the pleistocene (1.5 - 10,000 b.c) in northern america was tilled by a succession of great ice sheets. the area covered by glaciers in north america is estimated at 10.4 million km and about 20% of the u.s. is influenced by the deposits. the retreating glaciers created the unconsolidated rock surface east of the missouri river. it took the sediment numerous glacial advances and retreats beginning some 1.5 million years ago, and ending 10 thousand years ago to create glacial tilled soils. eastern south dakota has a thick layer of glacial sediment that is about 100 feet (30.5 meters). the glacial till is a result of the material deposited directly by the ice. the mixture of rock has a diverse range of particle size.