This paper presents results of study of the structural and overlap relations of several formations of Tertiary age in the region east of Washington, Baltimore, and Fredericksburg. Attention is also given to the relations of various formations of Cretaceous age (Upper and Lower).
The altitude of the base of the Calvert formation (Miocene) was determined at many places, and from these data structure contours were constructed as shown in Plate 1. These show that in a wide area the base of the Calvert has a fairly regular easterly slope of about 10 feet to the mile, with a somewhat steeper rate in the outliers west of Washington. In the Baltimore region this southeasterly slope averages nearly 15 feet to the mile, also with some steepening on the west. In the Fredericksburg region the slope is less than 10 feet to the mile, and farther south this rate carries the plane of Calvert overlap west onto the crystalline rocks.
The Econe succession which I named the Pamunkey formation in 1891 was later divided by W. B. Clark into a lower formation, Aquia, and an upper one, Nanjemoy, the latter having a basal member of clay, named Marlboro. Near Washington this clay member thins out to the west under the Miocene, the edge of which also extends considerably farhter west than the edge of the overlying greensand of the Nanjemoy. The last named gradually thickens to the east and south. South of Fredericksburg the Aquia formation overlaps the edge of the Potomac group slightly, onto the old crystalline rocks, and the Potomac in turn is overlapped by the Calvert formation (Miocene), the Nanjemoy formation with its Marlboro clay member having thinned out or been eroded.