The coordinates will lead you to the Mississippi River valley wall with a fine example of exposed Platteville Limestone. 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. The Platteville is buff in color and is erosionally resistant, which often causes it to overhang above the easily erodible Glenwood and St. Peter below it. The Platteville, like the Glenwood and St. Peter, is widespread across the Midwest and is approximately 30 feet thick in Minnesota. This carbonate formation is mostly made up of limestone with some dolomite in its lowest member and some shale beddings in its upper parts (1). The Platteville formed during the Middle Ordovician era in a shallow marine carbonate bank environment (2). Whereas the St. Peter was formed in a windy, shallow marine environment, the Platteville formed when the inland sea that once covered the land was deeper, in a marine shelf environment (3). At the time, the Twin Cities area was a marine basin known as the Hollandale Embayment. To the northwest laid the shores of the northeast-trending Transcontinental Arch and to the Northeast laid the shores of the northwest-trending Wisconsin Arch (2).
The limestone consists of mineral calcite, which formed from solidified calcareous ooze that existed at the bottom of the marine basin (4). Geologists now this because of the fossils in the limestone. Sea life was abundant during the Middle Ordovician, and several species of brachiopods, cephalopods, gastropods, bryozoa, crinoids, and trilobites sank into the layers of calcareous ooze to form fossils (1).
Terror of the Ordivician sea (1).
Four foot long Cephalopod found in the Platteville (1).
Some large cephalopods can be found in the Platteville, including some that are as large as 4.5 meters long and 25 cm in diameter (1). Geologists have found cephalopods 10 “ in diameter sunk into the ooze to the depth of two-thirds of their shell diameters. They found the exposed parts of the cephalopods planed off at the top of the ooze layer. This meant that exposed parts had dissolved in the carbonate sea before the deposition of next layer of ooze bedding and that long intervals must have occurred between beddings as layer after layer of ooze piled up (2). When you look at the limestone, you will see horizontal lines. These lines represent the bedding planes (or layers of calcareous ooze), which vary in thickness. Thin bedding planes indicate periods of less deposition and thicker planes indicate periods of rapid deposition. Cracks in the limestone that are parallel with the ground are called cleavage, and run along the bedding planes. Cracks that run perpendicular to the ground are called fracture (4).
To claim credit for this EarthCache, email us the answers to the following:
1. Estimate the height of the exposed limestone to determine the approximate Platteville accumulation at this location. Take an elevation measurement at the bottom of the limestone formation, and subtract that from another elevation reading at the rim of the valley, by the East River Parkway.
2. Examine the limestone. Which visible cracks are the longest? Those that run parallel to the ground or those that are perpendicular to the ground? What are these cracks called?
3. Look around you. What evidence do you see that would support the theory that the Platteville Formation was formed in a shallow marine bank environment?
1. Ojakangas, R. W., & Matsch, C. L. (1982). Minnesota’s geology. Minneapolis: University of Minnesota Press.
2. Webers, Gerald F. (1972). Paleoecology of the Cambrian and Ordovician strata of Minnesota. In Sims, P. K., & Morey, G. B. (Eds.), Geology of Minnesota: A centennial volume. St. Paul, MN: Minnesota Geological Survey. 474-484.
3. Mossler, J. H. (1985). Sedimentology of the Middle Ordovician Platteville Formation Southeastern Minnesota. In Sloan, R. E. (Eds.), Report of investigations (Minnesota Geological Survey). St. Paul, MN: University of Minnesota. 33.
4. 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.