Frontenac Geology EarthCache
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From this outcropping you can look out over the Mississippi River Valley and specifically Lake Pepin. At one point in time what you are standing on was an island in the Mississippi River.
The Mississippi River, like all rivers, is in a constant state of change. Various forms of the Mississippi River have flowed through our area for more than a million years, but the Upper Mississippi River Valley as we know it was primarily shaped during the most recent glacial stage of the Great Ice Age, The Wisconsin period.
The melting of those enormous ice sheets that, at their maximum, were 5,000 to 10,000 feet thick and covered hundreds of thousands of square miles, released tremendous amounts of water, forming huge glacial lakes. The largest of the glacial lakes, Lake Agassiz, covered northwest Minnesota, parts of North Dakota and the Canadian provinces of Manitoba, Saskatchewan and Ontario. The southern discharge outlet to this lake was called Glacial River Warren, which eventually excavated the valley now occupied by the Minnesota River. (The Mississippi River flows into the valley carved by the River Warren at St. Anthony Falls in Minneapolis.)
The St. Croix River which drained Lake Duluth, a glacial lake that covered the western Lake Superior basin, joined the River Warren about 30 miles downstream from the present confluence of the Mississippi and Minnesota Rivers. In Wisconsin, another glacial lake drained first into what is now the Black River and later the Wisconsin River, both of which emptied into the River Warren. During the 3,000 years that River Warren carried water from these and other smaller glacial lakes, the Mississippi River valley was carved bluff to bluff and the resulting valley was approximately 250 feet deeper than it is today.
However, there is also evidence that the main valley is much older than the River Warren and underwent several cycles of cutting and filling during the Great Ice Age. Even though the Coulee country escaped the land-leveling effect of the ice mass, the glaciers left their mark by carving the spectacular Mississippi River that cuts through the heart of this area with the torrents of water draining from the melting ice mass.
With the demise of the glacial lakes, the tremendous flow of water was diminished and the Mississippi River became the most important stream in the area. The many tributaries continued to bring in sand and gravel (sediment) into the wide deep valley of the Mississippi that the river could no longer carry away. Consequently, the sediment began to fill in the recently scoured river valley. At each point where major tributaries joined the Mississippi River, natural sediment dams eventually built up to pond the upstream portion of the river, forming a natural river lake. The largest is Lake Pepin which at one time extended all the way up to St. Paul.
To gain credit for this Earth Cache please email me the answers to the following questions:
1) What evidence do you see of this spot being an island.
2) What are the three rivers that are dumping sediment into the river and creating Lake Pepin.
3) Take an elevation reading here and at the base of the hill by the park entrance. What is the height of this bluff?
4) Add 250 feet to your answer in #3 and figure out how much erosion happened each year that the River Warren flowed through this Valley.
Additional Hints
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