The crystalline rocks of the Piedmont region in Virginia and Maryland are flanked eastward by an overlapping series of deposits of later Mesozoic and Cenozoic age, which extend thence to the Atlantic ocean, a distance averaging one hundred miles. This district has been designated the “coastal plain,” and is in general terms the continuation of the Piedmont plains which, with gradually decreasing elevation, finally pass beneath sea-level.
The eastward inclination of the crystalline rock-surface on which the coastal plain deposits lie is slight, and the width of the zone of overlap from the feather edges and outliers of the formations to the final disappearance of this floor below tide-level usually averages about ten miles.
The irregular western terminations of the various formations usually do not give rise to notable topographic features, and in the larger drainage depressions the crystalline rocks finally disappear below the clastics, generally at a Considerable . . .