According to the book I have resumed reading, Minnesota Geology, the Cenozoic was the era when Minnesota began to achieve its status as the “icebox of the nation”. The Rocky Mountains had formed, changing the climate. Dinosaurs had disappeared and small mammals predominated. Remains are found in the Badlands of South Dakota which unfortunately are not found in Minnesota. Glacial drift covers evidence of activity. Of course, subsurface exploration may one day reveal some of the specifics.