The Shawangunk conglomerate, basal Silurian formation in south‐eastern New York and in New Jersey, is markedly unconformable on underlying Ordovician rocks. This unconformity represents an interval of folding, uplift, and erosion, and this upheaval was apparently more intense in New York and has been there called the Taconic revolution. In southern Pennsylvania the Juniata sandstone lies conformably on the Martinsburg shale and is followed by the Tuscarora sandstone, and no apparent break in sedimentation follows either the Martinsburg or the Juniata. A reconnaissance survey of the upper contact of the Martinsburg shale was made across Pennsylvania for the State Survey to determine the nature and extent of the unconformity. It was learned that unconformable relations extend as far southwest as Susquehanna Gap, and that in Pennsylvania this unconformity consists chiefly of the erosion of the upper member of the Martinsburg shale and the overlap of the basal conglomerate . . .