STRUCTURE AND TECTONIC INTERPRETATION OF THE ROCKS OUTCROPPING IN THE ROSENDALE, NY AREA

Bedrock in the Rosendale, N. Y. area consists of Upper Silurian to Lower Devonian sedimentary rocks which lie unconformably on Ordovician black shales, known locally as the Hudson River Shale. These Ordovician shales were affected by the Taconic as well as the Acadian Orogeny.

The Upper Silurian and Lower Devonian rocks, in the vicinity of Rosendale, were deformed during the Acadian Orogeny. The Shawangunk Conglomerate thickens to the south forming Shawangunk Mountain, thins to the north and is not seen in the northern part of the study area. The most acute rock deformation in the area is observed right in the vicinity of the northernmost extent of the Shawangunk Conglomerate. The authors believe that the Shawangunk Conglomerate acted as a massif against which the more ductile rocks were deformed.

The area is characterized by tight asymmetrical to overturned folds, plunging to the north-northeast, with the steep limb on the western side of the folds frequently truncated by thrust faulting. Drag folds, associated with the thrust faulting, are commonly observed in this area as well. The faulting produces repetition of the outcrop sequence of the sedimentary formations in the area, including the Upper Silurian Rosendale and Whiteport argillaceous, dolomitic limestones, which were mined extensively for natural cement. These formations, together with the Cobbleskill Limestone lying between them, are sometimes referred to as the "Rosendale Cement Series" (RCS) The repeated exposure of the "Cement Series" at the surface, due to the intense folding and faulting, greatly influenced the way in which the rock suitable for the production of natural cement was mined and extracted. The widespread excavation of the Whiteport and Rosendale formations provides an unique opportunity to observe the structure of the Upper Silurian and Lower Devonian rocks of the area.

Slaty cleavage is very conspicuous in the shales in the study area, as well as the argillaceous Devonian limestones such as the New Scotland and Port Ewen formations.

Ductility contrast between the argillaceous dolomitic limestones of the RCS and the Cobbleskill limestone between them is indicated by pronounced mineralized jointing in the Cobbleskill, that is not observed in the enclosing cement formations.