MINNESOTA GLACIATIONS FROM OLDEST TO YOUNGEST
Only a modest number of magnetic polarity measurements have been done in the state; they show the great majority of samples are of normal polarity. Recent studies in the down-ice states of Iowa and Missouri show extensive pre-Brunhes deposits going back over 2 million years, but their equivalents in Minnesota appear to be thin and patchy, presumably eroded by later ice advances. This patchiness makes it difficult to correlate the oldest tills lithologically, especially because they tend to contain large amounts of local bedrock and saprolite.
Provenance differences become more distinct in the middle Pleistocene.Tills have been widely sampled, and are being correlated and formally named. Oxidized and leached zones tend to be thicker in these tills than in the late Pleistocene tills. A general pattern can be discerned of alternating northwest (Winnipeg) and northeast (Rainy or Superior) provenance, interpreted as couplets of northwest over northeast tills within a glaciation, similar to the pattern in the last glaciation. The northeast provenance tills tend to be thinner and thus perhaps less likely to be preserved. Scattered paleosols and organic-rich sediments have been described, but not a complete framework.
The Wisconsinan section is complex, but better studied than the older sediments. The time of the earliest ice advance is unclear. Loess stratigraphy in other states suggests that the Superior lobe was active in Stage 4, but no definitive dates in Minnesota confirm this inference. Ice margins and till sheets that have been dated in Minnesota are attributable to Stage 2. Overall, the picture is of northeast provenance tills overlain by northwest provenance. Till rich in Cretaceous Pierre Shale fragments (Riding Mountain provenance) entered Minnesota for the first time in the Wisconsinan, from ice streams sourced in the northern Great Plains.