ROSENDALE, NEW YORK: A CLASSIC AREA FOR A FIELD MAPPING COURSE

The bedrock in Rosendale, New York, consists of sedimentary rocks of Ordovician through Lower Devonian age. The formations outcropping in the area include argillaceous and arenaceous limestones, silty and arenaceous shales, dolostones, a thin to thickly-bedded, well-indurated sandstone (quartz arenite to quartzwacke), and a highly-indurated quartz-pebble conglomerate. Outcrops of the bedrock are very well exposed throughout the area. In addion to numerous road and railroad cuts, the Town of Rosendale was extensively mined for natural cement during the 19th and early 20th centuries. This has resulted in the excavation of two Silurian formations, the Rosendale and Whiteport Dolostones. The excavation of these units makes it very easy for students to observe the intense folding and faulting of the rock units, primarily associated with the Acadian Orogeny, and to map the structural geology of the area. Structural features observed in the area include: faults, slickensides, drag folds, slaty cleavage, as well as asymmetrical and overturned plunging folds. Many of the formations in the Rosendale area are highly fossiliferous, facilitating paleontological studies by students. Fossils observed in the area include: Halysites (Silurian), Gypidula Coemanensis, and numerous Crinoids and Brachiopods (Lower Devonian). Other primary features observed in the exposed formations include ripple marks, mud cracks, cross bedding, sole marks, and penecontemporaneous slumping (in the High Falls Shale). Shallow water ichnofaunal assemblages also characterize several formations. Depositional environments range from strictly continental to fluvio-deltaic to shallow marine with periodic incursions of the shallow transgressive sea. Stratigraphic contacts between formations are of sharp, inter-fingering and gradational nature. Several formations display a remarkable facies change laterally. Furthermore, tectonic overprinting on the depositional units is obvious as evidenced from the development of an angular unconformity between the Ordovician Hudson River Shale and Silurian High Falls Shale. All of these text-book quality features can be observed in an area of less than 4 square miles and offer an excellent field resource for paleoenvironmental interpretation from depositional, structural, and tectonic point of view.