ASSESSING THE RETREAT OF THE DES MOINES LOBE USING LIDAR DATA

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, we used high resolution LiDAR data to re-evaluate the subtle landscape of the lobe’s footprint in Iowa. Results indicate that ~90% of the lobe’s area consists of stagnation features. Some landforms are more prevalent than mapped previously, including eskers and subdued ice-walled lake plains. Importantly, subglacially formed minor moraines (a.k.a. washboard moraines), which resulted from sediment filling of transverse crevasses, cover ~75% of the lobe’s area. 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 during surging, so minor moraines are good evidence of Des Moines Lobe surges. Many areas have three sets of minor moraines indicating a surge history more complicated than one advance for each of the three major end moraines. Many end moraines have minor moraine sets associated with them, consistent with a surge-like advance. Others, however, have no minor moraine set and truncate minor moraines, indicating that these end moraines are probably recessional and superimposed on previously formed minor moraines. Stagnation and down-wasting, therefore, is an incomplete characterization of the retreat style of the Des Moines Lobe.