Name:
Geology 310
Professor Moosavi
Purpose:
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)
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?
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.
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?
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?
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.
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?
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
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
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?