Named the Pinesburg member of the Shippensburg formation in western MD and south-central PA for Pinesburg Station, Washington Co., MD. Unit is basal member of formation. Consists of dark gray, fine- to medium-grained limestone, cobbly below and platy above with three metabentonite beds. At base in type section, is dark gray to black, medium-grained, dark platy-weathering limestone with irregular partings; above is thicker unit consisting largely of dark gray, fine-grained, cobbly, blue-weathering limestone with prominent thick silty partings; at top is dark gray, fine- to medium-grained, platy to slabby limestone. Thickness is 21 to 238 feet (114 feet in type section). Disconformably overlies "Stones River" limestone and conformably underlies Fannettsburg member. The Pinesburg is of Middle Ordovician age.

Source: GNU records (USGS DDS-6; Reston GNULEX).