A HYPOTHESIS FOR THE ABSENCE OF DRUMLINS BENEATH THE SURGE-TYPE DES MOINES LOBE

The lowest reaches of the Des Moines Lobe (DML) seemingly behaved like a surge-type glacier. LiDAR-based mapping indicates that 90% of its footprint in Iowa, excluding modern drainages, consists of stagnation topography, such as hummocky moraines and ice-walled lake plains. Two-thirds of this stagnation topography consists of minor moraines; their geometry, sedimentology, and geotechnical properties point uniformly to their origin as crevasse-fill ridges—landforms diagnostic of post-surge stagnation. Moreover, most of 25 newly mapped end-moraine ridges have minor moraine sets associated with them, suggesting that surges were more numerous than indicated by the three primary end moraines of the lobe.

Notably lacking, however, are landforms streamlined parallel to flow that are commonly associated with surge-type glaciers in Alaska, Iceland, and Svalbard. These landforms developed in many cases in the absence of a cold-based glacier margin. Thus, lack of evidence for a cold-based DML margin is an insufficient argument for the lack of streamlined landforms such as drumlins, which formed beneath more northerly lobes.

A new mathematical model based on data collected from the active drumlin field at Múlajökull, a fully temperate surge-type glacier in Iceland, provides an alternative hypothesis for the absence of DML drumlins. In the model, drumlin relief develops during quiescent (normal) flow, when high effective stresses cause the till bed to behave rigidly resulting in slip over bed undulations, and when basal water moves in channels under low water pressure. These factors cause spatial gradients in basal effective stress that provide the instability for relief production (i.e., as drumlins grow, effective stress gradients that drive drumlin growth become larger). The DML, however, may have been so far out of balance with the prevailing climate that there was little or no ice flow after surging events, a conjecture consistent with geomorphic reconstructions of the DML’s profile. Resultant negligibly small basal slip velocity and resultant basal water flow insufficiently vigorous to focus in closely spaced, low pressure channels could have prevented the effective stress gradients necessary to form drumlins.