Sedimentary, Dear Watson EarthCache
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In order to count this Earthcache as a find, you must complete the following tasks and email the answers to me.
1. What is the elevation?
2. Over time, what continues to happen to the sandstone layer? What effect does this have on the falls?
3. What was the Platteville limestone used for?
4. How are the layers of sediment visualized in this park? (Hint: turn your back to the sign and look straight ahead)
5. What is the purpose of the man-made aprons?
This earthcache is located at Mill Ruins park in downtown Minneapolis. History of flour milling in the area can be learned from several different signs throughout the park. In addition, the ruins of several abandoned flour mills can also be seen in the park. Vistas of the Mississippi River and Saint Anthony Falls can be had here. Enjoy!
The Minneapolis-St. Paul area sits atop of three layers of sediment. At the bottom, St. Peter’s Sandstone is the thickest at 155 feet. Occupying the middle layer is Glenwood shale at sixteen feet thick. Platteville limestone is the top layer and is about thirty-five feet thick. These layers formed about 500 million years ago when an inland sea covered the area you in which you are now standing. The St. Peter’s Sandstone began as a sheet of sand in clear, shallow water. It is likely that the sandstone in the Minneapolis area originated from the sandy shore of the inland sea. During the time of its existence, the inland sea was filled with shell fish. When these organisms died, their skeletons drifted slowly downward--along with their calcium shells. This resulted in a slow, soft, invisible rain of tiny flecks of calcium carbonate that very, very slowly built up at the bottom of this sea. These fossilized shell fish created the hard limestone cap.
Fast forward to about 20,000 years ago when the inland sea no longer existed; instead, the Twin Cities area was covered by a massive glacier from the Laurentide ice sheet. After the Laurentide ice sheet retreated, the area was covered by a lobe (tongue-like extensions of the main ice sheet) of the Des Moines glacier. Beneath these ice sheet, tunnel valleys were created by water erosion. These valleys, which served as drainage for the melt water, cut through the limestone layer with significant force. This resulted in a series of channels in the limestone, which were filled by glacial till and outwash deposit when the glaciers receded. This has made the landscape what it is today.
Jump to 10,000 years ago--the time geologists hypothesize Saint Anthony Falls first appeared. These falls expose the three layers of bedrock. The difficult-to-erode Platteville limestone forms the cap of Saint Anthony Falls. The falls themselves have continued to move further upriver due to the erosion of the bedrock it sits atop.
NOT A LOGGING REQUIREMENT: Feel free to post pictures of your group at the area or the area itself - I love looking at the pictures.
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