The Prairie du Chien Escarpment is present in many areas in western Wisconsin. An escarpment is a steep slope or long cliff that forms as an effect of erosion or faulting and separates two relatively flat areas of different elevations. The Prairie du Chien Escarpment is formed by the differential erosion of the softer Cambrian Sandstone and the harder Prairie du Chien Dolostone which lies above the sandstone.
On the map above, the Prairie du Chien Dolostone in western Wisconsin is colored and present in southern Polk County, most of St. Croix County and Pierce County. The eastern edge of the Prairie du Chien Dolostone at the border between Dunn and St. Croix Counties is where the Prairie du Chien Escarpment can easily be observed. In order to understand the formation of the escarpment, the characteristics of the Cambrian Sandstone in Dunn County and the Prairie du Chien Dolostone in St. Croix County must be studied.
The Cambrian Sandstone is the bedrock found in most of Dunn County and about 25% of Wisconsin. Bedrock is the hard solid consolidated rock under the loose soil. The bedrock in an area can be observed along road cuts, in stream and river valleys and at escarpments. The most famous place to view Cambrian Sandstone is Wisconsin Dells where flood waters from ancient Glacial Lake Wisconsin carved the sandstone into many beautiful sandstone formations.
The Cambrian Sandstone found in Wisconsin represents a time in geologic history when shallows seas were beginning to cover the land and deposit beach sands. This deposition occurred for about 55 million years from about 540 – 485 million years ago. In Dunn County approximately 400 feet of beach sands were deposited, Since the beach environment did not protect the hard body parts of the prehistoric animals, not very many fossils are found in the Cambrian Sandstones.
The Cambrian Sandstone in Dunn County is honey-colored and composed of almost perfect spherical shape sand particles held together by cements like iron oxide (rust) and calcite. The sandstone weathers (breaks apart) fairly quickly on the geologic scale and usually results in fairly flat and level landscapes with gentle hills. This type of land can be seen throughout Dunn County and lends itself to large agricultural fields.
The Prairie du Chien Dolostone is bedrock found in much of St. Croix & Pierce Counties and is composed of three layers called the Oneote Dolomite, the New Richmond Sandstone, and the Shakopee Formation. It respresents a time when invading seas got deeper and more marine life existed in the environment. When the marine organisms died, their shells and outer coverings fell to the sea floor and accumulated for millions of years. The weight of overlying layers compressed the remains into calcite (limestone) which was later altered into dolomite (dolostone) by groundwater. These remains acculumlated for about 45 million years from about 485 – 440 million years ago and formed about 250 - 300 foot thick layer. Since this gentler marine environment did not destroy the hard body parts, many fossils are found in the Prairie du Chien Group.
The Prairie du Chien Dolostone is a harder rock composed of calcium magnesium carbonate and does not weather (break apart) very easily and hence forms an escarpment, a steep slope or long cliff. This earthcache will start you on the fairly flat Cambrian Sandstone and then have you drive up the escarpment to the fairly flat top of the Prairie du Chien Dolostone. It will also have you measure the height of the escarpment which in turn will give the thickness of the Prairie du Chien Group. You will also be able to observe and describe an outcrop of the Oneota Dolomite, the lowest layer of the Prairie du Chien Group.
The earthcache will have you stop first at the lower level of the Prairie du Chien Group where you will observe some exposed Oneota Dolomite of the Prairie du Chien Dolostone and record the elevation. To get to the second waypoint you will drive about 1 mile up the escarpment and arrive on top of the Prairie du Chien Group. Here you will record the elevation and calculate the thickness of the dolostone.
To claim credit for this earthcache you must complete the following tasks and email the answers to the cache owner using the profile link.
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Find the elevation at GZ (Waypoint 1), the bottom of the Prairie du Chien Group.
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Find the elevation at Waypoint 2, the top of the Prairie du Chien Group.
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Find the thickness of the Prairie du Chien Group (Waypoint 2 – Waypoint 1 = thickness)
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Compare this thickness to the thickness given in the earthcache write-up.
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Describe the following characteristics of the Oneota Dolomite at Waypoint 1 (shape, bed thickness, position, color, texture, etc.)
Optional: You may include a photo of you at Waypoint 1 with the nearby road sign in the background as a substitute to Requirement #5