Long-age puzzle of thin ice at the edge of the Laurentide Ice
Sheet The margin of the Laurentide Ice Sheet in North America
was generally composed of lobes, possibly as the result of rapid
movement or surges to the south. The Des Moines Lobe (DML) was
the largest of several lobes that extended into the mid-
continental region of the USA. The DML, determined by mapping
lateral and end moraines, was up to 250 km wide and covered more
than 100,000 km2 in southern Minnesota and north-central Iowa.
Reconstructions of the surface morphology of the DML at its
maximum, assuming that the thickness of the ice at the edge was
as high as the horseshoe-bounding moraine, have indicated that
the DML was probably thin and gently sloping.1,2 Such
measurements have produced a puzzle for glaciologists. The basal
shear stress, which depends upon the weight of ice and the sine
of the surface slope, is calculated to be very low—about 10
times smaller than values considered typical for glaciers.3
Thus, the driving force to move the