Minnesota Geology Pictures - 800 Million Year Old Unconformity

In the town of Gilbert, Minnesota is an outcrop of 2,700 million year old greenstone.  Lava initially erupted into an ocean was basaltic, after undergoing a weak metamorphosism involving higher pressures and temperatures.  Plagioclase and pyroxene were changed to green minerals like chlorite and epidote.  Deposited on top of the greenstone is the 1,900 million year old Biwabik Iron Formation partially consisting of red hematic chert.  The difference between the two units are 800 million years of time.

Glaciers originating from the Rainy Provenance during Late Wisconsin glaciation removed most of the Biwabik Iron Formation from this location.  While these glaciers came through the area they also left numerous glacial striations seen in these two pictures as well.

Minnesota Geology Monday - Magnetic Rock

Near the end of the Gunflint Trail, in northeastern Minnesota is a hiking trail that leads to Magnetic Rock.  The trail is approximately one mile in length and mostly uphill (at least on the hike in).  In the picture below, you can make out a small portion of the Gunflint Trail near the trailhead, the picture was taken about 1/4 mile into the hike.

Due to a forest fire in 2007, there are relatively few mature trees or vegetation along the trail.  This fire also did an excellent job of exposing bedrock outcrops throughout the area.  When looking north at several points along the trail, you see numerous exposures of the white, granitic Saganaga Tonalite.

The majority of the trail is on top of the 1,878 million year old Gunflint Iron Formation, which would have been deposited near the shoreline of an ancient sea.  The Gunflint Iron Formation consists of black to gray chert alternating with black iron-rich layers.  A goal this summer is to also acquire better photographs of the Gunflint Iron Formation, in particular the bedding planes, which is largely horizontal in the area.

Along the trail to Magnetic Rock, you are also able to view stromatolites within the Gunflint Iron Formation.  Though the day I traveled to the area a storm was coming in and I needed to complete the hike and set up camp, so I didn't have time to look for the stromatolites.  Maybe that can happen this summer.

Magnetic Rock is a large piece of the iron formation that has been turned vertical, probably by the southwestern trending Rainy lobe during the late Wisconsin glaciation.  Magnetic Rock is almost 25 feet from ground level to the top.  Due to it's height, the rock is visible for some distance along the trail before you actually reach it.

The bedding planes of the Gunflint Iron Formation is largely horizontal in the near vicinity of Magnetic Rock.  The bedding plane of Magnetic Rock is vertical, indicating that it has been moved from it's initial location.

Minnesota's Glacial Story (briefly)

Here's a brief (five minutes or so) narration and/or video diagraming major events in Minnesota's most recent glaciation that students may (or may not) use in preparation for their coming test.

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.

Geologic History of the MN River Valley - Part 1

On October 28, Delano Middle School 8th Grade students traveled to the Henderson area within the Minnesota River Valley for our annual field investigation.  Our goal that day is to acquire the observations and evidences necessary to determine the geologic history of the Minnesota River Valley and the adjacent Rush River County Park.  This post will be the first post of a series that examines and identifies the regions history.  This post will focus on what the students saw, what they did and what was recorded in their student lab journals.

Our first stop of the day was the bridge that spans the Minnesota River at Blakeley, Minnesota.  The day that we visited, the river was at 714.15 feet, making it the fourth lowest level recorded for the Minnesota River at this location.  Levels lower that this are 701.00 ft. on 1/1/1950, 713.29 ft. on 11/16/2001 and 713.36 ft. on 10/11/2000.  You can compare this to the all-time high crests of 740.08 ft. on 9/28/2010 and 739.65 ft. on 4/11/1965.  Source:  http://water.weather.gov/ahps2/hydrograph.php?wfo=mpx&gage=henm5&view=1,1,1,1,1,1,1,1. 

While on the bridge, students attempt to gauge the scale of the valley.  How deep is the valley?  How wide?  At this location, the valley is almost two miles wide and 250 feet deep.  In some locations, the valley is almost five miles wide and the same depth.

Students have been asked to always question what took place to form what they are seeing.  Very near where students get back on the buses, they are able to see a road sign that indicates past high water marks of the river.  It becomes clear to students that the river floods periodically to very high levels, at this point, many students begin to assume that erosion by the flooded river would be enough to form the valley.  Pictures of the river in flood were taken in the spring of 2009 and 2010.

The second stop of the day is in the town of Henderson, still inside the Minnesota River Valley, at the flood gates that the city is occasionally forced to close.  When the Minnesota River reaches 734 feet, the town closes itself off to protect the lives and property from the river.  While at this stop, students again realize that the river floods to very high levels.  When trying to determine what caused the valley to form, many students see the flooding river as a strong piece of evidence.

From Henderson, our third stop is at the Rush River County Park, a short drive away.  The Rush River is a small tributary to the Minnesota River, it flows primarily through farmland and it's watershed occupies parts of Sibley, Nicollet and McLeod counties.  While at the park, students do a variety of activities designed to give them real world experience with several of our units.  These units include Rocks, Water Systems, Erosion, among others.  The majority of our time in the park is spent near the river on a point bar, an area of deposition where the river deposits all types of sediment.

As students walk the point bar, one immediate observation made is that larger sediments are located upstream, while farther downstream the sediments get smaller.  The picture on the top is indicative of upstream regions of the point bar, the picture on the bottom demonstrates downstream regions. 

While walking the point bar, students begin to identify some of the rock types found.  Students identify many igneous rocks like basalt, rhyolite, granite, gabbro and sedimentary rocks like limestone, sandstone and shale.  We typically find several agates and occasionally find petrified wood.  These rock types are not native to the region.  Bedrock at this location is sedimentary in nature, though the bedrock is covered by glacial drift.  Students are asked to brainstorm how these non-native rock types could have been deposited at this location.  Students generally generate a list that includes:  water, wind, animals, and some think of the real depositional method of glaciers (we have not discussed glaciers at this point and will not for another month or so).

Our last activity at the Rush River is to notice and draw the cut face (an erosional feature) found directly across the river.  This particular feature is comprised of at least three different glacial advances and tills.  The uppermost till is representative of a Northwestern source area or the Riding Mountain Provenance.  The middle layer of till represents a Northeastern source area or the Superior Provenance.  The bottom most layer of till represents a much older glacial advance.

Back in the classroom at a later date, students will examine these different glacial tills that have been acquired from the Rush River County Park or immediate area.  Students begin to realize why they only find the volcanic rocks of the Mid-Continent rift system in the middle layer of till and why the gray, Pierre shale is only found in the uppermost layer of till in the region.  It's while examining these different glacial tills that students finally begin to realize the geologic story of the Rush River County Park and Minnesota River Valley.  They also begin to realize how valuable their observations of the area are in determining the history of the region.

In a later post, I'll go deeper into the glacial history of the area and how exactly the Minnesota River Valley formed.