COLLISION AND RISE OF THE TACONIC MOUNTAINS
Mountain-building intervals in eastern New York State record events that led to growth of the supercontinent Pangaea. The first was the collision of eastern Laurentia with a volcanic island arc that extended from western Newfoundland to Alabama in the late Ordovician (450 mya). This collision, the Taconic orogeny, produced the Taconic Range of eastern New York and western New England, which was first proposed on the basis of geologic relationships in the Taconic Range and foothills of eastern New York State. Volcanic ashes, now claylike beds up to 4 inches (10 cm) thick in Upper Ordovician limestones of the Mohawk Valley and Saratoga areas, are the first evidence for proximity of the volcanic arc. During the Taconic collision, deep-water sandstones and mudrocks deposited on the Laurentian margin were pushed into eastern New York State in front of the volcanic arc. These overthrust rocks comprise the slate-rich Taconic hills of Washington, Rensselaer, and Dutchess Co.s and are much more highly metamorphosed in Westchester Co. Further south, the highly metamorphosed Taconic overthrust forms the Manhattan Schist in Central Park.
The Taconic orogeny ended Ordovician shallow-water limestone deposition. East of Syracuse, the weight of the Taconic overthrust depressed the dearth’s crust. This deepwater tough was filled in black shales, seen in road cuts on the New York State Thruway from Amsterdam (Montgomery Co.) to Herkimer, and overlying sands and muds, like those seen in the Mohawk River cliffs downstream of Schenectady. These sands and muds were eroded from the Taconic Mountains. Vertical movement up to 1,310 feet (400 m) took place along old basement fractures late in the Taconic orogeny. Local uplifts of hard Grenville and Upper Cambrian rock define the canyons and “narrows” along the Mohawk River at Hoffmans (Schenectady Co.), Randall (Montgomery Co.), and Little Falls (Herkimer Co.).
The Ordovician period ended as the retreating seas deposited shallow-marine sandstones across the state. Sandstones of this type make up the Tug Hill Plateau, a giant mesa of flat-lying rocks between the Black River valley to the east and the Lake Ontario Lowlands on the west and southwest. The triangular plateau is a topographically high remnant of harder rock that was isolated by erosion before the Ice Age. Downward erosion by the Black River, which runs north south, and the Mohawk River, which runs west-east, cut through the hard sandstone cap of the plateau and isolated the area as a high-standing region. The latest Ordovician deposits are very shallow-marine to subaerial (land deposited) red shales and sandstones exposed along Lake Ontario from Rochester to the lower Niagara River. The retreat of the seashore west of New York State at the end of the Ordovician was due in part to infilling by the apron of sediment eroded from the Taconic Mountains. In addition, water evaporated from the ocean accumulated as ice caps in South America and northern Africa, then at the South Pole, and caused the global sea level to fall.