The Couteau des Prairies (Highland of the Prairie) is a 200 mile swath of Precambrian Sioux Quartzite, stretching from eastern South Dakota, through southwestern Minnesota and into extreme northwestern Iowa. Remnants of an ancient mountain range, this hard, metamorphic rock was deposited 1.5 billion years ago and has since undergone compression by overlying Paleozoic-Tertiary sediments, uplift, erosion and both sculpting and burial by the Pleistocene Glaciers. Even now, numerous streams erode the plateau, which divides the watersheds of the Missouri and Mississippi Rivers.
Today, the Couteau stands out as a network of ridges and low plateaus, rising 500-800 feet above the surrounding plains. Cliffs of the Sioux Quartzite are exposed throughout the uplift but, in many areas, are buried by up to 700 feet of glacial drift (both till and wind-blown loess). Buffalo Ridge forms the crest of the uplift, rising to nearly 2000 feet; it is composed of Cretaceaous shale and sandstone overlying the much older Quartzite.
The Couteau des Prairies is well known to naturalists for its remnants of virgin, tallgrass prairie, to geologists for its varied strata and glacial features and to anthropologists for its quarries of catlinite, seams of red mudstone within the Sioux Quartzite. Native Americans used this soft "pipestone" to produce peace pipes and other ceremonial artifacts; they also traded this carvable rock with other tribes, including the Anasazi of the Four Corners region. Pipestone National Monument, the first National Park Service preserve in Minnesota (1937), protects some of these quarries and harbors tracts of virgin and restored tallgrass prairie. The Nature Conservancy's Hole in the Mountain preserve, covering a valley in the Buffalo Ridge near Lake Benton, Minnesota, is another interesting site to visit.
