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.
They yield valuable coals, clays, marls and ganister.
It is marked by grey clays and sands, lignitic fossiliferous clays, beds of lignite or brown coal, sometimes 8 ft.