The Wyalusing River
The Wisconsin River is well known to schoolchildren in the state. It starts at Lac Vieux Desert on the Michigan border and flows into the Mississippi at Wyalusing. But geologists noticed something that caught their attention, actually a series of things.
From Wyalusing, the river valley gets wider as you go upstream. This is the opposite of how rivers usually work.
Where the Wisconsin enters the Mississippi, its should angle downstream as it enters the Mississippi, but it doesn't. Instead the valley curves upstream.
Tributaries entering the Wisconsin do so from valleys that curve to the east, indicating that the river flowed east when the valleys were formed. This is known as a barbed tributary and is a sign that the river has changed in some major way.
Military Ridge, a resistant formation topped with harder rock, runs through southern Wisconsin, but stops at the Mississippi River. But the same ridge starts on the other side of the Mississippi at Pike's Peak, Iowa. Did this once function as a continental divide, separating the waters flowing to the Gulf from those headed for the Atlantic.
The Mississippi River narrows south of Wyalusing, suggesting that the gap there is more recent and not subject to the same forces that carved the river north of there.
An extraordinary claim
Did the ancestor of the Mississippi River once take a turn at Wyalusing and run east through present day Portage and Green Bay to empty into the Atlantic Ocean via the St. Lawrence River? Wikipedia notes such a possibility. Such a claim requires extraordinary evidence. Thanks to Eric Carson and other geologists, that evidence was assembled.
Rivers often have terraces, which are former river beds. As the river erodes into the bedrock, these former river beds are sometimes left well above the new river. Geologists were able to identify three such terraces. They tilt to the east, indicating that the river once flowed that way. The difference is just inches per mile, but modern equipment can measure that precisely.
A valley buried under glacial debris was detected, running from near Portage through Green Bay, showing the rest of the river. It deepens as it trends to the east, showing the direction of the flow. Since the debris is sometimes hundreds of feet deep, they used data from wells to map the bedrock.
Records of sediment in the Gulf of Mexico indicate that the continental divide was south of Wisconsin at the time in question. The unique rocks of Wisconsin, Minnesota and North Dakota do not appear there.
When The Wisconsin River Flowed East. Geologists Delve Into An Ice Age Transformation That Reshaped North America. Eric C. Carson, Wisconsin Geological & Natural History Survey.
Carson, Eric & Mauel, Stephen & Bates, Benjamin & Ceperley, Elizabeth. (2018). GIS-BASED DELINEATION OF THE BURIED PRE-QUATERNARY WYALUSING RIVER VALLEY IN EAST-CENTRAL WISCONSIN, USA. 10.1130/abs/2018AM-321031.
Late Cenozoic Evolution of the Lower Wisconsin River Valley: Evidence for the Reversal of the River by Eric C. Carson and J. Elmo Rawling III
Late Cenozoic Evolution of the Upper Mississippi River, Stream Piracy, and Reorganization of North American Mid-Continent Drainage Systems - Eric C. Carson, J. Elmo Rawling III, John W. Attig, and Benjamin R. Bates, Wisconsin Geological and Natural History Survey.