MAPPING RECESSIONAL PHASES OF THE SUPERIOR LOBE ALONG MINNESOTA'S LAKE SUPERIOR SHORELINE

The recession of the Superior lobe was interrupted by numerous readvances. After the ice had receded into the Superior basin, each readvance incorporated glacial-lake sediment deposited in the basin during the previous retreat. The lake that was created each time is here considered to be a phase of glacial Lake Superior in a broad sense. Each readvance of ice covered a smaller area, reached a lower elevation, and deposited a finer-grained till than the previous one.

The first retreat into the basin did not create a large continuous lake, but rather a set of small ice-marginal lakes trapped between the ice and the sides of the basin. Meltwater escaped by a series of ice-marginal channels, building ice-contact deltas into ponds on its way out of the basin to the southwest. The next advance of the Superior lobe overrode these deltas and deposited a loamy 5YR till which ends at the Highland moraine. Another retreat uncovered a portion of Lake Superior about as far northeast as Two Harbors, followed by the Split Rock advance that spilled out of the basin to the southwest and deposited a silty 5YR till. Another retreat opened up a larger portion of Lake Superior, followed by an advance that deposited a clayey 2.5YR till and built the Nickerson moraine at the southwest margin of the basin.

The next retreat probably opened up the entire lake basin for about 1000 years, which allowed Lake Agassiz to spill east through Lake Superior. A considerable amount of suspended sediment from glacial Lakes Aitkin-Upham and Agassiz was deposited in the basin at this time. The last, or Marquette, advance filled the basin with ice again, depositing a very clayey 2.5YR till with few pebbles that extended as far as the proximal side of the Nickerson moraine.

This interpretation of Superior-lobe recessional history was created by combining earlier mapping in the region with recent detailed mapping of 1:24,000 quadrangles northeast of Duluth, supported by the USGS Statemap program. These maps are intended to help guide the rapid development now occurring along Minnesota’s Lake Superior shoreline.