GREEN BAY LOBE DRUMLIN MORPHOLOGY AND SPACING

The Green Bay Lobe (GBL) formed thousands of drumlins during its late Wisconsin advance. GBL drumlin composition varies including disturbed and undisturbed outwash, till and bedrock, which has been interpreted to indicate that at least some subset of the drumlins is erosional while others may be depositional. In addition, a wide range of morphologies are found in the GBL drumlins. Questions remain about the formation process of drumlins and which glaciological controls have a significant impact on their final form and spacing. A better quantitative assessment of the forms and spacing of the drumlins of the GBL may provide insights into the glaciological processes of the Laurentide Ice Sheet.

We have mapped over 13,000 drumlins in the footprint of the GBL using high resolution (2-m) LiDAR derived digital elevation models resulting in 3d morphologies for each drumlin. We determine best-fit ellipsoids for each drumlin to calculate height – length – width ratios that can be compared to spatial factors (e.g. ice advance phase, bedrock geology, depth to bedrock, and local slope). Then we focused on a subset of ca. 5000 drumlins located in Jefferson and Dodge counties where the variability in the geology and topography is minimal to determine the importance of various glaciological factors (i.e. time under the ice, total displacement, bed conditions, ice velocity) have on the final form and spacing of the drumlins.

We determine that in this simplified region time under the ice correlates well with drumlin form. Drumlins that have spent the longest time under the ice were found to have the largest length to height ratios indicating that formation was a continual process. Furthermore, where postglacial fluvial erosion was not significant in the simplified region, drumlin spacing was determined to be random with greater than 95% confidence. Random drumlin spacing is in agreement with the heterogeneous pattern of bed strength that is expected if melting permafrost beneath the ice served as the impetus for drumlin initiation.