The earlier wells in Pennsylvania consisted of three sections, the first formed of surface clays and gravels, the second of stratified rocks containing water, and the third of stratified rocks, including the oil-sands, usually free from water.
Clays of all qualities and colours abound.
The best soils are in the west section, where limestone clays or shell marls are common.
It is marked by grey clays and sands, lignitic fossiliferous clays, beds of lignite or brown coal, sometimes 8 ft.
The latest Cretaceous is the Ripley formation, which lies west of the northern part of the last-named, and, about Scooba, in a small strip, the most southerly of the Cretaceous - it is composed of coarse sandstones, hard crystalline white limestones, clays, sands, phosphatic greensands, and darkcoloured, micaceous, glauconitic marls; its greatest thickness is about 280 ft.