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St. Peter Sandstone
The 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 Arkansas and east-west from Illinois into Nebraska and South Dakota. The formation was named by Owen (1847)〔Owen, D.D., 1847, Preliminary report of the geological survey of Wisconsin and Iowa: U.S. Gen. Land Office Rept., 1847, p. 160-173.〕 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.〔Stauffer, C. R., 1934, Type Paleozoic sections in the Minnesota Valley: Jour. Geology, v. 42, p. 337-357.〕 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, which lies below a much thicker layer of Platteville limestone.〔Mossler, J. and Benson, S., 1995, 1999, 2006, (Fossil Collecting in the Twin Cities Area ). Minnesota at a Glance: Minnesota Geological Survey: University of Minnesota.〕 Examples can be seen at Minnehaha Falls in Minneapolis, the bluffs from downtown to Mounds Park in Saint Paul, and Minneopa Falls near Mankato.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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