Study of stratigraphic sections in the Green Pond-Schunemunk Mountain outlier and along the main Silurian-Devonian outcrop belt 25 mi to the northwest has resulted in alteration and refinement of regional stratigraphic relations of Upper Cayugan and Helderbergian strata in southeastern New York and northern New Jersey. Many well-established stratigraphic units of New York and eastern Pennsylvania can be recognized in the outlier. Two new units in the outlier are here proposed—the Berkshire Valley Formation and the Orrs Mills Member of the Rondout Formation. A tentative paleoenvironmental analysis has resulted in the recognition of eight facies: fluvial redbed; supralittoral; littoral lagoonal; sublittoral lagoonal; biostromal reef; shallow sublittoral (crinoidal meadow and sandstone); intermediate sublittoral; and deep sublittoral arranged in basinward order. Seven recognizable time horizons were used in the establishment of stratigraphic correlations. The approximate geographic distribution of paleoenvironments at eight successive horizons during the late Cayugan and Helderbergian are shown on a series of lithofacies-paleoenvironmental maps. It can be inferred from these maps that the report area was situated on the eastern side of a generally deepening northern Appalachian basin. The northeast-trending shoreline was predominantly located within the report area, but at times receded to the west or advanced to the east. A sediment source was continually active in eastern Pennsylvania south of the main outcrop belt, whereas a source east of the northern portion of the outlier was active only during the late Helderbergian. During the deposition of the Berkshire Valley Formation, the southern part of the outlier appears to have acted as a strait that connected the northern Appalachian basin to a sea located to the south-east. The southern part of the outlier was subjected to rather extensive pre-Oriskany (Deerparkian) erosion, whereas pre-Oriskany erosion in the northern outlier was considerably less. On the other hand, deposition continued uninterrupted along the main outcrop belt throughout late Cayugan, Helderbergian, and Deerparkian time.