Are pancakes really so flat? LiDAR and the recognition of subtle geomorphology

The Delmarva Peninsula that comprises the eastern landmass enclosing the Chesapeake Bay is one of those places where you can ride a bike for long distances without changing gears at all. This landscape is incredibly flat, and save for some small drainages here and there, the geomorphology simply fails to impress. This low-lying landscape was constructed over the course of many glacial-interglacial cycles from the Pliocene on through the Pleistocene. During glacial periods, sea levels were depressed by >100 m and erosional processes dominated the landscape, with river systems actively incising into their substrates. Large volumes of sand and gravel were deposited in the north-central Delmarva to form the uplands of this peninsula. During interglacial periods, sea levels rose several meters higher than today, and estuarine-to-marginal marine processes reworked these gravels and filled incised valleys with fine-grained sediments. The legacy of this ancient shallow marine setting is easily grasped in this "flat as a pancake" landscape that is seemingly featureless even to the attentive, through-going geomorphologist.

But the availability of LiDAR (light detection and ranging)- derived elevation data reversed this perception. LiDAR data is collected from airplanes to produce high-resolution elevation data over a landscape of interest. LiDAR data was collected along the coastline of the Delmarva Peninsula to forecast the continued effects of sea level rise. With surface elevations resolved to 30 cm accuracy, elevation data obtained by LiDAR serves this purpose particularly well and has been implemented by planners in coast-proximal counties. But when geomorphologists saw this imagery, they recognized an additional contribution of this data: a subtle yet complex geomorphology was recognized in this region. Suddenly an unremarkable bench in the landscape became a stage on which cold climate (periglacial) processes became active during the Last Glacial Maximum (LGM). While geologists have been dimly aware of some of these features for decades, their extent was under-appreciated and their origins were misunderstood before the availability of high resolution elevation data. These features have since been studied in detail at two areas located at the inferred southern extent of past permafrost: the areas around Snow Hill, Maryland and the Blackwater National Wildlife Refuge (BNWR) just ~50 km northwest.

In Snow Hill, MD a large sand sheet with a NW-SE flow direction blew across the uplands (Fig 2). The leading edge of this sand sheet overrides several elliptical basins that are present at all terrace levels above the modern floodplain. Both the sand sheet and the raised-rim basins have been interpreted to have eolian origins; the sand sheet is a depositional feature sourced from local stream valleys that were dry during the LGM, and the basins are interpreted to be blowout features formed by wind scour. Around the BNWR in west-central Delmarva, a dune field, several elliptical basins, and a large meandering channel exist in complex cross-cutting relations. Long, parabolic dunes emerge from the edge of the Chesapeake Bay and issue southeast (Fig 3-4). These dune forms are truncated both by a large meandering channel form and then elliptical basins. The large meandering channel flows from north to south in the western portion of figure 4, but there is no sign of this river north or south of this location; the landscape offers no clues to its watershed or its mouth. Slightly east, the dunes are cut by the edges of elliptical basins, much like the Snow Hill location. This relationship suggests that the dunes and the basins were active around the same time, and dune material may have been mixing with bottom sediments of these small basins. These, and features like it, are now being recognized along many parts of Delmarva and up to central New Jersey. Thanks to LiDAR data, what was once considered a flat, featureless landscape can now be re-interpreted to help inform about previous the climate and the geomorphic processes in action the last time the Mid-Atlantic was in a deep-freeze.

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