ST. PETER SANDSTONE

The white sandstone that forms the walls of this ravine was made from sand that originated in what is now the Lake Superior region. About 460 million years ago, this sand was carried southward by rivers and strong winds to the edge of a warm, shallow sea that covered much of North America. When the sand reached the sea, storm waves and nearshore currents dispersed it along the coastline. As the sea level changed over time, the shoreline migrated across the region of southeastern Minnesota. At that time, southeastern Minnesota may have looked like the sandy coast of the Gulf of Mexico today.

The St. Peter Sandstone was named after the St. Peter River (now called the Minnesota River). It is composed almost entirely of similarly sized and well-rounded quartz sand grains, which are pitted or frosted. The pitting and frosting probably occurred in large dune areas along the shoreline where the sand was blown by the wind before being submerged. The sandstone crumbles easily because its grains are poorly cemented together with minerals that were precipitated from water flowing between the grains. The St. Peter Sandstone is nearly 99 percent pure quartz (crystalline silica) and has been used as a raw material for glass manufacture and metal casting molds.

The St. Peter formation reaches a thickness of up to 58 meters in the Twin Cities area and has been excavated to hold an extensive system of tunnels for water, sewer, and communication cables. Extending across the Midwest and from Michigan south to Arkansas, the St. Peter Sandstone is one of the most widely distributed, high-purity quartz sandstone formations in North America.

Erected by the Geological Society of Minnesota in partnership with the Minnesota Department of Transportation, the Minnesota Geological Survey, and Ramsey County Parks and Recreation. 2003