Possible Tertiary Drainage in Wisconsin
The Tertiary drainage system of Wisconsin may have looked like this. Structurally, Wisconsin is an arch, with cuestas of resistant rock dipping away from the axis. The Western highlands are defined by resistant units in the Cambrian sandstones. In eastern Wisconsin, the Niagara Escarpment is defined by the edge of the Silurian rocks, and escarpments defined by the Sinnipee and Prairie du Chien dolostones are less evident because they are mantled by drift. The Wolf River flows parallel to the Prairie du Chien escarpment and its pre-Pleistocene location was very likely controlled by that escarpment.
It is reasonable to assume that the cuestas would have been more prominent and influential in the Tertiary before erosion, glacial scour, and drift mantling. This map shows the drainage pattern to be expected on a breached anticline with cuestas on its flanks. The drainage was southward along the axis of the arch through the now-buried Yahara River valley, thence to the Rock River drainage of Illinois. These deep valleys, now partially filled by glacial and fluvial deposits, are well documented from subsurface drilling.
The hypothetical drainage on the western side of the arch is now completely gone. It could have been disrupted in a variety of ways. If the Yahara-Rock valley was blocked by Pleistocene ice, the basin upstream could have flooded and cut new outlets over the drainage divide. Alternatively, G.L. Laberge proposes in Geology of the Lake Superior Region (Geoscience Press, 1994) that the present upper course of the Mississippi is an ice-marginal drainage on the eastern edge of an early Pleistocene ice lobe. If so, local ice-dammed lakes may have flowed east over the drainage divide, cutting valleys. Finally, during deep incision of the upper Mississippi Valley, streams flowing into the deep valley may have been able to incise their valleys faster than streams flowing south in Wisconsin, and may have pirated the drainage. Perhaps all three mechanisms may have played a role.
If the present upper Mississippi didn't exist in its present form before the Pleistocene, where was the master drainage? There are no deep hidden valleys running south across Minnesota and Iowa. The drainage could have been somewhere to the west, but the hypothetical drainage shown in western Wisconsin has to be considered a serious contender.
The problem with geomorphic history is that it's very easy to come up with plausible scenarios but very hard to prove them. Buried valleys are good evidence for ancient drainages, but completely-destroyed drainages are hard to document. Finding distinctive rocks of limited source area in sediments is one method, but the floods of sediment that have passed through this region since the Tertiary make that approach unlikely to succeed.
Created 27 Dec 1997, Last Update 04 April 2013
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