The Chazy Reef, which has been called the oldest reef in the world,
is located on Isle La Motte, a Vermont island on Lake Champlain.
However, there are two even older reefs on the island, which are the
subject of study by scientists.

UVM Professor of Geology, Charlotte Mehrtens, says that the oldest
reefs are found around “The Head” of the south end of the island,
slightly younger reefs are found at the Fisk Quarry and the youngest
(the famous coral reefs) are located in fields to the north. Together,
these three sites provide a unique narrative of events which took place
over 450 million years ago in ocean in the Southern Hemisphere, long
before the emergence of Lake Champlain – 20 thousand years ago.

Wikipedia Entry