Physical Geology-Earth Science Field Trip Photos
Steven Dutch, Natural and Applied Sciences,
University of Wisconsin - Green Bay
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||At Wequiock Falls, the Silurian Mayville dolomite overlies the Brainard and Fort Atkinson Members of the Late Ordovician Maquoketa Formation. 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 lip of the falls is formed by 3 to 4 meters of gray medium-to-coarse-grained dolomite characteristic of the Mayville dolomite (the light thick band at the top of the bluff). Beyond the falls, ice on the cliff shows where ground water is seeping out of the rocks.
||In this warm-weather view, water is not only flowing over the falls but running out of the rocks as well. The Maquoketa Formation below the lip of the falls consists of blue-gray shaly dolomite. The blue-gray color is due to volcanic ash derived from the Appalchians and transported far inland by winds and currents.|
||Like all waterfalls, Wequiock Falls retreats by undermining soft rocks, causing the lip of the falls to collapse on occasion. Here we see a recent collapse.|
||Looking down the gorge of Wequiock Creek. In 11,000 years, Wequiock Falls has retreated about 200 meters, an average rate of a meter per 55 years. In the same time, Niagara Falls, which has a bit more water, has retreated 11 kilometers, an average of a meter per year or 55 times faster. |
||A good view of the Bay and the Fox River lowland from the Escarpment. 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. The hilly terrain west of the Bay is due to glacial deposits on top of the bedrock.|
||This bench mark lies about 20 meters west of the end of the sidewalk along the shoreline at Bay Beach. Bench marks are the precisely-surveyed points that are the basis of all detailed maps.|
||In 1973 the shore areas of Green Bay were badly flooded. To guard against future flooding, this dike was built along the shore. The large blocks of stone and concrete rubble, termed riprap, serve to protect the dike from wave erosion.|
||Renard Isle, formerly called Kidney Island, is a confined
disposal facility (CDF) in the lower Green Bay, visible offshore to the right in this picture. The CDF is a depository for polluted dredged materials from the Green Bay harbor entrance channel. The island is 800 feet offshore from Bay Beach. The island protects the channel behind it from wave erosion, causing sediment to be deposited there. The reed-covered point has been deposited since the island was constructed, and if allowed to continue forming, will eventually join the island to the mainland.|
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. We do not enter the Duck Creek Quarry, but observe from a vantage point on the edge.|
||Water seeping out of the rocks shows the movement of groundwater in the area. Note that many seeps originate about 5 meters below the lip of the quarry at the base of one particularly thick layer. The quarry would fill with water if it were not pumped. A pipe runs up the right side of the quarry to carry pumped water.|
||A nice example of microclimate. Dandelions are growing on the sunny south slope of the berm but not the shadier north slope.|
||If not for pumping, the quarry north of the road would look like this one south of the road. This quarry was used for disposing of cedar bark from a fence-post industry, and tannic acid from the bark soon colored local wells like weak tea, harmless but annoying. Still, it's a potent illustration of why quarries can't be simply filled in with refuse. In quiet places on the water, patches of bubbles can be seen caused by methane gas rising to the surface from the decaying cedar bark.|
||Iron staining on the side of this dolostone block is due to the oxidation of iron sulfides. These blocks have weathered visibly since I first saw them in 1976; oxidation has become more apparent and fossils easier to see. Geologic changes can occur even in a human lifetime.|
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.
||Walking into the Pound Esker. The entire access road was once occupied by a gravel ridge that has been dug away.|
||Coarse gravel fills the center of the esker, where the water was moving fastest, and finer sands (mostly left behind) were deposited on the sides. Most students estimate the slope as being much steeper than it really is.|
||The pit was dug mostly to get gravel for concrete, and the sand was left behind. It's a popular place for dirt bikers. Students usually estimate the steep face of the pit as having a slope of 45 to 70 degrees, but this photo shows that slopes in unconsolidated materials are actually pretty moderate, typically about 30 degrees.|
The Hip Bone Connected to ...
||In 1997, a sharp-eyed student spotted a buried bone in the embankment along the southwest side of the pit.|
||The bone appeared to be located in a charcoal-lined pit. Because of the chance the remains might be human, we did not disturb the site. There are strict laws governing disturbance of Native American burials and disposition of human remains. The State Archaeologist was notified, and after viewing the site, judged that the bones were probably not human.|
||A close-up of the bone. The site has since been obscured by slumping or human disturbance. No radiocarbon dates are yet available on the bones.|
||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. This photo was taken in the woods along the stream.|
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.
||The soil here is rich in organic matter but quite thin. Note the sandy spot where plowing has exposed the underlying lake sands. This soil at this site has become noticeably lighter and thinner since I first saw it in 1976.|
||Drainage ditches like this lower the water table, making it possible to farm what would otherwise be marshy ground. Whether this would be allowed to happen today with regulations governing wetlands is not certain.|
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. 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
||At this stop, we 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. Note the two sets of joints, one along our line of sight and the other slanting from left to right.|
||A close-up of the gneiss. The gneiss probably started as a granite intrusion that was deformed and metamorphosed during the Penokean Orogeny, a collision of three plates about 1900 million years ago.|
||This area is also near the margins of the Wolf River Batholith and irregular masses of fine-grained pink granite intrude the gneiss.|
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.
High Falls "Rhyolite"
||This rock, the High Falls "Rhyolite", actually seems to be an unusual fine-grained intrusive rock. This rock is at the extreme northern end of the Wolf River Batholith. Note the rectangular joints and the beginnings of spheroidal weathering.|
||This circle once identifed a bench mark so mappers would be able to locate it on air photos. Unfortunately, souvenir-hunters like to take bench marks, despite the hefty fine for doing so. The remnants of the bench mark are still visible in the hole in the center of the circle.|
||This train of fractures, called crescentic gouges, formed when the glacier pressed a boulder into the bedrock surface with enough force to fracture it.|
||An end-on view of the crescentic gouges.|
||The rule with crescentic gouges is that the bow of the crescent points in the direction of ice movement (i.e. to the right). Here, surprisingly, that direction is northwest|
||This sort of feature is called by a variety of names, including whaleback. When ice flows over a jointed outcrop, it presses the joints shut on the upstream side and smooths the rock off, producing a streamlined shape. On the downstream side, it pulls the joints open and plucks blocks away, creating an irregular, blocky face. |
||This feature also indicates an ice movement direction toward the northwest. Other terms for this feature include roche moutonee (French: sheep rock) or stoss-lee feature. The stoss side (German stossen to push) is the upstream side, and the lee side, of course, is the downstream side.|
||The point in the reservoir at the end of the access road has a number of interesting bedrock and surface features.|
||Compare the dark color of this rock to the pink rock at the first outcrop. The magma on the margins of the Wolf River Batholith reacted with the surrounding rocks and became more ferromagnesian in composition. The rounded grooves have joints running along them and are probably due to chemical weathering. Parts of the rock surface have glacial polish but polish extends into recesses as well. It's unlikely that ice could polish rock in small openings, but wind abrasion is a likely possibility. As the ice retreated, there would have been little vegetation, strong winds off the ice, and lots of sand and silt to blow around, a perfect setting for wind abrasion.|
||On this glacially-smoothed surface there are crescent-shaped fractures called chatter marks. Chatter marks seem to result from intense friction and can be made artificially by using too-high pressure when polishing lenses. Unlike crescentic gouges, these have the horns of the crescents pointing in the direction of ice movement. Again, it's to the northwest. Thus, there are three different ice-flow indicators in a small area, all indicating ice flow in an unexpected direction: to the northwest.|
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 moraines and return to the lake plains.|
||On either side of the road in this area we see the characteristic irregular topography of moraines.|
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 the 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 (west) 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.|
||Just north of highway 64 we cross a small esker. Off the west (right) side of the road, especially, we can see the characteristic sinuous form of an esker.|
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Created 18 October 1998, Last Update
14 December 2009
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