Decollement in the Hudson River Valley
The Hudson River Valley is a conspicuous linear lowland which extends 260 km from New York City northward to Albany, New York, and the Mohawk River (Fig. 1A). The valley ranges from 6 to 25 km in width and has 30 to 175 m of relief. The area discussed here, from Ravena to Kingston, New York, is bordered on the east by the Taconic Mountains and on the west by the Catskill Mountains (Fig. 1B). Rocks of the Helderberg, Ulster, and Hamilton Groups (Fig. 2) have been folded and faulted by westerly directed compression. West of the Hudson River, the deformation is confined to a narrow linear belt know as the Little Mountains (the belt is now known as the Helderberg Escarpment) (Davis, 1882). This belt extends from about 3 km northeast of Ravena to about 13 km southwest of Kingston (Fig. 1B). The rocks of the Hamilton Group dip gently to the west; the Ulster and Helderberg Groups dip more steeply to the west and, in places, toward the east. The structural trend is not continuous; it is interrupted by a belt of relatively undeformed rock 2 km north of Kingston (Fig. 1B). At Mount Ida and Becraft Mountain, 15 km to the east, erosional “outliers” of the same sedimentary sequence are less deformed.