TROPICAL SEAS IN EASTERN LAURENTIA
Sea-level rise brought shallow marine waters across Duchess Co. relatively early in the Cambrian (543-489 mya); they completely covered New York State by the end of the period. Cambrian shoreline sandstones overlie the Grenville at Stissing Mountain to Dutchess Co, Skene Mountain in Washington Co, Ausable chasm in Clinton Co, and Chateauguay Chasm in Franklin Co. Overlying deposits that range high into the youngest Ordovician period (489-438 mya) are dominated by limestones (calcium carbonate rock often changed into the calcium-magnesium carbonate rock dolostone). Evaporite minerals (halite and gypsum) and mud cracks in parts of the Upper Cambrian-Lower Ordovician in the Mohawk Valley and Middle Ordovician on Isle La Motte in Lake Champlain, and coral reefs in the lower Upper Ordovician at Plattsburgh indicate tropical conditions.
New York State and eastern Laurentia were rotated 90o clockwise from their modern position and lay 35o south of the equator. These limestones are present as far sough as New York City, where the rock succession includes the Grenville (Yonkers and Fordham Gneisses), the Lower Cambrian Lowerre Quartzite, and Cambrian-Ordovician Inwood Marble. The latter easily eroded unit forms the beds of the lower Hudson, East, and Harlem Rivers.