Spatial analysis of Des Moines Lobe washboard moraines using LiDAR data

Washboard moraines are a characteristic landform of the Des Moines Lobe. Primarily composed of late Wisconsinan till and oriented parallel to and up-glacier from more conspicuous end moraines, washboard moraines are of low relief (1--2m) and appear to be regularly spaced. These moraines have been assumed in Des Moines Lobe reconstructions to have formed subglacially as basal crevasse fills, with crests developing perpendicular to the ice-flow direction when the lobe was at its maximum extent. Alternatively, the moraines are push moraines formed by seasonal advances of the lobe during its overall recession. The veracity of geomorphic reconstructions of the lobe's thickness relies upon the former hypothesis being correct.;The morphologies and spatial patterns of the moraines may help reveal their origin but are poorly characterized, owing to the topographic subtlety of the moraines. The acquisition of 1 m LiDAR over the Des Moines Lobe's footprint allows washboard moraines to be spatially characterized over broad areas for the first time. After mapping LIDAR-derived elevation data and identifying suitable tracts of moraines, spatial analysis techniques are applied to study moraine spacing and cross-sectional profiles. Using a chi-square test with a significance level of 95%, Fourier analysis of 400 topographic profiles constructed perpendicular to moraine trends reveals a dominant, statistically significant wavelength of 90--110 m. Uniform moraine spacing is expected for crevasse-fill ridges but not for end moraines. The cross-sectional profiles of individual, de-trended moraines are "stacked" to characterize the average moraine shape. This exercise indicates that they lack the systematic asymmetry typically displayed by push-style end moraines, which are generally steeper on their down-glacier sides. Analysis of abrupt changes in moraine trend, called "cusps," indicates that they occur preferentially in the vicinity of outwash trains of sand and gravel: all cusps point up-glacier, and 62.7% are coincident with surface outwash trains. Importantly, well logs indicate that remaining cusps are coincident with subsurface outwash overlain by till, indicating that the outwash pre-dates the last glacier advance and lending support to the hypothesis that subglacial outwash controlled the positions of cusp axes. Outwash trains may have supported anomalously low subglacial water pressures, thereby slowing basal slip in their vicinity and rotating basal crevasses to form the ridge cusps after glacier stagnation. These findings are consistent the Des Moines Lobe undergoing surge-like motion, with longitudinal extension creating transverse crevasses and stagnation allowing weak basal till to intrude upward into them.