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/). 

The Carver Rapids on the Minnesota River

On Thanksgiving, we heard some reports that water levels on the Minnesota River were low enough to expose the Carver Rapids, near the city of Carver.  Both the cities of Carver and Chaska, probably are located where they are because of the rapids.  Early in the history of Minnesota, travel was done via steamboat and steamboats were unable to move upstream past the rapids.  I have been unable to find a source that describes how often the river level is low enough for the rapids to be exposed, but it cannot be too often. According to the Minnesota Department of Natural Resources October 2011 Monthly Hydrological Report, late summer throught autumn 2011 precipitation totals rank among the lowest on record.  This Google Earth image, taken in March of 2005, shows the rapids and their effect on river ice.

Knowing that the forecast for Friday after Thanksgiving was unseasonable warm temperatures in the low 50s, it was time to view the rapids in person.  Just upstream of the rapids, the river guage located in Jordan indicated that the river was currently at 5.37 ft, (source http://water.weather.gov/ahps2/hydrograph.php?wfo=mpx&gage=jdnm5&view=1,1,1,1,1,1,1,1).  So the prospects seems good.  In 1920, Warren Upham said of the rapids:

At Little Rapids of the Minnesota River, adjoining the southeast quarter of section 31, Carver, a ledge of the Jordan sandstone running across the river bed causes a fall of two feet; and again about a quarter of a mile up the river its bed is similarly crossed by this sandstone, having there a fall of slightly more than one foot. In the stage of low water, these very slight falls prevent the passage of boats; but at a fuller stage the river wholly covers the ledges, with no perceptible rapid descent, being then freely navigable.

(source:  http://www.chaskaherald.com/view/full_story/13428744/article-The-reason-we-are-all-gathered-here?instance=article_results)

Being as prepared as possible, we were off.  Before actually getting too far, we suddenly realized that we would not be the only people in the woods this day.  Apparently, the Louisville Swamp (which is part of the Minnesota Valley Wildlife Refuge) is open to Archery-only deer season as well as small game.  Judging by the vehicles in the parking lot, others chose to enjoy this day outside as well, only we were not wearing blaze orange.  This directly affected our route to the rapids, we were now looking at a minimum of a three mile hike, one way, as shown below.

After about a mile and half hike, we crossed Sand Creek, which is a tributary into the Minnesota River.  The water level in the creek was very low.

What was interesting crossing the creek though, was the indicators of last springs (or even last fall's) high water marks.  The fourth highest Minnesota River mark at Jordan happened during the fall of 2010 and there was another flood event in the spring of 2011.

While crossing Sand Creek, we also noticed several beaver slides, where beavers slide down the slope into the creek bed.  Beavers at this location were apparently attempting to damn the creek as evidenced by the picture below.

After crossing the creek, our next stop was the Minnesota River and the rapids made of the Jordan Sandstone.  The lower rapids were seen first, the sandstone was not exposed at this location and the water was flowing over.

 After the brief stop at the lower rapids, we quickly moved upstream to the upper rapids where we hoped they would not be cover by water.  We needed to navigate a 10 foot descent down a slippery slope to be able to walk out on the rapids made of the Jordan Sandstone.  For a sense of scale, one of the below pictures has my 6' 5" brother standing on the rapids.

Besides seeing how the flowing Minnesota River has eroded and weathered the sandstone into some unique shapes, we also saw a very large number of Mussel shells in amongst the rapids.  Due to the large number of tracks, it was readily apparent the raccoons are also being drawn to the rapids with the low water levels.  They have been enjoying quite a feast at this location.

Knowing we had a little over an hour of daylight remaining and a long hike in front of us, it was time to leave.  Throughout the day, we saw evidence of numerous different animals.  Beaver and raccon have been mentioned earlier, but we also saw eagles, hawks, cardinals and other small birds, small rodents and finally lots of different tracks.  Tracks we found along the trail were of horses (it was a horse trail afterall), numerous deer, some dogs, and the track shown in the picture below.  If anyone knows what this could be, I'd be interested in hearing your opinions or knowledge, I have my ideas.  We saw numerous examples of this track and this isn't the largest example that we saw.

Minnesota Continental Divide Student Map

The state of Minnesota has four major continental divides which are shown on all Minnesota roadmap.  Because of these divides, Minnesota shares it's water with a large part of the North American continent as water flows downhill, based largely on water flowing away from these divides.

Students recently were required to map the location of these divides and other surface water features found within the watershed.  Some of the surface waters students needed to include on their map include major lakes (Mille Lacs, Upper/Lower Red, Winni, Superior, etc) and major rivers (Mississippi, Minnesota, St. Croix, Red, etc.).  An example of a student completed map is posted for reference.

Minnesota's Continental Divides

Water is one of Minnesota's most important natural resources.  Continental divides force water to that falls on one side of them to flow one direction, while water that falls on the other side flows the other direction due to gravity.  The majority of water that falls on Minnesota drains through a series of rivers to the Mississippi River and ultimately, the Gulf of Mexico.

In the southwestern portion of the state, another high area or divide, forces water that falls in that area to drain to the southwest towards the Missouri River.  Of course, the Missouri River ultimately flows into the Mississippi River and then the Gulf of Mexico.  Of course, this particular picture was taken in Minnesota, though it's a continuation of the same divide.  This picture was taken near Carroll, Iowa.

Across the northern portion of the state runs the Laurentian Divide.  Water the falls to the north of this divide flows to the north towards Hudson Bay, in Canada's far north.  Water falling to south of the Laurentian Divide flows either to the Mississippi River or towards Lake Superior.  Water that flows to Lake Superior ultimately flows through the Great Lakes to the Atlantic Ocean.

In places where two continental divides meet you can find triple divide points or triple points.  On of these three way continental divides occurs in Minnesota near Hibbing.  The exact triple divide point is located inside the private property of a taconite mine, so it is not accessible to the public.  Early Native Americans living in the area called this spot "the hill of three waters."

Looking at the picture below (source:  http://commons.wikimedia.org/wiki/File%3ANorthAmerica-WaterDivides.png) you can find several other triple divide points.  One of interest would be the one located at Triple Divide Peak in Glacier National Park.  Water falling on this location flows three directions; west to the Pacific Ocean, north to the Arctic Ocean, and east to the Atlantic Ocean.  This is apparently the only location on the planet where water flows into three separate oceans.

Most might find the pictures of continental divides as rather boring, especially when you compare the Minnesota pictures to the picture of the continental divide located at Loveland Pass in Colorado.  Loveland Pass is located on the Great Divide, this divide separates water that flows in the Pacific Ocean from that that flows in the Atlantic Ocean.  The Great Divide runs across a series of mountain ranges from North America all the way to the Andes in South America.

Our 8th grade students have been using Minnesota roadmaps this week to map out locations of continental divides located in our state.  Using this information, they then map major rivers and lakes and determine how waters flow throughout our state.

Help with Calculating the Mississippi River's Gradient

Take a look at the attached files and use these as a guide or homework check for you assignment that is due soon.

The gradient of a river measures the change in elevation over a certain distance.  All rivers flow downhill due to the force of gravity.  To calculate gradient, you must find the change in elevation between two locations and divide that by the change in distance.
Since we measure elevation in feet and distance in miles, the units for gradient will be feet per mile (ft/mile).

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.