Character and origin of De Geer moraines in the Seacoast region of New Hampshire, USA

Previously unrecognized clusters of regularly spaced, low-relief ridges were recently revealed by LiDAR data acquired in the Seacoast region of New Hampshire, USA. Based on their morphometry and geologic context, these features are interpreted as De Geer moraines that formed during regional deglaciation. While De Geer moraines are widely recognized as products of ice recession in glaciomarine settings, their formative processes remain debated. Here, we integrate geospatial mapping, morphometric analyses, stratigraphic investigations and ground-penetrating radar (GPR) subsurface profiling to characterize De Geer moraines in New Hampshire and assess their origin. On average, the moraines are 1.2km long, 28m wide, 1.7m tall and spaced 104m apart. Trenches expose stratigraphic units consistent with subaqueous deposition along a tidewater glacier margin, including subglacial traction till indicative of proglacial pushing, and gravity-driven flow deposits on an ice-distal moraine slope. GPR profiles reveal variable till thicknesses beneath the ridges and stratified sediment in adjacent swales. Our findings suggest De Geer moraine formation was driven by annual oscillations of the retreating Laurentide Ice Sheet margin. In this interpretation, average moraine spacing would represent a retreat rate of approximate to 100m a(-1), which is in line with previous estimates of recession rates across New England.