This doesn't make sense to me; the whole area was buried under an ice sheet during the most recent glaciation, and the hundreds of meters of moving ice would have scraped everything down to bedrock (and scraped off large amounts of the bedrock itself) and prevented any new soil from forming until the ice sheet retreated. Northern Minnesota has only been ice-sheet-free for a blink of an eye, geologically speaking - how did hundreds of meters of soil manage to form in such a short period of time?