Chazy Formation
（重定向自Chazyan）
![Stromatoporoid at Fisk Quarry, Isle La Motte, Vermont](/Images/godic/202501/04/Chazy_Reef_Isle_La_Motte4341.jpg")
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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.