THE LAND BETWEEN THE OROGENYS

The rock formations outcropping in the area of Rosendale, N. Y. are Upper Silurian to Lower Devonian in age. They unconformably overlie Ordovician shale, locally called Hudson River Shale that most workers in the field believe correlates to the Martinsburg formation. The Taconic Orogeny (Circa 470 - 440 MA) deformed the Ordovician shales and raised the mountains that were the source area for the Upper Silurian and Lower Devonian sediments. The Shawangunk Conglomerate, the basal unit of the Tuscarora Sandstone, is the oldest Silurian formation seen in the study area. It lies immediately above the Taconic unconformity in the southern part of the study area. It thins and is not observed in the northern part of the study area. There the High Falls Shale is seen forming an angular unconformity with the Ordovician shale below. All of the post Taconic formations in the study area were intensely deformed by the Acadian Orogeny (Circa 400 MA). The outcrop area of the Upper Silurian and Lower Devonian constitutes what the authors call “The Land between the Orogenys”. The authors believe that the ductility contrast between the Shawangunk Conglomerate and the other formations in the area is the cause of some unusual structural features and unique exposures. The excavation of two of the Upper Silurian formations for the mining of natural cement dramatically exposes these structural features. The structure and lithology of the area influence geomorphologic and environmental characteristics including topography, water and soil quality. This makes the study area particularly suitable for educational field studies by college students.