We went camping in The Islands this past weekend, about 5 miles from the Canadian border. We went to Isle La Motte and explored part of The Chazy Formation. The pictures are ours but the text comes from Wikipedia.
The Chazy Reef Formation is a mid-Ordovician limestone deposit that consists of some of the oldest reef systems built by a community of organisms. The reef structure was formed largely by bryozoans, but corals made an early appearance, as did the first stromatoporoids.
The formation is named for the small town of Chazy, New York, where the reef was first studied. The reef extends from Tennessee to Quebec, but its most easily studied outcropping is at Goodsell Ridge, Isle La Motte, the northernmost island in Lake Champlain; there, a 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 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.
The system formed in warm tropical waters of the Iapetus Ocean, in low latitudes of the Southern Hemisphere. The site was in shallow waters of a continental shelf: the continental craton of Laurentia lay to the west, beyond the lagoon system that formed behind the protective reef. The time was some 480-450 million years ago, at a period when global paleoclimate was so warm that the planet was all but ice-free. Atmospheric carbon dioxide was fourteen to sixteen times more plentiful than it is today.
The Chazy Reef Formation, which built up vertically from a muddy base catching fine dark silt as it grew, began as mounds that stabilized a muddy bottom, then built up into the water column to such an extent that the connected mounds modified their surrounding environment. As the reef aged, it began to offer an increasing variety of ecological niches, which fostered the first rich local biodiversity that has characterized all reef systems ever since. One dominant reef-building organism took the place of another, in a slowly evolving faunal succession.