Hill of Chalk

When William Clark climbed this conical hill in 1804, he correctly concluded that it was a natural landform rather than a human-created mound. Its central core is bedrock of Niobrara chalk that the continental ice sheet did not erode away. The chalk, made of fossil shells deposited in an ancient sea, is visible on the hill's north side. This remnant of eroded bedrock highlands predates the last glacial advance here 13,000 years ago. When the glacier advanced from the north over the highland, it removed some of the sediment, and as it melted, it left behind glacial till, a mix of clay, silt, sand, and other materials it was carrying.

South Dakota · United States