An upland capping of gravel and overlying silt (Pliocene ?) covers approximately 600 square miles of the Coastal Plain of southern Maryland. This sheetlike deposit, which successively overlaps older formations to the northwest, has a fairly constant thickness of 25–30 feet, and dips southeastward at approximately 5 feet per mile.
In the northern part of the upland, size and composition of the gravel show orderly changes eastward, which is also the direction of sediment transport as indicated by cross-bedding and gravel fabric. In the southern part of the area, facies changes are less orderly, and pronounced anomalies appear. These anomalies characterize both scalar and vector properties and therefore record a modification in conditions of deposition, probably induced by early Pleistocene climatic changes.
The upland deposits are fluvial in origin. They were probably deposited by a graded ancestral eastward-flowing Potomac River which, in the process of lateral corrasion, spread a veneer of channel gravel and flood-plain silt. Later side- and down-cutting combined with a gradual southeastward migration of the river, resulted in the present large, thin gravel sheet.