Fluvial and Glacial Landforms of West-Central Wisconsin

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1 Fluvial and Glacial Landforms of West-Central Wisconsin Geography 360 November 7, 2005 Shana Neumann Holly Johnson

2 Table of Contents Page Introduction...1 Stop 1: Turtle Lake Ice-Walled Lake Plain...1 Stop 2: St. Croix Falls Hospital Esker...4 Interstate State Park Visitor s Center...5 Eagle Peak...6 Summit Rock...8 Stop 3: Potholes...11 Stop 4: Cascade Falls in Osceola...13 Conclusion...14 References...14

3 Introduction On October 14, 2005, our geomorphology class, Professor Jol, and Adam Cahow went on an adventure into West-Central Wisconsin where we studied fluvial and glacial landforms. The morning was a cool 4.5 degrees Celsius with a light layer of fog and a rising sun. We left campus around 7:40 am, heading to Interstate State Park in St. Croix Falls. We went north on Highway 53 to Highway 8, where we headed west to Highway 63. We went south on Highway 63 to the town of Turtle Lake which is located on an ice-walled lake plain, our first stop. From there, we continued westward on Highway 8 to St. Croix Falls. Our second stop was at the St. Croix Falls Hospital Esker, located just off of Highway 35. After leaving the esker, we traveled across the highway to Interstate State Park. There we stopped at the Visitor s Center to learn about the St. Croix horst. From the Visitor s Center, we went deeper into the park to Eagle Peak, an exhumed landform. Our next stop was Summit Rock, overlooking the St. Croix River. There we looked at the formation of the St. Croix River gorge. After leaving Summit Rock, we headed for the Minnesota side of Interstate State Park. There we looked at the potholes formed by the St. Croix River. Finally, we headed south on Highway 35 to Cascade Falls in Osceola where we learned about under-fit streams and hanging valleys. The location of each of these stops can be found on Figure 1. Turtle Lake Ice-Walled Lake Plain We stopped at the Turtle Lake ice-walled lake plain at 9:05 am. It was sunny, breezy, and cool with a temperature of around 7 degrees Celsius. We stopped about 400 meters southwest of the south margin of the lake plain. From where we stood, we could see the top ridge of the ice-walled lake plain as shown in Figure 2. We were surrounded by rolling hills that were formed by sediment deposited by glaciers, depressions which are the areas where the last 1

4 3 2 1 Figure 1 This map shows the location of our

4 Figure 1 This map shows the location of our stops indicated by the yellow circles. Top ridge Figure 2 This picture shows the top ridge of the ice-walled lake plain located where the cows are standing. 2

ice blocks melted, and flat areas that are old lake beds. This area is part of the St. Croix Moraine which stretches southeast from Spooner, Wisconsin to the Twin Cities and into northeast Minnesota.

5 ice blocks melted, and flat areas that are old lake beds. This area is part of the St. Croix Moraine which stretches southeast from Spooner, Wisconsin to the Twin Cities and into northeast Minnesota. The St. Croix Moraine is a glacial feature that formed on the edge of the Superior Lobe. Ice-walled lake plains form in a stagnant, or dead, ice landscape. The stagnant ice that created this ice-walled lake plain was around 90 meters thick. Steep-sided holes formed in the ice and filled with water, creating ice-walled lakes. When the glacial ice melted, the lake drained and the lake bottom was left as a plateau-like feature, an ice-walled lake plain, made up of the fine-grained sediments, mostly clay and silt, which had been deposited on the lake bottom. This formation is shown by the sketch in Figure 3. The ice-walled lake plain is made up of the debris that covered the stagnant ice. The stagnant glacial ice was covered in soil and rock as a result of over-thrusting. Over a number of centuries, an active glacier was repeatedly over-thrusting, or covering, the stagnant ice. The active glacier had accumulated lots of rocks and boulders as it moved and these rocks were dragged along its bottom. When the active glacier covered the stagnant ice, the bottom of the glacier would melt and leave behind debris from the bottom of the glacier. As the climate warmed, the wastage process was accelerated. Glacial Sediment Ice-Walled Lake Plain Lake Rim Ice Silt and Clay Ice Silt and Clay Subglacial Till Subglacial Till Figure 3 This sketch shows how ice-walled lake plains are formed. 3

6 After we finished stop one, we got back on the bus at 9:25 am. We headed west on Highway 8 towards stop two in St. Croix Falls to look at another glacial feature. St. Croix Falls Hospital Esker Our second stop was at the St. Croix Falls Hospital Esker. We arrived at 10:00 am. It was around 13 degrees Celsius, sunny, breezy, and crisp outside. We climbed the 30 meter high esker and stopped on its top. This esker is one of the highest in Wisconsin. It is a long and narrow ridge made of sand and gravel stream deposit. The esker is also made of boulders that were carried by the river with its high velocity. It is steep-sided and winding. This esker was formed around 13,000 years ago when a glacier from the Grantsburg sublobe pushed in from Minnesota. Between 16,000 and 17,000 years ago, the St. Croix Lobe which had covered this area retreated. Around 13,000 years ago, there was no ice left here. Then a new lobe, the Grantsburg sublobe came into the area. It pushed up the hillside of Highway 8 and dammed up the St. Croix River and created a glacial lake, Glacial Lake Grantsburg, to the north of St. Croix Falls. A huge river, fed by Glacial Lake Grantsburg to the north, flowed under the glacier, reaching the bottom through fractures and holes in the ice. As this river flowed under the glacier, it melted the ice above it and formed a tunnel. The river carried so much sediment that it kept filling its bed. This resulted in the river melting more of the ice above it and rising in elevation. When the glacier melted, this 30 meter high esker was left behind. The water of the glacial stream used to flow on the top of this ridge. Figure 4 shows the formation of eskers. Having observed the esker and learning about its formation, we traveled to our next location of Interstate State Park at 10:25 am. 4

7 ice water flowing through the glacier on top of the sediment it had deposited glacial sediment Figure 4 This sketch shows how eskers are formed. Interstate State Park Visitor s Center We arrived at Interstate State Park Visitor s Center at 10:35 am. The visitor s center has numerous displays giving information about the geographic features found within the park. One of the main features displayed was the St. Croix horst. Around 1.1 billion years ago, the Mid-Continental Rift was being formed as land was torn apart by tectonic forces. Molten rock rose up from below the surface; the lava flows were up to 4600 meters thick. The lava spread in layers which could extend over entire counties. These layers are stacked on top of one another. The land from Lake Superior to Kansas was affected by these forces and the entire area except St. Croix and northward was buried under sediment. The rift was active for a 100 million year period. The St. Croix horst was formed by the Mid-Continental Rift. When the lava flows first formed, a high plateau of basaltic rock was not formed but rather the area was sinking. The crust beneath the lava flows was sinking as fast as the outpouring was increasing in height. The stress placed on the rocks was great enough to create a parallel pair of faults. The land between these two faults was uplifted, forming a horst (Christopherson, 2003). Figure 5 shows how this process. This became the St. Croix horst. The St. Croix River flows on the St. Croix horst. After we left the Visitor s Center at 11:25 am, we boarded the bus and drove into the park where we hiked to the top of Eagle Peak. 5

8 upraised portion = horst basaltic rock faults Figure 5 This sketch shows the horst as the upraised portion between the two faults. Eagle Peak We arrived at our fourth stop, Eagle Peak, at 11:30 am. The sun was shining and it was breezy but warm, around 16 degrees Celsius. Eagle Peak is an exhumed landform. Around 500 million years ago, this area was made of layers of volcanic rock. Eagle Peak was formed through water erosion of this basaltic rock 500,000 years ago during the Precambrian era. Because there were no land plants to hold soil in place in late Precambrian time, the weathering process was much more rapid than it is today. Thousands of feet of the basaltic rock were worn away. Seas then invaded the area, making Eagle Peak an island. Pieces of rock were torn loose and stones formed a basal conglomerate until there was enough sediment in the sea to bury the hill. The land sank for millions of years while covered by the sea and over 30 meters of sedimentary rocks. In the Wisconsin area, the crust was arched up to raise the land above sea level in an action called doming. This created the Wisconsin Dome. This area of sediment became dry land near the end of the Paleozoic Era. The area was covered by glaciers during the ice age. The valley that eroded in pre-glacial times was filled in and exhumed twice: in the Precambrian era and then after the glacial period. 6

9 The valley was filled in with glacial deposits and then large glacial floods came through and washed out the sediment. However, not all of the sediment was washed away; Eagle Peak is still partially covered by sedimentary deposits which can be seen in Figure 6. Eagle Peak also has a microclimate. The prickly pear cactus grows on the top of the peak as shown in Figure 7. There is limited moisture availability. This is because there is no soil at the top of the peak, meaning there is no holding area for moisture so other plants can t grow there. The environment is not favorable to any other plants. The north side of the peak is home to some moss and lichen. We left Eagle Peak at 1:10 and headed to our next stop where we observed the St. Croix River from Summit Rock. Figure 6 This picture shows the top of the partially exhumed Eagle Peak. 7

Figure 7 This is a picture of the prickly pear cactus which is part of the microclimate on the top of Eagle Peak. Summit Rock We reached Summit Rock, our fifth stop, at 1:25 pm.

10 Figure 7 This is a picture of the prickly pear cactus which is part of the microclimate on the top of Eagle Peak. Summit Rock We reached Summit Rock, our fifth stop, at 1:25 pm. The sky was clear with a temperature near 16 degrees. Walking to Summit Rock, we traveled through a homoclinal valley, which was dotted with boulder lag from when the river flowed through this area. The homoclinal valley displayed an escarpment and a dip slope which were formed by the creation of faults during the formation of the Mid-Continental Rift. The dip slope was the area of the trail that we walked on to get to Summit Rock, along with the area to the east. The escarpment (located to the west) was from the next basaltic rock layer up and sloped around 15 degrees towards Minnesota. The valley ridges are asymmetrical in cross section. The bedrock along our path and that makes up the St. Croix River valley is around one billion years old, formed when the Mid-Continental Rift was being formed. The rocks along our 8

11 path were very jointed. These joints, or fractures in the rock, are weak spots. The rock is easily weathered at these points because the surface area exposed to chemical and physical weathering is increased (Christopherson, 2003). Physical weathering is occurring in these rocks, as can be seen by the trees growing within the rocks shown in Figure 8. The rocks also face physical weathering from the freeze-thaw cycles faced in this area. The rocks with rounded edges have been chemically weathered. Those with sharp edges have not yet been weathered. The high ridges we stood on at Summit Rock overlooked the St. Croix River gorge. The present day St. Croix River is an under-fit stream, meaning that today s river is too small to have cut the gorge. The valley was actually made by catastrophic glacial flooding. During glacial times, the water from the glacial ancestor of Lake Superior, Glacial Lake Duluth, along with about a half dozen other glacial lakes drained into the St. Croix River valley. Huge bursts of water came from the lakes when the glacier retreated, unblocking the lakes and allowing the water to drain. Glacial Lake Duluth formed and disappeared four different times during glacial times due to the retreating and advancing of the glacier. When the glacier retreated, the lakes were able to drain. Then the glacier would advance again and block drainage. Then the catastrophic flooding began. As shown in Figure 9, the gorge was carved along the fault zone created with the Mid-Continental Rift where the rocks were weak and eroded quickly during the catastrophic floods. The gorge was cut within hundreds of years. At the end of the glacial period, the river was as large as the Mississippi River in the New Orleans area. The water level was very high during that time but has gotten shallower over the years. Having observed the St. Croix River gorge, we crossed over to the Minnesota half of Interstate State Park to observe the potholes which were formed by the river. 9

The valley was only as deep as where we are standing around 12,000 to 9,500

12 area of physical weathering Figure 8 This picture shows the physical weathering by the vegetation growing in the joints. The valley was only as deep as where we are standing around 12,000 to 9,500 years ago. Fault line Figure 9 The St. Croix River follows the fault line where the rocks were easily eroded. 10

13 Potholes We arrived at the Minnesota side of Interstate State Park at 2:20 pm and went to investigate the potholes in the area. Potholes are silo-shaped holes in the bedrock created by grinders. Three conditions are needed to create potholes: a bedrock bottom, fast flowing water, and stones or sediment (grinders) being carried by the water to grind the holes in the bedrock. Once all three conditions are in place, the formation of potholes can begin. The potholes along the St. Croix River were created when the terraces from upstream came to grade with the river. The St. Croix River used to fill the full width of the river valley. Huge swirls were created on the sides of the river as the water hit the bedrock walls or outcroppings along the edge of the water, slowing the water. The fast current caught the slower moving current and caused the water to move in a circular manner, creating eddies or whirlpools as shown in Figure 10. This happened for decades. The force of the water was so strong that it would move rocks in a circular manner as well and the rocks would wear downward into the bedrock, creating the siloshaped potholes. The potholes along the St. Croix River have different shapes and sizes. Figures 11 and 12 show some of the size variations found within the park. Some of the potholes in this park are over 25 meters deep and 8 meters in diameter while others were very shallow. There are hundreds of potholes in the park. They formed where the bedrock had a weak fracture or intersecting fractures. Potholes sometimes occur in a chain and occasionally they merge with one another. When we finished looking at potholes, we got back on the bus at 2:45 pm and headed to Osceola for our final stop. 11

Figure 10 This picture shows some eddies being created, as

14 Figure 10 This picture shows some eddies being created, as indicated by the arrows, as the faster moving water catches up with the slow moving water. Figures 11 and 12 These pictures indicate size variations in the potholes of Interstate State Park. The Figure on the left shows a small pothole whereas the Figure on the right indicates a much larger pothole. 12

15 Cascade Falls in Osceola Our final stop was at Cascade Falls in Osceola, Wisconsin. We arrived at 3:05 pm to a sunny, warm afternoon of 20 degrees Celsius. We went down to the water and observed Cascade Falls and Osceola Creek. Osceola Creek is wider and deeper after the falls than upstream. The creek has a pronounced, deep valley after the falls and the riverbed is very rocky. Osceola Creek is located in a hanging valley. The hanging valley formed when a secondary channel of the glacial river flowed through the valley to join the larger glacial river forming the St. Croix River valley. The secondary river was not able to erode the landscape as much as the larger river, causing its valley bottom to be higher in elevation. Therefore, a hanging valley formed. Osceola Creek drops around 30 meters from the town to the St. Croix River where the creek is at grade with the river; St. Croix River is the local base level. Osceola Creek has eroded its knickpoint back from next to St. Croix River to its present location of Cascade Falls as shown in Figure 13. The falls represents the maximum valley lengthening retreat. It is a knickpoint. As the knickpoint erodes back, the valley will lengthen. There is another small knickpoint farther upstream, on the other side of bridge. Osceola Creek is an under-fit stream, meaning that it is too small of have created the valley it now occupies. The creek s valley was cut by a secondary channel of the glacial St. Croix River. The large amount of water flowing from the glacial lakes flooded this area as it rushed through and created the deep valley. The water came along the upper terraces. We left Osceola Creek at 3:45pm and headed back to Eau Claire concluding our trip for the day. 13

16 Figure 13 This picture indicates the knickpoint at the current location after having eroded back from the St. Croix River. Conclusion Our trip into West-Central Wisconsin took us to many different locations showing us fluvial and glacial landforms. We learned about ice-walled lake plains in Turtle Lake, and then traveled to St. Croix Falls where we learned about eskers, the river gorge, exhumed landforms, and potholes. Our last stop was in Osceola where we learned about under-fit streams and hanging valleys. References Christopherson, R.W., 2003, Geosystems: an introduction to physical geography: New Jersey, Pearson Education, Inc., 660 p. Wisconsin Online, Wisconline Maps [online]. Available from: [Accessed 3 November 2005]. 14

mountain rivers fixed channel boundaries (bedrock banks and bed) high transport capacity low storage input output

mountain rivers fixed channel boundaries (bedrock banks and bed) high transport capacity low storage input output strong interaction between streams & hillslopes Sediment Budgets for Mountain Rivers Little