The Upper Cambrian Gatesburg Formation of northwestern Pennsylvania (Erie, Crawford, Mercer Counties) is almost 1,000 ft thick and consists of oolitic sandy dolomite; two 100- to 150-ft thick sandstone units, previously called "Upper Sandy and Lower Sandy members," occur at the top and middle of the formation. One hundred twenty-five miles southeast at outcrop in central Pennsylvania, the Gatesburg is 1,500 ft thick and is similar in lithology to the northwestern Pennsylvania strata. Recent drilling between these two areas indicates that the Gatesburg thickens to more than 1,900 ft and is of different lithology in the intermediate area. The two sandstone units of northwestern Pennsylvania are replaced by dolomite, and a sandstone body, 200-350 ft thick, occurs stratigraph cally below the position of the sandstone units of northwestern Pennsylvania. Apparently no strata represent this thick sandstone in northwestern Pennsylvania. The additional thickening and the different lithologic sequence of the Gatesburg strata in this intermediate area are the result of deposition in a northeast-trending basin whose western edge is interpreted to be a growth fault.

Lower Ordovician (Beekmantown) dolomites and limestones thicken from zero in northwestern Pennsylvania to more than 3,500 ft in central Pennsylvania. The thickening also results from a growth fault which trends northeast and lies east of the fault in the Upper Cambrian rocks.

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