Physical Geology-Earth Science Field Trip Stops

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Wequiock Falls

This is the first of two exposures of Paleozoic bedrock that we will be visiting. At Wequiock Falls, the Silurian Mayville dolomite overlies the Brainard and Fort Atkinson Members of the Late Ordovician Maquoketa Formation. (Please refer to the Table, Paleozoic Formations of Wisconsin.)

The rocks of the Maquoketa Formation are generally easily eroded and less resistant than the overlying Silurian dolomites. This outcrop is one of the better and more accessible of those exposed as streams flowing over the escarpment cut away the most resistant Silurian beds. The Fort Atkinson Member of the Maquoketa Formation represents the last carbonate deposition before the Silurian. The Brainard Member is largely soft green mudstone with thin interbeds of dolomitic shale and argillaceous dolomite containing brachiopods, arborescent bryozoans, cornulites worm tubes, and occasionally stromatolites.

About 2 meters of greenish-gray thin bedded dolomite lies above the mudstone. This unit forms the lower part of the step valley walls just west of the bridge and the middle part of the face of the falls. The contact between the Maquoketa Formation and the Mayville dolomite is tentatively placed below this unit. The top of the section is 3 to 4 meters of gray medium-to-coarse-grained dolomite characteristic of the Mayville dolomite.

We will observe the following features:

1. Nearly horizontal sedimentary rock layers

2. Vertical fractures (joints)

3. Thin soils

4. Stream erosion features

Points for discussion:

1. What does the rock type imply about the Late Ordovician and Silurian environment?

2. What is the ultimate fate of the falls?

3. What is the dip of the bedrock?

4. Is the soil residual or transported? How do you know?

*Excerpted from the Guidebook, 44th Annual Tri-State Geological Field Conference, 11-12 October 1980, p. 79-81.

We turn onto Highway 54 and proceed toward Green Bay. There is a good view of the Green Bay lowland as we cross the Escarpment. The straight trace of the escarpment continues another kilometer south, then there are no Silurian outcrops until Scray Hill about 15 kilometers away. Well data show that there is a deep buried valley in between, up to 200 feet deep. This is one of a series of deep, pre-Pleistocene valleys that cut the Escarpment.

Exit for Bay Beach and park on the bay shore just west of the park.

Renard Isle

Renard Isle, formerly called Kidney Island, is a confined disposal facility (CDF) in the lower Green Bay. The CDF is a depositor for polluted dredged materials from the Green Bay harbor entrance channel. These materials are considered polluted by the Environmental Protection Agency (EPA), but not hazardous. The cap placed by the U.S. Army Corps of Engineers over the filled island will consist of unpolluted materials.

Renard Isle is 10 feet above low water datum (lwd) for Lake Michigan. Lwd for Lake Michigan is 576.8 feet above mean water as determined at the International Great Lakes datum in Quebec. The island is county owned, but under the U. S. Army Corps of Engineers' control until all work is complete. The island is 800 feet offshore from Bay Beach. Bay Beach and the adjacent Wildlife Sanctuary are both city owned.

1. Speculate on how the island influences water circulation in the Bay.

2. Comment on the effect of the island on sedimentation in the Bay.

3. What happens when the island is full? What are some alternatives to building new islands for disposal of dredge spoils?

Return to I-43 and cross the Tower Drive Bridge, getting a good view of the Bay and the Fox River lowland. Note the rising, hilly terrain west of the Bay. The Great Lakes did not exist before the Pleistocene, and the Niagara Escarpment was probably west of the present bay. Glacial scour planed back the Escarpment and excavated the soft Maquoketa Formation beneath. The west side of the Bay is a gentle dip slope on the top of the Platteville-Galena dolomites. From well data, the contact with the Maquoketa Formation is probably about at the Fox River. There are a number of outcrops and quarries near the west shore of the bay; our next stop is one. The hilly terrain is due to glacial deposits on top of the bedrock.

Beyond the bridge we pass the Pulliam Power Plant and a large fill area. The fill area, projected as an industrial park, was formerly the Atkinson Marsh, the loss of which is still a bitter memory for local conservationists. Exit at Velp Avenue and head west. Cross Duck Creek, turn right on County J, then left at the River Bend Supper Club. Continue straight west to the large quarry.

Duck Creek Quarry

This exposure of Ordovician-age bedrock is the Galena dolomite. For the time sequence of rock units, please refer to the Paleozoic Formations of Wisconsin. As we travel north, the bedrock gets progressively older until we encounter outcrops of the crystalline Precambrian bedrock near Mountain, Wisconsin. We will not enter the Duck Creek Quarry, but observe from a vantage point on the edge.

Observe the following features:

1. Nearly horizontal sedimentary rock layers (dolomite or dolostone)

2. Vertical fractures (joints)

3. Evidence of rock displacement (faulting)?

4. Thin glacial till overlying bedrock

5. Sharp soil/bedrock contact

6. Abandoned quarry (across the street)

7. Level of the ground-water table

Points for discussion:

1. Speculate on the causes of rock fractures.

2. What caused the staining of the rock walls?

3. Determine the depth of the local water table.

4. What are the limits to the size of this excavation?

5. What is an environmentally sound use for an abandoned quarry?

We now travel north on Highway 141 for a long distance (punctuated by a rest stop) to Pound. The terrain is mostly flat with a few sandy hummocks. The flat terrain is former lake plain, the hummocks are mostly old sand dunes. A few are glacial deposits. At Pound we continue straight on a cutoff while the highway turns. Cross Highway 64 and park in the gravel pit.

Pound Esker

Lobes of glacial ice have advanced and receded over Wisconsin numerous times over the past 1 to 2 million years. The last major advance began about 25,000 years ago and the ice reached its most southerly extent about 18,000 years ago. The glacier than began an oscillatory retreat until the ice finally withdrew from the Great Lakes region for the last time about 10,000 years ago. Advancing glacial ice sculptured the landscape leaving behind a variety of streamlined land forms including drumlins and stoss-lee topography. Receding (that is, melting) glacial ice also impacts the landscape but primarily through the erosive action of melt-water. The esker into which we venture is an example of a glacio-fluvial landform.

We will observe the following features:

1. Sinuous shape of the esker

2. Wide variety of rock types--igneous, sedimentary, metamorphic

3. Sediment size and shape

4. Cedar swamp

Some discussion questions:

1. What other land forms are associated with eskers?

2. What is the economic value of an esker?

3. What type of soil might develop on a glacio-fluvial deposit?

4. Explain why the coarsest sediments form the core of the esker

Continue west on Highway 64. In a mile or so the terrain changes dramatically from flat to very irregular as we begin crossing moraines. About 5 miles (8 km) from the last stop, where 39th Road crosses Whiskey Creek, there are a few low ledges in the stream (barely visible from the highway). This is one of only two localities where Cambrian rocks are exposed in this part of the state. In another 5 miles (8 km) the terrain again flattens dramatically. We will stop at the farms in the large open area.

Glacial Lake Oconto (Williams' Farm)

Advancing lobes of glacial ice not only excavated the land but also disrupted drainage patterns. In some cases, ice blocked rivers and streams causing water to flood vast areas and creating large lakes. Later, when the glacial ice melted and drainageways were restored, lake levels dropped and in some cases lakes completely dried up. We will view the remains of the northerly margin of a large glacial-age lake that extended southward into the Stevens Point area.

Some notes on the Williams' farm:

Observe the following features:

1. The striking change in landscape as we approach

2. Composition of the soil - organic-rich

3. Composition of the sediment layers exposed in the drainage ditch (organic layer over sand)

4. The use of the land for agriculture

Some discussion questions:

1. Identify the features that suggest that this area was formerly occupied by a lake.

2. What can we infer from the sediment layers regarding the geological history of the area?

3. Is this an example of eutrophication? If so, how does this type of eutrophication compare to "cultural" eutrophication?

4. Speculate on the ultimate fate of the organic layer.

Continue west on 64 and turn right where it joins 32. A few hundred meters later, in a clump of trees on the right side of the road, is the first outcrop of Precambrian rock.

Green Lake: Lunch

On our way to Green Lake, we will observe the first outcrops of crystalline bedrock of Precambrian age. In preparation for the geology of the next three stops, you may wish to review Bowen's Reaction Series, Metamorphic Index Minerals, and the description of the "Brief Overview of the Precambrian of the Mountain-Crivitz Area, Wisconsin" by Professor Steve Dutch. Green Lake itself is a typical glacial kettle pond.

Continue north on Highway 32-64. A mile or so north of Green Lake there is a large jagged granite outcrop on the right side. This outcrop is smooth and streamlined on the north side and jagged on the south, a typical example of glacial sculpting. We will see a smaller version of this topography at High Falls Dam. The granite is part of the northern extremity of the Wolf River Batholith. From there to Mountain, we pass outcrops of granite and greenstone alternating in a very irregular way.

Turn right onto County W at Mountain. After three miles (5 km), bear right onto Bear Paw Road. Stop at the large outcrops on the right about 200 meters down the road.

Macauley Granite Gneiss

In the Mountain area there is a wide variety of rock types and structural features exposed within a relatively small compact area. At this stop, we will head into the woods and ascend over an extensive outcrop of the pink to grey Macauley granite gneiss. Locally, the rock is fractured and intruded. Also, the surface of the outcrop provides an excellent illustration of primary succession and the first stages of soil development.

Observe the following features:

1. The relationship between the host bedrock and intrusions

2. Evidence of glacial activity

3. Differential (unequal) rates of weathering

4. Soil development

5. The relationship between fracture patterns and intrusions (veins, dikes)

Some discussion questions:

1. This outcrop has been exposed since deglaciation about 10,000 years ago and yet soil has been slow to develop. Why?

2. How different would this area appear if glacial ice had not completely buried the area?

3. What factors control the rate of erosion?

4. What does the rock fracture pattern suggest about the regional stresses?

Return to W and continue east. The topography is deep sandy glacial outwash with numerous kettles, some with ponds, others dry. Bedrock knobs stick through the outwash in places. There is a prominent one just before Crooked Lake. Continue 12 miles (19 km) to Parkway Road and turn north (left). Cross the bridge, and continue up the hill. Just after the top, turn right at the sign for High Falls Flowage. Park at the fence and walk to the large rocky knobs.

High Falls Dam and Reservoir

This stop provides us with an opportunity to view a variety of crystalline rock types as well as a number of different glacial ice direction indicators. We also will observe the effect of a dam and reservoir on the regional drainage pattern and land use.

Observe the following features:

1. Changes in the appearance of rock (texture, mineralogy)

2. Reconstructed directions of glacial ice movement

3. The effect of rock fracturing on the rate of weathering

4. The alteration of the natural environment by the dam and reservoir

Some discussion questions:

1. In this portion of the state, thin layers of sand and gravel overlie impermeable bedrock. What does this imply for groundwater quality?

2. Speculate on how hills composed of resistant rock might influence the direction of glacial ice motion.

3. Speculate on why the rock type changes from one place to another within this relatively small area.

Return to Parkway Road and turn right. In a few hundred meters, turn left into Veterans' Memorial Park and take the trail to the stream.

Veterans' Memorial Park

This is our final stop of the day. The bedrock in this area is greenstone, a metamorphosed basalt. It is the oldest rock that we have seen today (

1.9 billion years old) and is a member of the Waupee Volcanics. Be sure to note:

1. the characteristics of the stream channel

2. bedrock texture and composition

3. the differential weathering

Retrace your route to Highway 64 and continue east. Note the large cleared fields and center-pivot irrigation system about a kilometer east of Parkway Road. Five miles (8 km) from Parkway Road, the terrain drops dramatically as we leave the morains and return to the lake plains. Note the small hills that rise abruptly here and there; the sharp breaks in elevation are old shorelines. These were glacial hills that protruded as islands and were eroded by wave action. Just beyond Crivitz Intergalactic Spaceport (actually a small light airport), Highway 64 turns abruptly north for a mile, then sharply east again. If you continue north up he small dirt road, you arrive at Sandstone Rapids Dam, the other place in this part of the state where Cambrian rocks are visible. Nice outcrops and occasional bald eagles along the river make this a worthwhile stop. The outcrops are only visible from the catwalk in front of the dam.

We take a short rest stop in Crivitz, then return to Green Bay via Highway 141. Just north of highway 64 on the right side of the road is a Geographic marker, marking the halfway point between the equator and North Pole (why does the pole seem so much closer most of the time?). The latitude is not 45 degrees, as you'd expect, but 45 degrees, 8 minutes, 45.7 seconds. The discrepancy is due to the ellipsoidal shape of the earth, so that degrees of latitude are slightly shorter at low latitudes than high. It is 3107.47 miles to both equator and pole.

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Last Update February 21, 1997