<P> Deposits left by the continental ice sheets advancing from these three centers reflect the characteristics of the rocks over which they passed . The Keewatin ice encountered the Cretaceous limestones and shales of Manitoba and the Red River Valley, whereas the Patrician and Labradorian ice moved over iron - rich Pre-Cambrian crystalline rocks of the Canadian Shield . </P> <P> There are few areas in which the earlier drifts from the glacial deposits of the Pre-Ilionian or Illinoian stages are exposed at the surface . The extreme southeastern and southwestern portions of Minnesota (Driftless Area) have extensive areas of pre-Wisconsin drifts, but they are masked almost everywhere by surficial covering of loess (wind - blown silt). Furthermore, these regions of older drift are maturely drained, because the streams have had a longer time to evolve into an efficient drainage system compared with the streams flowing in areas covered by younger glacial deposits . Howard Hobbs has proposed that the Pre-Illinoian glacial deposits in southeastern Minnesota are actually younger Illinoian glacial deposits . </P> <P> As the ice sheets moved into the central portion of North America, the rivers that used to flow from the Rocky Mountains to the northeast into the Arctic Ocean found their valleys choked with ice . The rivers had to divert around the farthest extensions of the ice . When the ice retreated, the new valleys eroded into the landscape kept the rivers from moving back to their old positions . </P> <P> The Wisconsin glacial episode, the most recent glacial period, has been subdivided into four substages, each representing an advance and retreat of the ice . The substages, named from the oldest to the youngest, are the Iowan, Tazewell, Cary and Mankato . Only the Iowan, Cary and Mankato are recognized in Minnesota, but studies indicate that the Tazewell drift may be present in southwestern Minnesota . </P>

What parts of minnesota were covered with the glacier