 Its bedrock is formed of Cretaceous shale, sandstone and clay that lie above the pinkish-red Upper Precambrian Sioux Quartzite.These units are covered in most areas by thick deposits of glacial drift, which consist of up to 800' (244m) of pre-Wisconsin age glacial till left after the glaciers receded.The inner coteau is made up of extremely stream-eroded glacial deposits of pre-Wisconsin glacial drift, which is then covered by a 6-15' (1.8-4.6m) thick deposit of a wind-blown silt called loess.