CHANGING STYLE OF DEFORMATION AT THE NORTHERN LIMIT OF THE EXTENT OF THE SHAWANGUNK CONGLOMERATE; ULSTER COUNTY, N.Y

The Upper Silurian Shawangunk Conglomerate thins to the north and does not outcrop north of the Town of Rosendale, N. Y. It thickens to the south to approximately 20 feet just east of the location of the former Williams Lake Hotel in Rosendale. It is observed to be still thicker farther to the south. The Shawangunk Conglomerate and, in the northern part of the study area, the High Falls Shale, lie unconformably over the Ordovician Hudson River Shale. The Upper Silurian and Lower Devonian rocks of the Helderberg sequence, as well as the Ordovician shales below, were intensely deformed during the Acadian Orogeny. The Ordovician rocks were deformed during the Taconic Orogeny as well. Detailed field mapping in the vicinity of Fourth and Fifth Lakes in Rosendale, N. Y. confirms a distinct difference from the surrounding area, in the style of deformation at the northernmost extent of the Shawangunk Conglomerate, where décollment type structures are observed. This appears to be due to a ductility contrast between the Shawangunk Conglomerate and the surrounding rock units.