Using LiDAR Digital Elevation Model data to map Lake Agassiz beaches, measure their isostatically-induced gradients, and estimate their ages

High-resolution LiDAR Digital Elevation Model (DEM) data were used to create DEM mosaics and hillshade maps of a large region in North Dakota for better defining and measuring the strandlines of glacial Lake Agassiz. Although many of the strandlines in this region had been identified and named more than a century ago by Warren Upham, LiDAR images clearly reveal their complexity, and identify additional small and low relief ridges not previously mapped. The elevations of the crests of six well-developed strandlines – the Campbell, McCauleyville, lower Blanchard, Emerado, upper Ojata, and lower Ojata beaches – were measured. For each of these strandlines, north–south gradients were calculated, which were produced by differential isostatic rebound. The steepest gradient is on the uppermost (Campbell) beach in the sequence and the gradient on each beach below is progressively lower, confirming that these beaches were deposited in sequence from oldest to youngest as the level of Lake Agassiz declined. Because there were no anomalous beach gradients in this sequence, the Ojata beach formed after 9400 14C (10,630 cal) BP, which is when the higher-elevation Campbell beach was deposited, rather than at ∼10,000 14C (11,480 cal) years ago as some have suggested because of the age of organics below the Ojata beach. Those organics and the subaerial surface on which they lie survived the Moorhead transgression to the Campbell beach, and the Ojata beach must have been deposited over the older organic unit later, as lake level regressed. A simple extrapolation model was used to estimate the ages of beaches in the sequence.