The following description was published in GM15 Geologic Map of the Georgetown Quadrangle, Delaware, Ramsey, K.W., 2010:
Bioturbated, dark-greenish-gray silty clay, banded light-gray, white, and red silty clay, and glauconitic, shelly, very fine sandy silt. In the Georgetown Quadrangle, the St. Marys Formation is capped by about 5 to 15 ft of bioturbated, dark-greenish-gray silty clay. A distinct burrowed horizon separates the clay from the underlying banded clay that consists of a 10- to 15-ft thick, compact, color-banded silty clay with scattered white clayey concretions. The banded clay has a sharp contact at its base with underlying glauconitic, very fine, sandy silt. The sandy silt contains shells of the gastropod Turritella. The entire thickness of the St. Marys Formation is less than 100 ft in the Georgetown Quadrangle, thinning from its thickest in the southeast corner to about 50 ft thick in the northwest corner of the map area. Interpreted to be a marine deposit of late Miocene age (McLaughlin et al., 2008).
The following description was published in GM14 Geologic Map of Kent County, Delaware, Ramsey, K.W., 2007:
Light reddish-brown to gray, fine to very fine, silty sand and clayey silt. Discontinuous beds of fine to medium quartz sand are common. Base of unit in the Milford area (Ramsey, 1997) is a medium sand bed ranging from 10 to 15 feet thick. Found in the southeastern portion of Kent County. Patchy in distribution where it occurs beneath Quaternary deposits. Thickness ranges up to 30 feet. Interpreted to be a shallow marine deposit.