Beginning about 2 million years ago, continental glaciers moved generally southward across North America, covering eastern South Dakota several times. As each ice sheet advanced, it transported large volumes of rock debris frozen into the lower layers of ice. If the ice sheet was very thick and heavy, the glaciers scoured and smoothed off the terrain. In contrast, where the ice was thin, the glaciers overrode obstacles rather than planing them. As the ice melted, sediment called glacial drift was left behind.