Recent mapping adventures have reminded me just how much I enjoy studying the St. Peter Sandstone. This sandstone was named by Arkansas’ first State Geologist, David Dale Owen, for exposures on the St. Peter River, now called the Minnesota River, in southern Minnesota. The sandstone is Middle Ordovician in age (around 460 million years old) and during that time it extended all the way from Minnesota into Texas.
It is easy to recognize the St. Peter Sandstone whether you are in Minnesota or Arkansas – clean, sugary, white sandstone. In fact, here is a photo taken from the place where it was first described, known as the type section, under a bridge near Fort Snelling, in St. Paul, Minnesota.
In Minnesota the sandstone easily falls apart. In Arkansas, the surface of the outcrop is case- hardened meaning there is a hard rind on the rock that forms due to iron-rich water percolating through it and depositing iron on the surface as the water evaporates. Where this rind is broken, the sand grains fall apart more easily, as at the type area.
The contact of the St. Peter Sandstone with the underlying Everton Formation is particularly interesting. It is unconformable which means there was a period of non-deposition and erosion before the St. Peter was deposited. It is also undulatory with as much as 20 feet of relief in Arkansas. The relief is the difference between the top and bottom of an undulation. Flint, 1956, reports that these undulations can reach up to 200 feet in Wisconsin. Pretty amazing!
In the photo above, the relief at the contact is probably around 5-6 feet. Note the rock hammer for scale. The rock above the hammer is the St. Peter Sandstone while the rock the hammer is resting on is the Everton Formation. Also notice the curvature of the contact. The reason for the unconformable undulating contact is that the sand in the St. Peter was deposited upon the karsted Everton surface. Karst forms when rock such as limestone is exposed to slightly acidic rainwater or groundwater and develops sinkholes, caves, and enlarged fractures. Since this karst surface has been buried by the St. Peter Sandstone, it is considered paleokarst.
The geologic story goes something like this. After the sea that deposited the Everton Formation retreated, the limestone at the top of the formation was exposed. Weathering and erosion lasted for up to tens of millions of years, during which time an extensive karst surface developed (Palmer and Palmer, 2011). Sand was brought into the area from the source area to the north (Great Lakes region) by rivers and wind. Later, as the sea advanced again, it spread the sand over the area filling in the depressions and forming a thick deposit covering a large portion of the mid-continent.
The St. Peter Sandstone is relatively resistant to erosion compared to the surrounding rocks; therefore, it is usually a bluff-former. The tallest bluff I have seen crops out at Blanchard Springs Recreation Area near the group camp and the amphitheater.
Enjoy these photos of the St. Peter Sandstone and hope to see you in the field!
References and other sources on the St. Peter Sandstone:
Flint, A.E., 1956, Stratigraphic relations of the Shakopee Dolomite and the St. Peter Sandstone in southwestern Wisconsin: Journal of Geology, vol. 64, no. 4, pp. 396-421.
Giles, A.W., 1930, St. Peter and older Ordovician sandstones of northern Arkansas: Arkansas Geological Survey Bulletin 4, 187 p.
Palmer, A.N. and Palmer, M.V., 2011, Paleokarst of the USA: A brief review; in U.S. Geological Survey Karst Interest Group Proceedings, Fayetteville, Arkansas: U.S. Geological Survey Scientific Investigations Report 2011-5031, pp. 7-16.