Six Coastal Plain formations and one group crop out in the Round Bay quadrangle near the inner edge of the Atlantic Coastal Plain physiographic province. The quadrangle lies astride the Severn River, in Anne Arundel County, near Annapolis, Md. The seven stratigraphic units aggregate as much as 128 m in outcrop. In ascending order, the units are: the upper part of the Potomac Group and the Magothy, Matawan, and Severn Formations, all of Cretaceous age; the Brightseat and Aquia Formations of Paleocene age and the Calvert Formation of Miocene age.
Quaternary deposits are thin and cover only small areas; they are all mapped under one unit. Several small, thin deposits of Tertiary alluvium are mapped separately.
The largely unconsolidated Cretaceous and Tertiary formations consist chiefly of quartz, glauconite, clays, muscovite, chlorite, lignite, feldspar, and pyrite. Quaternary sediments are mostly locally derived sands, silts, and clays with some gravel and, in the finer sediments, considerable amounts of organic matter.
The Cretaceous and Tertiary units strike generally northeast; the younger the formation, the more easterly it strikes. Dips are gentle, 3.6 to 15 m per kilometer toward the southeast, and decrease upward through the section.
The Round Bay quadrangle is near the southern limit of several formations that thin progressively toward the southwest from New Jersey. Some pinch out between Betterton, on the eastern shore of Chesapeake Bay, and Round Bay, on the western shore, whereas others are present only as thin remnants 1-2 m thick. Resources of the quadrangle include abundant ground water, sand, and high land values near water.
