The Genesee is the remaining western branch of a preglacial system, with rock layers tilted an average of 40 feet (12 m) per mile, so the river flows across progressively older bedrock as it flows northward. It begins in exposing the Allegheny Plateau's characteristic conglomerates: sandstones and shales in the rock columns of the Mississippian and Pennsylvanian subperiods. Thereafter, further downstream as it traverses the area known as The Grand Canyon of the East, where it falls (three times) through over 600 feet (180 m) as to passes through the gorges in New York's Letchworth State Park, the river also often exposes older rocks such as shales (some rich in hydrocarbons ), siltstones and some limestones of the Devonian period at Letchworth and, at other canyons with three more waterfalls at Rochester cuts through the Niagara Escarpment exposing limestones and shales of Silurian age in the rock column. With cuttings in the geologic record showing so many early ages, the river area has a great variety of fossils for paleobiological and stratigraphic analysis.
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