New data on the Ordovician-Silurian relations in Pennsylvania make possible reinterpretation of long-known facts. These data were obtained through the reopening of abandoned railway tunnels in south-central Pennsylvania and in highway construction elsewhere. It appears now that the term Oswego was misapplied in Pennsylvania. There is probably little or no true Oswego in the State. For beds commonly referred to by that name, the term Bald Eagle is revived and treated as basal Juniata. There is a disconformity between the Bald Eagle and the Tuscarora east of the Susquehanna River and either a disconformity or unconformity below the Bald Eagle or Tuscarora or Shawangunk from the Susquehanna River into southeastern New York. Either hiatus might be used as a convenient Ordovician-Silurian line of separation. If the lower is accepted, it puts the Juniata in the Silurian; if the upper be used, then the Juniata becomes Ordovician. The magnitude of the lower hiatus and the structural data associated with it indicate that the Taconic disturbance affected Pennsylvania probably as far west as the Susquehanna Valley.

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