Back on October 28th, 167 8th grade students from Delano Middle School visited two sites in the Minnesota River Valley and the Rush River County Park with a long-term goal of discovering the basic geologic history of the area. At this point of our year, we are most interested in the recent glacial history of the Rush River area.
The Rush River is located entirely within Sibley County, though its watershed includes a portion of two surrounding counties. The Rush River flows for a distance of 20 miles with an overall change in elevation close to 259 feet. The calculated gradient of the Rush River is then 12.95 feet per mile.
Within the Rush River valley there are numerous examples of large, rocky point bars that are comprised of nonnative rocks including, basalt, rhyolite, granite, shale, limestone and a few Lake Superior agates. In many or most cases, these rocks have been deposited hundreds of miles of their original location. Students on this day spent a large amount of time identifying these different types of rocks and discovering the source areas for these rocks within the region.
The source of the rocks that make up the rocky point bars are found within the river valley. Glacial till is being continually being eroded from the valley walls. The picture below shows a prime location of this erosion. This particular location is comprised of at least three distinct till layers, each with a different source location. The top two layers contain tills from the late Wisconsin glaciation.
The uppermost layer has its source in what is called Riding Mountain provenance and is commonly called the Des Moines lobe. Till or sediment deposits from the Des Moines lobe are at or near the surface for a large portion of the state of Minnesota. The color of the till is commonly buff or a yellowish brown. A distinctive characteristic of the till is the presence of a large amount of Cretaceous Shale, the gray Pierre Shale. Carbonate rocks, like limestone, are also found commonly within this till layer.
The middle till layer seen in the picture above is derived from sediments from the Superior provenance and is commonly called the Superior Lobe. Till from the Superior lobe is much redder in color and tends to contain more clay material. Rock types present within the till are indicative of the source area, a large grouping of crystalline rocks including basalt, rhyolite, granite and gabbro and some sedimentary rocks including red sandstone and limestone. Also found within this layer of till and occasionally on the point bars at the Rush River are Lake Superior Agates.
The lowest layer of till on the picture above (very near the surface of the river) was deposited before the late Wisconsin glaciation and is often referred to as the old, gray till. This till layer was not used in class and/or referred to often.
Students in our 8th grade Earth Science classroom have recently been completing lab work on identifying general characteristics (texture and lithological) of four known Minnesota glacial tills (Superior, Wadena, Rainy and Des Moines lobe) from the late Wisconsin glaciation. When students have identified characteristics of these four known glacial tills, they use this information to identify the source of two unknown glacial tills from the Rush River County Park in Sibley County, Minnesota. The two unknown tills represent the upper and middle till layers described above or the Des Moines and Superior lobes.
That the Superior lobe advanced on what is now the Rush River County Park first and was followed by the Des Moines lobe is just part of the geologic history of the area. To complete the story, the relatively high gradient of the river, at least for rivers in the area, needs to be explained thoroughly during a future post on Minnesota’s glacial history. For a quick (and non-illustrated) version, near the end of the late Wisconsin glaciation, an immense lake called Glacial Lake Agassiz formed from meltwa
ter. This lake catastrophically discharged forming what is called Glacial River Warren that carved a valley (now occupied by the Minnesota River) across Minnesota several kilometers wide and at least 100 meters deep. This large valley created ‘knick points’ which resulted in large changes in river/stream channel slopes. Since the incision of the valley by Glacial River Warren, rivers and streams have been eroding to the base level of the new valley floor in an attempt to level this steep slope. Since Glacial River Warren carved a valley with steep sides, rivers (including the Rush Rivers) flowing into this valley have higher gradients that also increases their erosional energy.