A Cambrian basin lies beneath the Appalachian Plateau of western Pennsylvania; it trends northeast-southwest between shelf sediments of northwestern Pennsylvania, where most of the Cambrian wells are drilled, and the overthrust outcrop belt of central Pennsylvania. The western edge of the basin is believed to be bounded by growth faults which restrict to the basin a thick sandstone herein named the Olin Sandstone. Evidence for growth faults is derived from many changes in regional stratigraphy of deep wells in Potter and Armstrong Counties, Pennsylvania, and Marshall County, West Virginia. The presence of growth faults also is postulated in Lower Ordovician rocks east of the faulting in the Cambrian strata. By projecting the Cambrian and Early Ordovician growth faults to he Precambrian structural contour map of Beck and Mattick, a Precambrian basin is indicated with structural relief of several thousand feet (about 1,200 m).