MULTIPLE INDEPENDENT LINES OF EVIDENCE FOR THE SYSTEMATIC ADVANCE AND RETREAT OF THE DES MOINES LOBE, NORTH-CENTRAL USA

Debate over the Des Moines Lobe (DML), one largest outlet glaciers/ice streams of the Laurentide Ice sheet, still persists in the literature despite a century of research. In this investigation we use multiple independent lines of geomorphic and sedimentological evidence that allow us to address several long-standing issues associated with DML chronology, dynamics, and origin of landforms. These issues include: the timing of the advance to the maximum limit along the left lateral margin of the lobe, origin of landforms suites including widespread washboard moraine and subglacial esker/tunnel valley complexes, and systematic longitudinal variation in till texture and composition.

The left lateral margin of the DML advanced into central and eastern Minnesota not as a broad ice sheet but rather as a series of individual lobes focused through narrow troughs in the pre-existing Alexandria moraine. These lobes then coalesced as a series of piedmont lobes, the most prominent of which are the Grantsburg Sublobe and the Bonanza Valley Lobe. Relationships indicate the DML advanced to its maximum extent in central and eastern Minnesota and western Wisconsin by about 14 ka BP as the lobe reached its maximum limit in central Iowa.

Contrary to some interpretations, evidence indicates that the DML systematically advanced and retreated as a single integrated lobe punctuated by episodic surges. Subglacial tunnel valley/esker complexes draining the DML on its right lateral margin were continuously occupied for at least 2000 years (and possibly longer) during overall ice retreat from 14 ka to 12 ka BP. Sub-parallel, low relief ridges of till oriented transverse to ice flow evident on 1 meter LIDAR as well as the prominent recessional moraines within the Minnesota River Valley indicate the DML has an observable recessional geomorphic record throughout its extent in south-central MN. Lastly, lithologic variability of DML till over the areal extent of the washboard moraines forms an integrated pattern indicating that the DML deposited a single till sheet in a single glacial advance.