Chazy Formation

The Chazy Reef Formation is a mid-Ordovician limestone deposit in northeastern North America.

It consists of some of the oldest reef systems built by a community of organisms rather than the deposit of a limited range of similar organisms, such as Stromatolite mounds deposited by ancient cyanobacteria. The reef structure was formed largely by cryptostome and trepostome bryozoa, some of the oldest known bryozoans, but corals made an early appearance, and stromatoporoids.

The formation is named for the small town of Chazy, New York, where the reef was noted by James Hall in Palaeontology of New York (vol. I, 1847) and the fossils first studied by the Canadian paleontologist Elkanah Billings (1858, 1859). The reef extends from Tennessee to Quebec and Newfoundland, but its most easily studied outcropping is at Goodsell Ridge, Isle La Motte, the northernmost island in Lake Champlain; there, gentle uplift has tilted the sediments: the bedding planes now dip slightly to the north, revealing sequences of horizons in exposed rock. The black limestone of Isle La Motte takes a polish, revealing the white "bird's-eye" markings of embedded fossil shells, notably the spirals formed by sliced gastropod shells. Rock of the Chazy Formation was quarried from the nineteenth century at the Fisk Quarry, Isle La Motte, the oldest quarry in Vermont. Portions of the exposed reef on several islands in Lake Champlain were dedicated as the Chazy Fossil Reef National Natural Landmark in 2009.

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