St. Peter Sandstone
Stratigraphic range: Middle Ordovician
Old mine entrances in the St. Peter Sandstone formation in Pacific, Missouri, where it is still actively quarried nearby.
Type	Formation
Unit of	Ancell Group
Underlies	Dutchtown Formation, Glenwood Shale, Joachim Dolomite, and Wells Creek Formation
Overlies	Beekmantown Dolomite, Everton Formation, Shakopee Dolomite
Location
Region	Midwest
Country	United States
Extent	Arkansas, Illinois, Indiana, Iowa, Minnesota, Missouri, Wisconsin and West Virginia
Type section
Named for	St. Peters River (now Minnesota River), Minnesota[1]
Named by	David Dale Owen

The St. Peter Sandstone is an Ordovician geological formation. It belongs to the Chazyan stage of the Champlainian series in North American regional stratigraphy, equivalent to the late Darriwilian global stage. This sandstone originated as a sheet of sand in clear, shallow water near the shore of a Paleozoic sea and consists of fine-to-medium-size, well-rounded quartz grains with frosted surfaces. The extent of the formation spans north-south from Minnesota to Arkansas and east-west from Illinois into Nebraska and South Dakota. The formation was named by Owen (1847)^[2] after the Minnesota River, then known as the St. Peter River. The type locality is at the confluence of the Mississippi and Minnesota Rivers near Fort Snelling, Minnesota.^[3] In eastern Missouri, the stone consists of quartz sand that is 99.44% silica.

Outcrop

In Minnesota, the soft St. Peter Sandstone can be observed at the bluffs of the Mississippi River valley beneath a very thin layer of Glenwood Shale and a much thicker layer of Platteville limestone.^[4] Examples can be seen in the Mississippi River Gorge and at Minnehaha Falls in Minneapolis, the bluffs from downtown to Mounds Park in Saint Paul, and Minneopa Falls near Mankato. In Illinois, Castle Rock is a large bluff of St. Peter Sandstone, and Starved Rock State Park and Matthiessen State Park feature numerous outcroppings and canyons.

Commercial use

St. Peter sandstone, also called "Ottawa Sand" in commercial applications, has a relatively uniform size and shape for each grain. It is used for the manufacture of glass, for filter and molding sand, and for abrasives. Its purity is especially important to glassmakers. It is also important, as "frac sand" in oil and gas drilling – loose sand pumped in a liquid mix under high pressure into a well where the sand grains wedge into and hold open any fractures in the rock, enhancing the extraction of hydrocarbons. The uniform particle size also makes the sand useful for laboratory experiments.

Mining locations

St. Peter sandstone is or has been mined

• Arkansas: Guion, Arkansas
• Illinois: Ottawa, Illinois, Sheridan, Illinois, Wedron, Illinois, Oregon, Illinois, and Naplate, Illinois
• Minnesota: Kasota, Minnesota and Ottawa Township, Minnesota
• Missouri: Pacific, Festus, Crystal City, Augusta, and Pevely

The Unimin Corporation is a large producer of commercial sand and operates surface mines in many of these locations.