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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
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