About, 20,000 years ago, a series of glaciers scoured and scraped their way across South Dakota, creating in their wake the Coteau des Prairies. As the glaciers retreated, they left a deposit known as glacial moraine - consisting of soil and rocky material - that created the Coteau des Prairie. The Coteau is a wedge-shaped area stretching 250 miles from the North Dakota border to Sioux Falls, SD.
In several places, large chunks of ice broke away from the glaciers. These ice chunks caused the land to slump, forming basins to catch the water as the ice melted. These areas formed the glacial lakes seen throughout the Coteau des Prairie.
Another important geological feature in the region is the Northern (or Laurentian) Divide, which runs within a few miles of Fort Sisseton. Water that falls on the north side of the divide flows into the Hudson Bay and water that falls on the south side of divide flows into the Gulf of Mexico.