Geology 310

                                                                                                Professor Moosavi


Minnesota River Valley – Morton/Jeffers Petroglyph Field Trip




On this field trip we will the opportunity to examine both ancient and modern geologic processes crucial to determining the character of inland continental realms such as Minnesota. Specifically, we will examine remnants of the Archean age accretion of continental crust contained within the crystalline basement rocks of North America.  We will see how these ancient rocks continue to play a role in determining the structure of the landscape today by how they affect weathering and erosion. Ancient and modern day depositional and erosional processes found in river valleys will also be examined in the Minnesota River Valley and at Jeffers Petroglyphs.  The biological and anthropological affects of these rock outcrops will also be examined.


The BIG QUESTIONS! (pertaining to this trip)


What was Minnesota like, long, long ago?


What determines the ecosystem found in a place?


How will the land change in the future?


What role do humans play in that future?


EN Route: Middle Minnesota River Valley


Take US169 South from Mankato to MN 68 West toward New Ulm. Take Nicollet County Road 24 North toward Courtland.



Why is the Minnesota River in a trough?


What keeps the Minnesota River from eroding deeper into its bed?


Was is the bed of the river composed of in this section?


What are the walls of the valley composed off?


Stop 1: Courtland Boat Landing



What is the river doing in this stretch?


Can you identify any point bars or cut banks?


Is there a levee visible?


What is the small pond on the south bank and how did it get there?


Where does erosion appear to be occurring?


Where does deposition appear to be occurring?


In case of a flood, how would the distribution of erosion and deposition change?


What is forms the substrate/basement of the river in this section?



En Route: Continue examining the structure of the Minnesota River Valley.


Stop 2: Meander Loop


Take Nicollet County 24 to US Highway 14.  Follow this west toward New Ulm ,but continue west on Nicollet County 21 rather than crossing the river into New Ulm. Get off at the boat landing where Nicollet County Road 21 meets Nicollet County Road 13.



What is the river doing in this stretch?


Can you identify any point bars or cut banks?


Is there a levee visible?


Where does erosion appear to be occurring?


Where does deposition appear to be occurring?


Stop 3: Harkin Store


Continue west on Nicollet County 21 past the Harkin store. Stop by the small creek.


What is the nature of the small creek found here?


What about the wetlands?


Would this area be good for farming if properly tiled?  Why or why not?


Why is the Harkin store placed where it is?



Stop 4: End of Pavement


Continue west on Nicollet County 21 until it becomes Renville County 5.  Stop just past the end of the pavement and look back down the valley.





Stop 5: Renville County Park 1


Continue west on Renville County 5 until the access road to the county park appears on the left. Enter the access road and park at the cabin. 


·      Examine the floodplain you have entered.

·      Describe the nature of the substrates, level of the water table, presence or absence of creeks, type of vegetation, etc.  Look for floodplain valley features. Explore along the roadside both behind and ahead of the van.

·      Specifically examine the rock exposed next to the cabin.

·      Describe its structure and attempt to identify it.


What kind of rock is this and what does it indicate about this region in the past?


Why does one not see this type of rock in most of southern Minnesota?


Based on your answer, how old would you say this rock must be, relatively speaking?


In what environment must this rock have formed?


What role, if any, does this rock play in the current Minnesota River Valley’s structure?


How is the forest adapted to this environment’s unique hazards/benefits?


Down at the riverside, what potential problems do the restroom facilities pose in this enrivonment?


·      Brief Discussion.




Stop 6: Renville County Park 2


Continue along Renville County 5 until the second park appears on the left. Drive into the picnic area and park.


·      Spend some time walking around the hill at the center of this park.

·      Examine the pink and black rocks found here.

·      Attempt to identify the rocks.


What kind of rock is the pink rock and is it native or imported? If imported how did it arrive here?


What kind of rock is the black rock and it is native or imported? If imported how did it arrive here?


What is the structure along the river bank and how did it form?


·      Brief Discussion.





Stop 7: Valley Wall Outcrop


Continue along Renville County 5 until it becomes Renville County Road 51. Stop at the outcrop on the right about 1 mile past the intersection.


·      Describe this rock outcrop.


What kind of rock is it and how did it form?


Stop 8: Morton Cemetery


Continue on Renville County 51 until it joins MN 19. Take this through the town of Morton. Note the mine tailings at the edge of town. Process through town and turn right on US 71 toward Olivia.  Proceed up the hill ~1 mile and exit on the right to enter the Morton cemetery. 


·      Look over the ridgeline at the shape of the Minnesota River Valley and the Morton outcrop within it.


What would this feature have been at a time of higher water flow?


What does it suggest about the reasons for the valley’s depth to be at the current elevation?


·      Examine some of the gravestones in the cemetery.


What type of rock do these appear to be made of?


Stop 9: Morton Mall Site


Drive back down US 71 into Morton.  Turn left on Century Drive and proceed to the Morton Mall (old school) and park.  Walk out onto the outcrop behind the school.


·      Take a fair amount of time to examine this rock outcrop.


·      Try to determine the type of rock exposed here.


What does this indicate about the environment in which this rock was formed?


What must have occurred for us to be able to examine this rock so conveniently?


·      Look at the features found on the surface of the rock.


·      You should find 2 features that seem out of place on this erosional high.


·      Draw pictures of the rounded depressions.


How did these features form and where might you find similar examples forming today?


What do they indicate about this area in the recent past?


Why is there so little soil found on the rock outcrop?


·      Look at the ecology of the site.


What fundamental process appears to be occurring here?


Does the nature of the rock aid or inhibit this process?


What unusual plant (for MN) do you find in cracks in the rock?


What does indicate about the state of this environment?


·      As a class we will proceed to the first quarry. BE CAREFUL OF THE HOLES in this area.


·      Carefully examine the rocks in this area.  You will see some interesting boundaries in the rock.


What are these intrusions called?


What kind of rock are they and what do they indicate about this region in the past?


Stop 10: Ramsey Park, Redwood Falls


Take US 71 to Ramsey Park in Redwood Falls.

Stop at the overlook of the Redwood River by the golf course.


·      Describe the cliff you see here, particularly, the nature of the material from which it is made.


·      Drive down below the cliff and park. Examine the cliff from this side.


What do you believe the cliff is made of?


In what environment must this material have been created?


What is the river doing to the cliff?


·      Walk up the road to the road cut to the west.

·      Examine the deposits closely.


What is this material composed of?


What would you suspect its parent material is?


·      Given your answers, why does this material remain in this currently “incomplete” form? (Think of what we have said about chemistry.)

·      Proceed up the valley to the waterfall.

·      Examine the rock surrounding the waterfalls.


What does it appear to be made from?


What does it indicate about this environment in the past?






Consider the role of the tile systems we will pass on the landscape. Specifically:


How would this change the amount and timing of water movement through the soil? 


How might this affect the hydrograph, flooding and droughts on the Minnesota River? 


What would the tiles and interconnecting channellized streams and drainage ditches do to the level of soil erosion in such areas?

Stop 11: Jeffers Petroglyphs


Take US 71 south to Cottonwood County 45. Look for signs to Jeffers Petroglyphs.


Jeffers is a unique site for us because its geologic, ecologic and archeologic resources have been put into a museum format for us. The topics you should observe include:


Tall Grass Prairie Remnant                                         Rock Cycle

Role of Fire                                                                 Braided Stream Deposits

Prairie Ecological Processes                                        Effects of Metamorphism

Petroglyphs                                                                 Glacial Erosion

Ancient Animal Assemblages                                      Role of Buffalo

Plant Succession


            What kind of rock is exposed here?


            In what environment was it deposited and how do you know?


            How has the rock been altered since the time of its deposition?


            Would you expect to find fossils in this rock? Why or why not?


Why are some rocks scratched in long parallel lines?


How old is the rock beneath this part of Minnesota?


Where did the prairies come from & where did they go?


What keeps a prairie from becoming a forest?


Did ancient people leave evidence of their presence in Minnesota?


What plants and animals lived here in the past?


Has farming changed the life of the prairie lands?


Is wildfire good or bad for the land?


How do plants come to grow on rock?


What overall impression do you have of about the scale of geologic, biologic and archeologic processes resulting from this field trip?


·      Long Discussion.

Key to Jeffers Petroglyphs


1.   Down the Drain - Tile Systems

Much of the farmland in southern Minnesota uses under ground drainage pipes, tiles, connected to ditches to remove excess water from the soil.  This was believed to be necessary to create sufficient agricultural land because fully half of southern Minnesota was covered by wetlands prior to European settlement. Wetlands filter nutrients and pollutants out of runoff and act as storage basins for water during times of surplus and as water sources to local streams during times of drought.  They also have many benefits for wildlife, especially waterfowl migrating along the Great Plains fly-way. Consider the following questions:



2.   Ancient Past

At Jeffers Petroglyphs the metamorphic bedrock, which underlies the younger sandstones and glacial till usually observed crops out at the surface.  The Sioux Quartzite outcrop forms a long low ridge.  Originally, the rock was deposited as sand and mud in the beds of braided streams draining a mountain range to the north.  Evidence for this is contained within the quartzite as ripple marks and mud flats left behind from the original sands deposited 1.6 billion years ago.  After deposition these sands were compressed by their own weight into sandstone.  Later metamorphism during the Algoman Orogeny converted the sandstones to quartzite.  The pink/purple color arises from the hematite coating the individual sand grains.  The quartzite remained buried for millions of years until recent tectonic activity brought it nearer to the surface where erosion could expose the ancient beds.


3. Icy Past

The Sioux Quartzite, as a resistant layer rising slightly above the surrounding landscape, became an obstruction to the movement of glacial ice out of Canada during the Pleistocene ice age of the last 2 million years.  Unlike the depositional environment which covers most of southern Minnesota, this is an erosional landscape. Slow movement of the mile thick ice sheet over the quartzite allowed rock material entrained in the ice to scratch the rock below. The resulting glacial striations indicate the direction the ice sheet was moving.  Since 2 separate patterns of striations exist, we can conclude that the ice sheet advanced over this area in 2 separate waves from 2 separate directions (north and northwest) The most recent of the striations were scratched into the surface a mere 12,000 years ago.  Where else in Minnesota might you expect to see glacial striations?


4. The Prairie

Following the retreat of the glaciers at the end of the ice age, southern Minnesota was a barren landscape.  Plants had to recolonize the rock explosures and glacial till, eventually building up soil.  That succession continues today on the exposed face of the Sioux quartzite.  Lichens, a symbiotic organism combining fungi and algae are slowly covering the rock face.  As they help to build soil other plants with greater soil requirements can move in.  Over a few thousand years this process caused southern Minnesota to be covered by a parkland of open boreal forest similar to what one finds in northern Canada and Alaska today.  Evidence for this can be found in pollen records in bogs and glacial lakes that extend back to that time.  As the climate warmed and dried, prairie vegetation expanded over the site. This tall grass prairie once covered huge areas from central Illinois and central Minnesota to the east down into Texas in the south, Nebraska and the Dakotas in the West, and Saskatchewan and Manitoba in the north.  This ecosystem is too dry for sustained growth of trees and is dominated by a thick mat of grasses, which hold and build the soil.  Wild fire helps to preserve this ecosystem by removing weedy, shrubby trees, which compete with the grasses and by returning nutrients stored in the plant litter to the available soil pool.  Grazing and trampling by herd animals, most especially the buffalo, played a major role in determining the mixture of grasses and other plants found on the prairies. The buffalo rub gives a sense of how many of the animals must have migrated across this area for millenia. The result of climate, fire, and animal disturbance is the rich black soil seen on so many of the fields en-route to Jeffers.  You can see the difference this makes in the quality of the grasses found on the recently burned and unburned parts of the grassland.  (Intentional fires are set periodically to recreate the original fire regime of this ecosystem.)


5. Human Past

An intense debate rages within the scientific community about the timing of the arrival of humans in the Americas from Eurasia. Regardless of how this debate is resolved, humans were definitely present on the prairies by 5000 years ago as evidenced by carvings in the form of petroglyphs found in the rock here.  From the petroglyphs you observe, what can you say about the animals that once lived on the plains?  What do these images tell us about the native Americans who once inhabited this region? What do the more recent human markings tell us? European settlers began arriving in the prairies in numbers only in the last 150 years.  This had a dramatic effect on the landscape as prairie was converted to farmland, wetlands were drained by tile systems and the buffalo were nearly exterminated.


What is left of the tall grass prairie today?  Of all the major ecosystems in North America, the tall grass prairie has experienced the greatest conversion into farmland. Only the very thin soils on the ridge top prevented plowing of Jeffers allowing for a small section of the original prairie to be preserved. Without the prairie, will the huge buffalo herds ever return?  How will this change the long-term nature of the land?  How will the soil retain its nutrient levels if crops and their residues are continually removed instead of being returned to the soil by fire?