Steven I. Dutch
The Silurian Escarpment is a prominent cuesta formed where resistant Silurian dolomite crops out around the periphery of the Michigan Basin (Figure 1). Along the western shore of Lake Michigan it first appears from beneath Pleistocene deposits near Fond du Lac, forms the eastern shoreline of Lake Winnebago and the western shorelines of the Door and Garden Peninsulas, and is again concealed by Pleistocene deposits on the Upper Peninsula of Michigan.
Figure 1. The Silurian Escarpment
The overall form of the Silurian Escarpment in this region is a very straight line of bluffs or steep slopes punctuated by occasional gaps (Figure 2) . The most conspicuous of these gaps is Sturgeon Bay, which almost cuts through the Door Peninsula (the channel is completed by a canal. For some state regulatory purposes the upper Door Peninsula is considered an island!). It has been suggested that the preglacial course of the Menominee and Peshtigo Rivers was through Sturgeon Bay (Martin, 1916, p. 271). The streams which originate near these gaps, within a few miles of the escarpment, flow to Lake Michigan; not eastward, down the dip of the Silurian rocks, but southeastward at about a 45 angle to the dip direction.
The very straight form of the Silurian Escarpment in eastern Wisconsin could be due either to glacial scour or to joint control, or a combination of the two. Regional joint data are summarized in Figure 3. There are some joints which trend parallel to the escarpment, but the major joint sets appear to trend ENE and SE or SSE. The case for joint control of the escarpment is open to question. Note that there appear to be no major geomorphic features related to the prominent ENE set.
The bedrock topography of this region is shown in Figure 4. The most striking feature is the presence of a large number of partially or completely filled valleys which trend SE, at about 45 to the strike of the Silurian rocks. Comparison of Figures 3 and 4 shows that these valleys are parallel to the SE-trending joint set. It is also curious that the acute angles formed by the intersection of the southern sides of the valleys and the escarpment are rounded in at least half a dozen localities, and that the escarpment to the south of each major valley shows a gentle curvature whereas the escarpment north of each valley is quite straight. The rounding extends to the highest elevations on the escarpment, so it does not seem likely that draining of impounded glacial lakes through the gaps is responsible. Streamlining by glacial scour seems a much more likely agent. One could envision the bedrock forming an originally acute-angled promontory. As the ice advanced, it parted around the bedrock, with some ice flowing southeast through the gaps in the escarpment but most flowing south down the present Fox River valley. Flow of ice around each promontory could have produced the observed rounding.
During the final retreat of the Green Bay Lobe, about 14,000-11,000 y.b.p., melt waters were ponded against the terminal moraine to form a large lake, Glacial Lake Oshkosh. This lake originally drained southwest into the Mississippi via the Wisconsin River at Portage. Retreat of the glacier uncovered successively lower outlets at Brillion (A on Figure 2), through the Manitowoc River; at Bellevue (B, subject of this field trip) through the Neshota and West Twin Rivers; at Dyckesville (C) through the Kewaunee River; near Brussels CD) through the Ahnapee River; and finally Sturgeon Bay (E, Thwaites and Bertrand, 1957).
Some erosion at each outlet did occur, but the bedrock valleys cannot be entirely attributed to it. The outlet at Bellevue, for example, is now at an elevation of about 765 feet, about 40 feet below the surface on either side but well above the bedrock surface. Such a shallow channel in easily eroded drift must mean that the Bellevue outlet had a low discharge (perhaps into a lake ponded against the Lake Michigan Lobe) or else carried water for only a very short time. Thwaites postulated a small lake. Lake Shoto, in the lower part of the Twin River Valley which may have limited the flow through this outlet.
Only tentative interpretations of the geomorphic history of this area are possible at present. It appears likely that the pre-Plelstocene drainage was southeasterly, controlled by the SE-trending joint set. Pre-Wlsconsinian glacial history is entirely conjectural, but the bedrock valleys through the escarpment were very likely widened and deepened. The "corners" where the southern side of each valley meets the escarpment were probably rounded by glacial scour. The valleys were largely filled with outwash or drift during the Wisconsinian advance, but not entirely, and they served as outlets for Glacial Lake Oshkosh.
SE 1/4, NW 1/4, Sec. 26, T24N, R21E, Brown County, Wisconsin, Green Bay East 7.5-Minute Quadrangle.
Proceed out the southeast entrance of the University and turn left on Bay Settlement Road. Park about 100 yards to the north in a small turnout on the west side. Small outcrops of buff Silurian (Mayville) dolomite occur just west of the turnout in a low embankment. The moderately thin-bedded dolomite is typical.
Bay Settlement Road parallels the Silurian Escarpment, which trends NNE. The escarpment is very straight and unbroken for about 6 miles to the north, before It begins to curve eastward south of Dyckesville. Dyckesville marks the next major gap in the escarpment north of Green Bay. Well data show that the Dyckesville gap is also a filled bedrock valley (Figure 4).
The outcrops at this stop are the last exposures of Silurian dolomite along the escarpment until Scray's Hill, some 7 miles to the SSW. The straight trend of the escarpment can be followed about 1/2 mile further SSW from this stop without outcrops, however. Joints in the dolomite here trend 065 and 158. There do not appear to be any joints parallel to the escarpment, though joints of NNW-SSW orientation occur sporadically elsewhere.
Proceed south on Bay Settlement Road and turn west (right) on Highway 54-57. The road travels down a gentle slope which marks the escarpment and Its apron of glacial deposits. Note the straight trend of the escarpment to the north (right) and a short distance (1/4 mile) to the south. The hills further south jut westward beyond the line of the escarpment and are made up entirely of till and outwash. Note, however, that their tops are level with the escarpment. Above the hilltop you. may be able to glimpse the TV towers on Scray's Hill. South of Highway 54 along the slope is a pond which fills a large pit that was used as a source of fill during construction of 1-43. The excavation penetrated 20 feet or so of glacial deposits and then entered soft limy mudstones of the Maquoketa. Formation.
Figure 5. Sketch map of UWGB Silurian Dolomite exposure
SE 1/4, SW 1/4, Section 34, T24N, R21E. Brown County, Wisconsin. Green Bay East 7.5 Minute Quadrangle
One of the few outcrops between UWGB and Scray's Hill is located here, where blue-gray shaly dolomite of the Brainard member of the Maquoketa Formation is exposed in the stream bed. The bluish color is mostly due to thin clay interbeds. Fossils are quite uncommon here. The surrounding hills are dissected Pleistocene outwash. The summits are at about 760 feet, roughly level with the edge of the escarpment at UWGB, but the bedrock surface is only at about 660 feet. Thus about 100 feet of Pleistocene deposits fill the buried valley here.
50 m downstream (northeast) of the railway bridge is a leftward bend in the stream. The lower 2 m of the far side of the stream is smoothly curved, probably by stream abrasion. The overhang above shows a joint set striking 055-060 and 130-135. The lower wall of the stream curves smoothly into a 135 joint plane downstream.
About 50 m further downstream is a 1-m fall. The lip of the fall is controlled by joints which strike 035 (parallel to the escarpment) and 160. In outcrops on the far (northeast) side of the stream conjugate joints of this set are well exposed. The 160 joints occur farther upstream but not the 035 joints. Note also the large caved block on the SW side of the stream.
The route from here to the next stop is over the flat upland formed by the thick blanket of unconsolidated material which fills the gap in the escarpment. We will drive through, but not stop at, two valleys cut into the unconsolidated deposits.
(DRIVE THROUGH) SW 1/2, SW 1/4, Section 24, T23N, R21E, Brown County, Wisconsin. Bellevue 7.5-Mlnute Quadrangle
The gentle depression east of Bellevue marks a probable outlet of Glacial Lake Oshkosh. To the south, streams flow southeastward to Lake Michigan, to the north, the streams flow northwestward into Green Bay. Well data indicate about 100 feet of drift over bedrock in this area. The very shallow gap in the drainage divide, considering the unconsolidated material here, certainly does not suggest an enormous or prolonged flow through this outlet. The form of the contours in the area about 1.5 miles east of Bellevue perhaps suggests a wide braided flow until this channel became entrenched.
(DRIVE THROUGH) South edge of Section 26, T23N, R21E, Brown County, Wisconsin, Bellevue 7.5-Mlnute Quadrangle.
The route crosses the upper part of one of the deep ravines which drain NW to Green Bay. Note the meandering, highly underfit stream, the drift walls of the valley (no bedrock in sight) We are only a mile from the drainage divide. Obviously there was a lot of water cutting these valleys. What was its source? Why didn't one of these streams capture much of the SE drainage east of here?
SE 1/4, NE 1/4, Section 29, T23N, R21E, Brown County, Wisconsin. Bellevue 7.5-minute Quadrangle.
This stop is on the north side of Scray's Hill. The Fort Atkinson member of the Maquoketa Formation here forms a resistant shelf with a low escarpment. The gentle slope to the south is underlain by less resistant rocks of the Brainerd member. The steep rise near the top of Scray's Hill is formed by the resistant Silurian dolomite.
This is the southern edge of the buried valley. Note that there are no outcrops evident in the lower area northeast of the quarry. The stream valleys are cut into Pleistocene deposits and the streams are underfit.
The rock at this quarry is a bluish-gray dolomite which weathers to buff near the surface. It lacks the clay interbeds which were seen at the last stop. Fossils are abundant here, especially in the rubble pile on the north side of the quarry. Brachiopods are most common, and cephalopods, crinoids, and trilobites also occur. The broad shelf on the south side of the quarry is a good place to obtain samples of glacial polish and striations. Striations trend about 030. The polished surface often truncates brachiopod shells and stromatolites. A few mud cracks occur in some bedding planes.
Joints at this quarry strike 025-030, 050-055, and 070-075. The 025 and 070 joints form a conjugate set that appears to predate the 050 set.
The low escarpment to the north of the quarry appears joint controlled, but it trends 053, parallel to the least Important of the three joint sets which are evident in the quarry.
SE 1/4, NW 1/4, SW 1/4, Sec. 7, T21N, R23E, Denmark 7.5-mlnute Quadrangle.
This stop contains one of the few bedrock exposures in the Neshota River Valley with rather extensive exposures of medium to thin-bedded gray Silurian dolomite for several hundred yards downstream from the bridge. The east side of the stream is lined by low bluffs and the course of the stream here is clearly joint-controlled. The joints which control the stream trend 060. Other joints trend 154 and 115. The limestone strikes 150 and dips 5E. Note the remains of a lime kiln south of the road.
Small outcrops occur on a hllltop 0.9 miles northeast of here, and in the bed of Devils River, 0.6 miles to the southwest. Also, steep bluffs occur 1-2 miles southwest at Maribel Caves County Park (the "caves" are small and extend only 20-30 feet into the bluffs). Thus the bedrock walls of the valley are only about 1.5 miles apart. This narrowness contrasts with the form of the valley farther northwest, where the gap in the escarpment is 7 miles wide and southeast, where the buried bedrock walls of the valley are as much as 3-4 miles apart. Why is the valley so narrow here?
Evidence of solution occurs not only at Maribel Caves but in the woods about 200 yards or so southeast of the bridge. A number of troughs and small valleys oriented along the 060 joint set seem to be collapse features. A striking overhanging promontory has formed where one of the collapse troughs meets the stream. If time permits we may visit these features. in that event, be careful of the electrified fences!
Figure 7. Sketch map of joint-controlled collapse features, Neshota River valley. Area shown is about 100 m across.
There are many unanswered questions about this area. When did the bedrock valleys form, and how? What filled the head of the valley at Bellevue? How did the deep ravines on the west side of the drainage divide form?
A reasonably good case for joint control of the deep bedrock valleys can be made. The case for joint control of the escarpment is less satisfying. A few joints of the right orientation are found, but many Silurian outcrops right on the edge of the escarpment lack them. Nevertheless, the escarpment seems too straight for too great a distance to be explained solely in terms of erosional agencies without structural control.
This concludes the field trip. We will disperse from here. Follow the trip leader back to Highway 141. Participants whose destination is south may continue southeast (left) on Highway 141 to connect with 1-43 or continue on 141 south along the lakeshore. Those whose destinations are north or west should return to Green Bay (right) on 141. The most direct route out is probably Mason Street (State 29) west across the city to US 41, then north or south to the desired route. -~\"M.
Created 25 August 2004, Last Update 17 November 2011
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