Assessing the style of advance and retreat of the Des Moines Lobe using LiDAR topographic data
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Successive advances of the late-Wisconsinan Des Moines Lobe to form three major end moraines in Iowa--sequentially the Bemis, Altamont, and Algona moraines--are thought to be the result of the lobe surging out of balance with a warming climate. Various styles of hummocky topography, collectively sometimes called stagnation moraine, are interpreted to be the result of widespread stagnation and down-wasting of ice following surges. Alternatively, end moraines could be recessional--a result of incremental back-wasting of the glacier margin and unrelated to surging.
To study the retreat style of the Des Moines Lobe, high resolution LiDAR data were used to re-evaluate the subtle landscape of the lobe's footprint in Iowa. Results indicate that ~90% of the lobe's area, excluding major Holocene stream drainages, consists of stagnation features. Some landforms are more prevalent than mapped previously, including eskers and features interpreted to be subdued ice-walled lake plains. Importantly, subglacially formed minor moraines (a.k.a. washboard moraines), which resulted from sediment filling of transverse crevasses, cover ~60% of the lobe's area with stagnation landforms. Also, ~25 previously unmapped end moraine ridges have been identified.
Transverse crevasse-fill ridges in the forefields of modern glaciers form due to longitudinal ice extension associated with surging and are not found in the forefields of non-surge-type glaciers, so minor moraines are good evidence of Des Moines Lobe surges. Most end moraines have minor moraine sets associated with them, consistent with a surge-like advances, and many areas have multiple sets of minor moraines indicating a surge history more complicated than one advance for each of the three major end moraines. Therefore, asserting stagnation and down-wasting after three surge-like advances provides an incomplete characterization of the Des Moines Lobe's advance and retreat. The surge-type Bering Glacier in Alaska is a good but imperfect modern analog for the lobe.