FUN FACTS FROM THE WASHINGTON EAST 1:100,000 QUADRANGLE

The Washington East 1:100,000 quadrangle stretches from the eastern part of Washington, D.C., to the western part of the Delmarva Peninsula. About 2 km² in the far northwestern corner is mapped as the metasedimentary Laurel Formation of the Piedmont. The remaining 99.96% is within the Coastal Plain: the western half is a dissected plateau with exposures from Cretaceous to Miocene; the eastern half includes the Chesapeake Bay and low, estuarine, bay-facing terraces. Fluvial gravel underlies muted uplands of Delmarva. Two buried Mesozoic basins, trending northeast, underlie parts of the map area.

Newly exposed mammal-dominated tracks near the north-central margin of the map area are from the youngest Potomac Group, thus, Late Cretaceous. Football fans will note that latest Cretaceous and Paleocene sediments underlie FedEx Field. Paleontologists worldwide flock to the Calvert Cliffs, spectacular Miocene exposures along the western shore of the Bay, to find shark teeth and macro- and microfossils. Successively younger Quaternary terrace deposits step down to the rivers and the Bay, recording sea-level highstands. New evidence from cores and geophysics documents a complex network of paleochannels during lowstands and allows nuanced interpretation of Quaternary history in general and of the Bay in particular. For our mapping, LiDAR has been transformational, permitting the identification of features such as marine scarps; windblown dunefields; and elliptical, raised-rim, shallow basins. Combining LiDAR with field observations has allowed new insights on freeze-thaw processes and deposits. The area is more structurally complex than previously thought; high-angle Coastal Plain faults are related at depth to Mesozoic rift basin margins. Recent assessments indicate potential for undiscovered natural gas reserves in these buried basins. Paleochannels from ancestral Hudson/Delaware, Susquehanna and Potomac Rivers are locally dominated by sands and gravel, easily mined, resulting in landfill sites. Although most of the islands within the Chesapeake Bay are losing land area, Poplar Island is an example of artificial fill and an engineered wildlife sanctuary; it is being rebuilt using material dredged near Baltimore.

Not fun, but valuable to know, is that the Eocene Marlboro Clay is susceptible to landslides.