The Hudson River flows through five physiographic regions. Above Glens Falls, it courses through the ancient crystalline rocks of the Adirondacks; from Glens Falls to Newburgh, it follows the Great Valley of limestone and shale; south of Newburgh, it leaves the broad Appalachian Valley to cross the Highlands, through a relatively narrow, crooked, and steep-sided gorge; from a point just southwest of Peekskill to the city of New York, it follows a contact between the metamorphic rocks on the east and the overlying Triassic on the west; below New York Bay, its channel is cut in the submerged Coastal Plain.
The Hudson Highlands are a block of northeastward-pitching pre-Cambrian crystalline rocks, separated from the Great Valley, on the northwest, by a thrust fault, and from the Piedmont, on the southeast, by a normal fault. For the purposes of this paper, these crystalline rocks may be divided into Grenville gneiss . . .