THE LOCATION OF THE LABRADOR AND KEEWATIN ICE CENTERS SHIFTED FROM ONE GLACIATION TO THE NEXT

The subsurface Pleistocene stratigraphic record in Minnesota indicates that ice sourced from the northeast (Labrador sector) and northwest (Keewatin sector) alternately covered central Minnesota during at least 5 glaciations. Each glaciation is recorded by a till pair or composite till pair. There are two "types" of pre-Late Wisconsinan age till pairs: 1. Red sandy till rich in Keweenawan volcanic and sedimentary rocks overlain by gray clayey till rich in Paleozoic carbonate, or 2. Sandy till rich in Archean greenstone and granite overlain by clayey till containing moderate amounts of both Paleozoic carbonate and Cretaceous detritus. The repetition of distinct pairs of northeast- and northwest-provenance tills shows that the Keewatin and Labrador sectors were enduring phenomena of Laurentide ice. Moreover, the switch between till-pair types in Minnesota reflects continent-scale changes in ice-flow directions, implying that the location of the Keewatin and Labrador ice-accumulation centers shifted significantly throughout the Pleistocene. The effects of these shifts in ice-center location between glaciations should be recognizable in other Pleistocene glacial sequences across North America, providing a possible means of long-distance correlation of scattered and poorly dated pre-Wisconsinan units.

In contrast to the sedimentary record of the pre-Late Wisconsinan, the Late Wisconsinan record in Minnesota does not yield any evidence of vegetative development between advances of Labrador- and Keewatin-source ice. This implies that during at least some earlier glaciations in Minnesota, the climate was warmer, or experienced greater fluctuations.