St. Peter sandstone is an Ordovician formation in the Chazyan stage of the Champlainian series. 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 Missouri and east-west from Illinois into Nebraska and South Dakota. The type locality for the formation is St. Peter, Minnesota. In eastern Missouri the stone consists of quartz sand that is 99.44% silica.
In Minnesota, the soft St. Peter Sandstone can be observed at the bluffs of the Mississippi River valley beneath a layer of Platteville limestone
. Examples can be seen at the Minnehaha Falls
near Minneapolis and Minneopa Falls
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 a "frac sand" in oil
and gas drilling
– loose sand is pumped in a liquid mix under high pressure into an 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.
St. Peter sandstone is or has been mined
The Unimin corporation is a large producer of commercial sand and operates the surface mines in many of these locations.