I continue to read about glaciers in the Minnesota geology book. The Laurentide Ice Sheet covered all of Minnesota except, perhaps, a small area in the southeast. Tongues of ice extended into other states. The center of the sheet was Hudson Bay. It is possible to reconstruct the flow of glaciers by measuring the direction of striations and grooves and by tracking erratics back to their bedrocks. Two troughs directed the course of ice streams from the glacier: the Superior-Minneapolis and the Red River and Minnesota River. These were straddled by higher, more erosion resistant rock types. The troughs, on the other hand, were made up of more erodable materials. Ice bulges developed at the perimeter and it's possible to study the advances and retreats of glaciers by studying the kinds of till left behind. (There were also earlier glacial periods). I think the Laurentide Ice Sheet deserves an in-depth study of its own.