Older glaciations in Minnesota either eroded away or are buried under later deposits.  It is hard to figure out what happened but a few conclusions have been drawn.  One is that the ice must have been very thick (at least a thousand meters).  Another is that, based on the number of wind-polished and faceted stone surfaces, dust bowl conditions worse than those of the 1930's once took place.

 Limestone and shale deposits from the Cretaceous lie under the younger drifts.  Some contain plant fragments carbon-dated to 40,000 years ago.  In some cases the sands and soils were subjected to intense chemical weathering so that even the toughest rocks became soft clay.  How long this takes to happen is debatable.