Minnesota was covered by glaciers many times during the last million years, with ice flowing across the state from both the northeast and the northwest and depositing thick layers of glacial drift. About 25,000 years ago, the Superior lobe advanced from the northeast, eroded mostly igneous bedrock in Ontario and the Lake Superior region, and left a reddish, sandy, rocky drift that produced soils of limited fertility. About 16,000 years ago, the Des Moines lobe advanced from the northwest, carrying sedimentary marine shales and limestone from the Winnipeg Lowlands in southern Manitoba and depositing a clay-rich, lime-, magnesia-, and potash-rich drift that produced fertile agricultural soils. In the Elk River region, deposits from both ice advances lie close together, and much of the fertile surface material was laid down by the Grantsburg sublobe, an eastern offshoot of the Des Moines lobe, while older Superior lobe deposits remain near the surface and may be mixed with them, causing soil fertility to vary. Sand and gravel river terrace deposits are also common near Elk River, and the Mississippi River took its present course during the glaciers’ final decline as meltwater followed different routes before cutting the valley the river now occupies.