The coordinates will lead you to the banks of the Mississippi River with a fine example of exposed St. Peter Sandstone. The bluff face along the river is comprised of three main layers. Underneath the top soil and glacial drift, the three layers from top to bottom are the Platteville Limestone, Glenwood Shale, and St. Peter sandstone (1). You can tell the St. Peters layer by its appearance. It is gradually eroded, unlike the vertical, sheering Platteville Limestone that caps the bluffs. The Glenwood Shale is a thin, dark layer between the St. Peters and Platteville that easily erodes (2). The St. Peter Sandstone formation covers 225,000 square miles of the upper Mississippi Valley, with an average thickness of 100’. The thickness of the St. Peter in the Twin Cities area ranges between 80 and 160’ (3). It is prized for its pure white sand, and is important as a conduit for groundwater (2).
Its white color is a clue to how it was formed. The sand grains that make up the sandstone are fine-medium size quartz sand, and are well-sorted and well-rounded (3). In order for it to acquire its pure white color, impurities had to have been sorted and removed from the sand. This suggests that the rock has an aeolian deposition, or that wind was the primary erosion force in its creation. Wind is the only erosive force capable of sorting finely rounded sand from subangular sand (3). Furthermore, the grains’ textual maturity (round shape), was caused by the grains colliding with each other in the air (2). Water erosion does not create enough force between the grains necessary to create that level of textual maturity (2). Since wind was the primary erosive force, it would seem that the environment was rather windy during the St. Peter’s formation, like a desert. However, the geometry and thickness of the formation, and the presence of trace fossils suggest that the sandstone may have formed in a coastal or shallow marine environment, such as the inland sea that once covered the land (3). The transition from St. Peter Sandstone to Glenwood Shale shows that the St. Peter Sea became deeper (3).
To claim credit for this EarthCache, email us the answers to the following:
1. Estimate the height of the exposed sandstone to determine the approximate St. Peter Sandstone accumulation at this location. Take an elevation measurement at the river level, and subtract that from another elevation reading where the sandstone transitions to shale.
2. Examine the sandstone grains. What shape are the grains? Are they uniform in size and shape? Or do they have different sizes and shapes? What would explain the grain texture you see?
3. Look around you. What evidence do you see that would support the theory that the St. Peter Sandstone was formed in a coastal, shallow marine environment?
4. Although not required, photos of your visited are encouraged. There are some fantastic views at this location!
1. Ojakangas, R. W., & Matsch, C. L. (1982). Minnesota’s geology. Minneapolis: University of Minnesota Press.
2. GEO 1001/1101: Earth and Its Environments (2010). River bluffs outcrop. Accessed June 30, 2010 from http://www.geo.umn.edu/courses/1001/campus/pages/river/river.htm.
3. Mazzullo, J. M., & Ehrlich, R. (1987). The St. Peter Sandstone of Southeastern Minnesota: Mode of deposition. In Sloan, R. E. (Eds.), Report of investigations (Minnesota Geological Survey). St. Paul, MN: University of Minnesota. 35, 44-51.