Broad paleostructural development in south-central Pennsylvania and Maryland was similar to that of central and eastern New York during the deposition of lower Middle Ordovician beds. Thick marine limestones of the easternmost belts of outcrop in the Cumberland Valley indicate the southward extension of the Champlain miogeosyncline. Thin and, in part, coarse-textured limestones in belts of outcrop just west of the Cumberland Valley indicate the extension of the Adirondack arch. Intermediate correlation shows the development of a relatively shallow platform east of the Adirondack arch and marginal to the Champlain miogeosyncline during part of the Bolarian (Kay, 1947) and Trentonian epochs.

Detailed description, definition and correlation of the stratigraphic units are presented. The Pinesburg and Fannettsburg members (new) of the Shippensburg formation (redefined), the Housum and Kauffman members (new) of the Mercersburg formation (redefined), the Oranda formation (Cooper and Cooper, 1946) and the lower Martinsburg exhibit westward thinning by overlap or convergence or both from Champlain miogeosynclinal sections in eastern belts to sections on the Adirondack line in Path Valley. The Doylesburg member (new) of the Shippensburg is believed truncated to extinction southeastward from Path Valley by pre-Mercersburg regional unconformity. Argillaceous material in southeasternmost exposures of Oranda begins the Middle Ordovician transgressive clastic deposits resulting from the Vermontian disturbance.

Correlations indicate Shippensburg is post-middle Chazyan to pre-Chaumont in age, Mercersburg is Rockland and Kirkfield, Oranda is Shoreham, and lower Martinsburg is Denmark.

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