The late-glacial (<∼>15 kyr B.P.) advance of the soft-bedded Des Moines Lobe, the marginal portion of an ice stream of the Laurentide Ice Sheet, deposited regularly-spaced, transverse moraines. These so-called washboard moraines have relief of 1-5 m, an average spacing of <∼>105 m, and are distributed over the footprint of the Des Moines Lobe. Determining the processes responsible for the formation of these moraines may help illuminate aspects of Des Moines Lobe dynamics and origins of submarine transverse ridges adjacent to modern ice sheets.

Washboard moraines may have formed as a result of weak basal sediments extruding into transverse crevasses in basal ice as the lobe underwent flow-parallel extension. To test this hypothesis, a cross-section (70 m long and up to 4 m high) was exposed across a prominent washboard moraine crest that is transected by 610^th Avenue, 16 km southeast of Ames. The section was described, and sediment samples were collected to study their density, grain-size distribution, and preconsolidation pressure. Anisotropy of magnetic susceptibility (AMS) of 3200 intact till samples was used to characterize till fabrics with orientations of principal susceptibilities. AMS fabrics were interpreted using ring-shear experiments that allow fabric orientation to be calibrated to the state of till strain.

The exposed cross-section revealed till with irregular, isolated lenses of silt, sand, and gravel that generally dip steeply up-glacier. Preconsolidation pressures, measured from intact till and silt samples, and till densities indicate that these sediments were under the glacier, precluding the possibility that they were deposited supraglacially at the glacier margin. Moreover these sediments could not have been pushed into a moraine ridge at the glacier margin without resetting preconsolidation pressures to significantly lower values. AMS fabrics in the moraine are consistent with flow-parallel shear, with shear planes inclined mildly up-glacier, together with flow-parallel shortening accompanied by vertical and lateral extension. This strain field is consistent with shearing and extrusion of weak basal till and subglacial fluvial sediments into basal crevasses during slip of the lobe <—> a conclusion supported by previous less detailed studies of several nearby moraines. These results provide support for reconstructions of the Des Moines Lobe and imply that the distribution of these landforms was spatially rather than temporally controlled.