The following description was published in Bulletin 20 Stratigraphy of the Post-Potomac Cretaceous-Tertiary Rocks of Central Delaware, Benson, R.N., and Spoljaric, N., 1996:
It is a clayey, calcareous, shelly, glauconitic (10-20 percent) silt. Its colors range from greenish-gray and gray-green to brownish-gray and light gray. It is rich in calcareous and siliceous microfossils. The matrix mineralogy shows a high calcite component, except in the lower part of the formation which is within a calcite dissolution interval. In the lower half of the formation quartz is predominant.
Aragonite is rare or several levels except for the uppermost part of the formation where it shows a significant peak as the Deal grades upward into the Piney Point Formation. Marcasite and, to a lesser extent, pyrite also show major peaks at the same level. Feldspar, dolomite, pyrite, siderite, hematite, analcine, jarosite, alunite, goethite, vivianitc, and phillipsite are present in trace amounts at various levels.
A significant feature of the opaque heavy mineral assemblage is the increase in the amount of andalusite, which replaces tourmaline as the dominant heavy mineral in much of the Upper (middle Eocene) part of the Deal. Kyanite, zircon, rutile and garnet are subordinate constituents, and silliminite increases in the uppermost Deal. Clay minerals are glauconite/smectite, kaolinite, chlorite/smectite, and illite/smectite.