red streaks show hematite carried downstream from sandstone eroding in South Dakota's Black Hills. The sandstone also contains mudrock seams, chunky white sediment from pieces of eroding riverbank picked up by moving sandbars and later deposited within the stone.