Glacial History of the Rush River County Park

Back on October 28^th, 167 8^th 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 8^th 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.

Some Examples of Weathering

Weathering can be defined as the gradual breakdown of rock materials.  It primarily results from the physical breakdown of rock material (mechanical weathering) or via the chemical breakdown of rock through chemical reactions (chemical weathering).

A nice example of mechanical weathering (especially pertinent for places like Minnesota) is through an action called ice wedging.  As liquid water flows into the cracks of rock materials and freezes during periods of low temperatures, the frozen water expands, widening the crack.  This action can reduce very large boulders to much smaller remants as shown in the pictures below.

These large granite boulders are found just outside Pipestone National Monument near Pipestone, Minnesota.  Granite is not native to the area and would have been deposited there after transport by glaciers.  The boulders are called the 'three maidens', at one point in time there would have been just three boulders of granite, but the repeated freezing and thawing of water have split the boulders into many pieces.  Largely because of how out of the ordinary granite is to the area, a Native American legend grew out of these large pieces of granite.  Native Americans believed that the granite boulders held the spirits of three maidens who required offerings before the quarrying nearby of catlinite (or pipestone) in what is now the National Monument.

Another form of mechanical weathering is abrasion, which is the grinding and wearing away of material through the action of wind or water.  The photographs below show great examples of abrasion at Iona's Beach, a Scientific and Natural Area maintained by the Minnesota Department of Resources along the Lake Superior shore.  On the north end of the beach a large rhyolite flow is found.  Waves break this rhyolite flow down and largely through wave action, these smaller pieces of rhyolite are rounded and smoothed before eventually being deposited on the beach.

Chemical weathering is the breakdown of rock material through a chemical reaction.  This occurs largely through weak acids that are found naturally in our rain or snow and through the oxidation of other materials.  The picture below (taken in Summit Cemetery, Waukesha County, Wisconsin) is a nice example of chemical weathering, over the last 150 years the rock has been exposed to a large portion of natural acids through precipitation.  A closer look at the headstone proves that the original carving into the stone has become much more difficult to read.

Another form of weathering is called differential weathering, which refers to how different rock materials weather (or breakdown chemically or mechanically) at different rates.  The two photographs below show a nice example of differential weathering, the pink feldspar crystals weather more slowly, and as such, seem to stand out from the rest of the granite.

Another very nice example of differential weathering is Devil's Tower National Monument in Wyoming.  The land area around Devil's Tower is comprised of sedimentary rocks, which weather (and then are eroded or transported away) at a much faster rate than the igneous rocks that comprise the monument.  The igneous rocks are much more resistant to weathering than the sedimentary rocks.  Devil's Tower formed as an intrusion of igneous material that, after the surrounding sedimentary rocks weathered and eroded away, was left standing over 1,200 feet above the immediate area.

In our classroom, students recently examined some examples of both mechanical and chemical weathering.  We used different rock types (limestone, rhyolite, basalt, sandstone, marble, gabbro) in our weathering lab.  Students placed these different rock types in weak solutions of carbonic acid to determine the effects of weathering.  The next class period, mechanical weathering through the process of abrasion was explored before comparing both activities.

Working with Stream Tables

These last few weeks our 8th grade students have been working with a stream table designed to simulate and teach basic river principles, including: how river channels form and change over time and how sediment is transported and deposited within river systems.

The stream table was built with an old wood household door that was no longer being used as the base.  It has the dimensions of 1.91 meters long by .85 meters wide.  There are numerous coats of silicon to prevent the leaking of water, these coats are especially thick near joints (after three years of use, there haven't been any leaks yet).  At any given time there is also 25-30 gallons of water being circulated throughout the system by a submersible pond pump.

The modeling media inside the stream table is manufactered thermoset plastic from Composition Materials Company (http://www.compomat.com/) in Milford, CT.  It is sold by them as 'Stream Table Mix' and consists of various sizes and densities that do an exceptional job of modeling on sediment is transported and deposited in natural river systems.  We use anywhere between 50-80 pounds of plastic within the stream table for student use.

The idea of using a large stream table came from seeing an example created by the folks at Little River Research & Design (http://www.emriver.com/).