Cretaceous "Windrow" Formation

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay

cretaceous "windrow" formation Cretaceous "Windrow" Formation
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Cretaceous "Windrow" Formation

During the Cretaceous, a seaway extended from the Gulf of Mexico up the middle of North America to the Arctic. It extended as far east as Minnesota. The transition between marine and nonmarine rocks in Minnesota is pretty well defined and the seaway very likely never extended as far east as Wisconsin. The Cretaceous shorline was probably more or less coincident with the present eastern edge of the Cretaceous marine rocks. Cretaceous rocks overlie the iron formations of the Mesabi Range in northern Minnesota and the marine-nonmarine transition is well documented there.

On the map, the marine rocks are indicated in Minnesota only. They should not be confused with Paleozoic rocks of the same color in Wisconsin. The legend has been omitted but the thumbnail sketch maps are retained to help interpret the Wisconsin geology.

Cemented upland gravels and regoliths on high summits in Wisconsin were named the Windrow Formation, after Windrow Bluff on Fort mccoy. They are similar enough in composition and elevation to be equivalents of the nonmarine Cretaceous rocks in Minnesota, although no fossils have been found in them. This is the only deposit in Wisconsin between the Paleozoic and the Pleistocene.

If there are any dinosaur fossils in Wisconsin, they will probably be here. It's a long shot, but not out of the question that there may be a few fossil fragments. Given the location of the Windrow formation on very high summits, it's pretty unlikely that any Cretaceous rocks might be hidden further east beneath glacial deposits. Erosion is a fact of life in geology, and when the last outcrop of a unit is eroded away, it's gone.

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Created 11 Sep 1997, Last Update 14 January 2020