Absent drumlins beneath southern lobes of the Laurentide Ice Sheet: A new hypothesis based on Des Moines Lobe dynamics inferred from landforms

Spatial distributions of drumlin fields encode information about ice sheet dynamics. No drumlins formed beneath the most lobate parts of the Laurentide Ice Sheet's southern margin, in South Dakota, Iowa and Illinois, whereas ice lobes to the northeast generally produced drumlins. This pattern may have resulted from northerly ice overriding permafrost. Here we propose a new hypothesis for this pattern by constructing a LiDAR-based landform map and applying a model of drumlin formation to account for the absence of drumlins beneath the largest of the ice sheet's southern lobes, the Des Moines Lobe. Broad belts of hummocky topography, ice-walled lake plains, doughnuts and minor moraines, which together cover 90% of the lobe's upland area in Iowa, attest to widespread ice stagnation, as does the lobe's scarcity of eskers. Most stagnation topography is subtle, with insufficient relief to have obscured drumlins that might have formed before stagnation. Minor moraines are crevasse-squeeze ridges diagnostic of surging, and their ubiquity indicates that during surging, the lobe's soft bed was weak nearly everywhere. End moraines are generally parallel to a minor moraine setup-glacier, implying that surge-driven advances were more numerous than indicated by the three major end moraines of the lobe. In the only physically based model of drumlin formation that includes surging, till deposition occurs during surges when effective pressure is uniformly low, whereas drumlins develop during quiescent flow between surges, when basal slip and low-pressure R-channels create the spatial gradients in effective pressure necessary to sculpt drumlins by differential erosion. Landforms of the lobe, however, indicate stagnation and down-wasting during quiescence, without significant basal slip or hydraulic potential gradients necessary for R-channels. We hypothesize that for the three southernmost lobes of the Laurentide Ice Sheet, surging followed by widespread down-wasting of stagnant ice prevented drumlin formation, whereas beneath northern lobes it was permitted by climatically-forced ice advance. No drumlins formed beneath the Laurentide Ice Sheet's southernmost margin, including beneath the Des Moines Lobe, whereas more northerly ice produced drumlins. Inferences from new a LiDAR-based map of Des Moines landforms indicate that the lobe surged and stagnated many times. These observations, interpreted within the context of a model of drumlin formation, indicate that along the southernmost ice-sheet margin, surging and stagnation may have prevented drumlin formation whereas to the north, it was permitted by climatically-forced ice advance.image