RAINY-SUPERIOR LOBE INTERACTIONS NORTH OF THE CONTINENTAL DIVIDE IN NORTHEASTERN MINNESOTA

Recent mapping of Quaternary deposits in the Babbitt-Isabella area of northeastern Minnesota has clarified the interactions between the two major Wisconsinan-age lobes in the region: the Rainy lobe and the Superior lobe. Rainy lobe ice flowed generally from the northeast across the Canadian Shield, composed mostly of Archean igneous and metamorphic rock. The Superior lobe also flowed generally from the northeast following the Lake Superior basin, which is rimmed by Mesoproterozoic lava flows and intrusions, and floored by Precambrian red sandstone. The two lobes were thus parallel and confluent, but can be distinguished by their distinctive clast suites.

Superior-lobe morphology in the area is dominated by ice-walled lake plains in the Highland moraine, and by streamlined bedrock and till (Highland flutes) behind the moraine. Rainy-lobe morphology is characterized by drumlins south of the continental divide (Toimi drumlin field), by recessional moraines and outwash plains north to the Vermilion moraine, and by flutes and Rogen moraine north of the Vermilion moraine. Rogen moraine is restricted to areas where the ice was advancing up an adverse slope.

South of the continental divide, the outwash of both lobes extends southwest from the ice margins. As the ice retreated however, meltwater was unable to flow south, but instead ran along the outer margin of the Rainy lobe to lower ground westward. A series of channels and outwash plains followed the retreating ice margin. Most of the meltwater was sourced from the Superior lobe. Evidence of catastrophic outbursts from under the ice includes two separate erosional gorges, and large clasts deposited many miles from the source.

Most of the Superior meltwater drained between the Vermilion moraine and earlier recessional moraines. Outwash surfaces are moderately to highly pitted in most places, but include some smooth areas from which the original surface gradient can be reconstructed. The meltwater built deltas into glacial Lake Dunka and flowed across the lake floor after the lake drained. This outwash continued west, building a delta into glacial Lake Norwood, which was confluent with Lake Koochiching, an early eastern arm of Lake Agassiz.