The presence of Cambrian and early Ordovician sediments on the northwest edge of the Piedmont Province of Pennsylvania and Maryland has long been known, but it has not been heretofore suspected that younger Ordovician strata not only overlap these early Paleozoic sediments, but also extend far southeastward onto the crystalline schists of the Piedmont.
The Piedmont upland of eastern Pennsylvania is traversed by a limestone valley which extends southwestward from Schuylkill River, north of Philadelphia, to Littlestown, near the Maryland State line. From the Schuylkill to Quarryville it is narrow and is called Chester Valley. Beyond Quarryville it expands northward into the wider Lancaster Valley around Lancaster. West of the Susquehanna it again becomes narrow, is known as the York-Hanover Valley, and extends to Littlestown. The valley is underlain by Paleozoic limestones. It is bordered on the north‐west in part by the overlapping Triassic sediments and in part . . .