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Figure 2. Detailed LiDAR-derived hillshade maps of the three remnant segments of the Bridgeport strath as identified in Figure 1, showing (A) the west-
ernmost segment; (B) the central segment; and (C) the easternmost segment. Red squares identify locations of Geoprobe coring to determine bedrock
surface elevations. Blue diamonds identify locations where Paleozoic bedrock crops out, verifying that the surface is a strath terrace. Blue dashed line
in (A) identifies the location of the Bridgeport moraine, which represents the farthest west extent of a pre-Illinoian glaciation that advanced out of Min-
nesota and Iowa (Knox and Attig, 1988). All images are shown at the same scale.

the modern Wisconsin River. Reversed METHODS AND RESULTS As expected, individual coring sites
polarity to the remnant magnetism of this reveal considerable variability below the
sediment indicates it was deposited prior Testing this alternate hypothesis (upper) trend line of the original strath
to ~780,000 years ago. They hypothesized required coring through the unconsoli- surface owing to localized erosion fol-
that this glaciation blocked the mouth of dated sediment on the terrace to establish lowing abandonment. However, the trend
the Wisconsin River and caused a tempo- bedrock elevations at numerous points of the strath dips to the east, in the oppo-
rary reversal of flow to the east. along the length of the terrace. This was site direction of flow of the modern
accomplished using a combination of Wisconsin River, with an estimated gra-
An alternate hypothesis to the pre- high-resolution LiDAR-derived digital dient of 0.15 m/km (Fig. 3A). The gradi-
sumption that the lower Wisconsin River elevation models to precisely identify ent of the strath surface estimated from
valley was incised through the late ground-surface elevation to within ~5 cm coring is consistent over a broad scale
Cenozoic by a westward-flowing river and Geoprobe direct-push coring to pre- with many other mid-continent streams,
and experienced a temporary reversal of cisely identify depth to bedrock to within and close to the gradients of the modern
flow at the time of the “Bridgeport” gla- ~2.5 cm. The strath surface is comprised lower Wisconsin River floodplain and
ciation is that incision of the lower of glauconitic units of the Cambrian associated MIS 2 outwash terraces.
Wisconsin River valley to the level of the Tunnel City Group, which facilitated Within the context of the westward-dip-
Bridgeport strath was accomplished unambiguous recognition of the transition ping late Quaternary surfaces in the
through the late Cenozoic by an eastward- between Quaternary sediment and the lower Wisconsin River valley, the east-
flowing river. A subsequent stream piracy strath. Cores were collected from 62 sites ward dip of the Bridgeport strath stands
event caused a permanent reversal to the on the strath surface on an ~60-km transect in stark contrast (Fig. 3B). The inescap-
modern westward flow. The test of this (Fig. 2; GSA Data Repository Table 11). able conclusion to be drawn from the ori-
hypothesis is to identify the direction of The highest bedrock elevation points were entation of the strath is that the lower
dip of the bedrock surface of the connected, based on the assumption that Wisconsin River valley was carved to the
Bridgeport strath, which necessarily dips they represent a good proxy for the origi- level of the Bridgeport strath by a river
in the direction of water flow at the time nal, un-eroded bedrock surface (see Data flowing to the east.
it was the bedrock floor of the valley. Repository Fig. 1 [see footnote 1]).

1GSA Data Repository Item 2017404, supplementary core and well log data and methods used to support interpretations, is online at www.geosociety.org/
datarepository/2017/.

6 GSA Today | July 2018