Minnesota River Valley Bedrock

MN DNR Minerals Education Workshop 2005

Field Trip guide by Dr. Dean Moosavi and Rick Ruhanen 

The Minnesota River Valley - much too large for the size of the river today. The Minnesota River flows inside a valley carved about 10,000 years ago from the melt water of the last glacier. As the glacier melted a very large lake was formed, now called Glacial Lake Agasiz. The Red River of the North flows north from Browns Valley to drain what was this former lake bed. During the time of the glaciers the river flowing north was blocked by the glacier so the Lake went over the boundary between the Red River and the Minnesota River at Browns Valley MN to drain much of this glacial lake forming what is now the Red River Valley. The large, fast flowing river eroded the glacial till down to the bedrock that is Morton Gneiss between Granite Falls and Morton MN.

The gravestone below shows some of the prized features typical of the gneiss found near Morton and Redwood falls. The beautiful swirls in the rock are the result of an igneous rock (granite) being metamorphosed by heat and pressure. Two periods of metamorphisis are believed to cause the gneiss. The first evidence is a period ending about 2650 million years ago called the Algoma orogeny. Partial melting is strongly suggested by flow features in the fabric of the rock. A second less intense period occured about 1800 million years ago at the time of the Penokean orogeny. Diabase dikes typical of the Penokean orogeny are found in the region and are the source of the date of this event.

The potholes cut into this very hard rock are evidence of very fast moving water. These potholes are very much like those at Interstate State Park near Taylors Falls in the St. Croix River. These potholes while small in comparison to those at the St. Croix are only possible with fast moving and swirling water. The potholes far above the current river show that an older river made great changes in this landscape. The potholes here, while old from our perspective are very young compared to the rock into which they are cut.
Morton Gneiss rock formations near Morton and Granite falls have large areas with very little soil. This means that many times in the summer the rate of water evaporation exceeds the rate of precipation. This is the technical definition of a desert and like the desert areas of the Southwest United States this area has cacti. These cacti are small and rare and are testiments to adaptations that plants have made to survive in otherwise hostile locations.



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