New York State is widely known for its wealth of fossils, especially those of organisms that lived in shallow seas that covered much of the state during the Paleozoic Era. Many of the rocks laid down during the Ordovician, Silurian and Devonian periods include beds full of wonderfully preserved remains representing a diversity of brachiopods, mollusks, bryozoans, corals, trilobites, and crinoids. So outstanding are two western New York localities that they lent their names to major divisions of geologic time used in North America: the Niagaran is the middle part of the Silurian Period, and the Erian (for exposures along the Lake Erie shore) is the middle part of the Devonian Period. We even have an official state fossil: the eurypterid.
Geological Background of New York State
New York State got its beginnings about 1.1 billion years ago in the Late Proterozoic Eon, when the collision of North America with another continent (Grenville Orogeny) metamorphosed a variety of rocks now comprising the state's Pre-Cambrian basement. It is believed that all the then-extant continents became united into a single supercontinent, which later rifted apart into smaller continents such as Laurentia (of which New York was part), Baltica, and the giant Gondwanaland. Subsequent global sea level rise flooded much of North America with vast, relatively shallow epicontinental seas. It was in these seas that the marine organisms preserved as fossils in New York State lived and died.
Things were relatively quiet during the Cambrian and much of the Ordovician periods as continents continued to drift apart and the newly formed Iapetus Ocean grew ever wider. Cambrian rocks include the Potsdam Sandstone and a limestone that would later be altered to become the Little Falls Dolostone, source of the familiar double-terminated quartz crystals called "Herkimer diamonds." Ordovician rocks include fossiliferous limestones and black shales exposed east of Syracuse. North America then reversed direction and headed back toward the other continents. The Taconian Orogeny marks its encounter with a large island arc complex, with mountains rising from New England as far south as Georgia. Vast quantities of sand and mud eroded from these mountains gradually filled in the sea across New York: this sediment now makes up the red Queenston Shale that underlies the Ontario Lowlands north of the Niagara Escarpment.
While Early Silurian sea level rise again flooded western New York, sediments derived from the eroding mountains were shed across the state. Resulting rocks include the red Medina Group sandstones so prominently displayed in the Rochester and Niagara gorges. Contained fossils are mainly trace fossils, but occasional brachiopods, nautiloids, and even rare jellyfish imprints may be found.
Western New York was subtropical, perhaps 30 to 35 degrees south of the equator, during the Middle Silurian. The sea extended across most of the state and was deepest along a subsiding foreland basin passing through central New York. Highly fossiliferous limestones and above them the Rochester Shale, so famous for its treasure of fossils, were laid down. Eventual shallowing was accompanied by extensive stromatoporoid and stromatolite reef development in western New York. Transformation of the resulting limestones to dolostones may have been related to activity associated with the Late Paleozoic Alleghenian Orogeny; the Lockport Dolostone today forms the caprock of the Niagara Escarpment from near Rochester westward into Ontario and beyond.
The sea became restricted during the Late Silurian Period, with evaporation of sea water causing precipitation of vast amounts of halite and gypsum. Mixed with muds derived from mountains to the east, these Salina Group deposits have significant economic value. Near the end of the Silurian the sea became deeper but apparently remained very salty, and the shallowest areas, were home to a strange fauna dominated by eurypterids.
Erosion accompanying low sea level during the Early Devonian produced a major unconformity. In the shallower western New York area, Middle Devonian rocks rest directly on eroded Late Silurian rocks. To the east, where the sea was deeper, the gap represents much less time and lies within a sequence of Early Devonian rocks that includes the Helderberg and Tristates Groups.
With the onset of the Middle Devonian, sea level rise was accompanied by deposition of the Onondaga Limestone across the state. The most fossiliferous parts include coral reefs (bioherms) and inter-reef deposits of shallower bottoms; deeper-water limestones contain abundant chert (flint) nodules.
Sea level continued to rise as Avalonia and other small continental masses collided with eastern North America. Mountain-building (Acadian Orogeny) provided sands and muds that make up the Hamilton Group, one of the most fossil-rich rock units in eastern North America. The Appalachian Foreland Basin, site of the deepest (to perhaps 100 meters) part of the sea where black shales accumulated, trended northeast through today's Finger Lakes region. Relatively clear waters over the shallower shelf to the west favored a profusion of brachiopods, bryozoans, corals, crinoids, and trilobites. Mollusks (gastropods and pelecypods) were more prominent east of the foreland basin (closer to shore) where silts and sands accumulated under more turbid waters. Further east was a coastal plain on which some of the first forests grew, as indicated by fossils of trees and other organisms that lived in the Gilboa area not far from Albany.
Sands and muds derived from the Acadian mountains led to westward expansion of the coastal plain as the sea was gradually filled in. The resulting upward-coarsening rock sequence, making up New York's "Southern Tier," is sometimes referred to as the "Catskill Delta" and contains both marine (brachiopods, mollusks, echinoderms, glass sponges) and terrestrial (dominantly plant) fossils.
With the exception of a brief submergence of extreme southwestern New York during the Mississippian Period, almost all of the state remained above sea level since the end of the Devonian. Very localized Cretaceous exposures on the western tip of Long Island represent the only other pre-Quaternary marine sediments in the state.
The record of post-Devonian non-marine rocks in the state is not much better. The Pennsylvanian Period, is represented by spotty outcrops of river-deposited conglomerate that make up some of the "rock cities" near Allegany State Park. Rifting of the supercontinent Pangaea in the Late Triassic Period was accompanied by creation of fault-block basins that filled with sediment as they dropped. The resulting red rocks, found in the Newark Basin in southeastern New York, contain our only evidence of dinosaurs in the form of footprints of the small theropod Coelophysis. Dinosaur remains are found nowhere else in the state because all other non-marine Mesozoic sediments that may have been deposited have been removed by erosion.
New York State does have fossils of organisms that lived during the latest part of the Cenozoic, from the time of the Pleistocene ice age. Fossil mastodons, bison, and other mammals can be found in some places where unconsolidated deposits veneer the surface. Freshwater snail and clam fossils occur along old drainage systems.