Minnesota Geology Monday - St. Peter Sandstone

The St. Peter Sandstone outcrops at numerous locations around Minneapolis and St. Paul, including Minnehaha Falls and throughout the Mississippi River valley.  The sandstone is world famous as a nearly pure sandstone, it is over 99% quartz.  The sandstone is not well cemented and as such, can be easily eroded.  The sandstone was deposited 458 million years ago during the Ordovician Period.  The sandstone was named for outcrops near the confluence of the Mississippi and Minnesota rivers at Fort Snelling.  An early name for the Minnesota River was the St. Peter River, this name was later given to the sandstone.

In the Twin Cities, the sandstone is overlain by the greenish Glenwood Shale and the Platteville Limestone.  The St. Peter Sandstone was deposited over a large area of the midwest United States, in Minnesota it is at the northernmost extent.  The average thickness of the sandstone, in the Twin Cities, is 155 feet.

Nearly all the sand grains within the sandstone are rounded, the result of the mechanical weathering process of abrasion.  Predominantly made of the resistant mineral quartz, four other minerals are present in the sandstone in small quantities, tourmaline, zircon, rutile and garnet.  Radiometric dating of the zircon indicates that they are derived from rocks 2,600 to 1,000 million years in age.  These ages leave plenty of time for the mineral grains to be weathered into the nearly round shapes they are in today.

Because the sandstone is not well cemented, it is easily excavated by humans and animals.  Birds use the sandstone as nesting burrows, humans have used it for numerous purposes.  From 1926-1959, the (now closed) Ford Twin Cities Assembly Plant excavated the sand to produce window glass for their vehicles.  In St. Paul, humans excavated caves in the sandstone to be used as storage spaces, growing mushrooms and during Prohibition, a restaurant and nightclub known as the Wabasha Street Speakeasy.