Butlers Bluff Member (Mixon, 1985). Pale-gray to light yellowish-gray, fine to coarse, cross-bedded, pebbly sand and sandy gravel comprising surficial deposits of upland (altitude 35-40 feet.). Diverse molluscan assemblage in lower part of unit, including Marginella, Mulinia, Nassarius, Spisula, Pleuromeris, and Olivella, indicates a shallow, nearshore-shelf depositional environment. Unit was deposited as a southward-building complex of spit-platform sands and shallow shoals and is as much as 60 feet in thickness. In subsurface, unit overlies 140 feet, or more, of pebbly to cobbly sand, clay silt, and muddy fine-grained sand of the Stumptown Memberof the Nassawadox Formation, which fills a late Pleistocene paleochannel of the Susquehanna River system.