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Lake Champlain's Isle La Motte is rich in marine fossils, some of which are 450 million years old. (Robert Lyons)

Paleozoic Vermont

What's the world's oldest communal ocean reef doing in the Green Mountain State?

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Nelson Fisk, who was Vermont's lieutenant governor from 1896 to 1898, was also the owner of a quarry on Isle La Motte, in Lake Champlain. His business card read: "Isle La Motte Grey and Black Marble Quarries." He was overselling. The rock was limestone.

Fisk limestone was loaded onto boats and floated down the lake to the Hudson River and points south, where it was used in the construction of the Brooklyn Bridge and, in Washington, D.C., the National Gallery of Art, among other structures. The darker Fisk limestone came to be known as "radio black" because it was used in Radio City Music Hall. Stone from the quarry was covered with odd swirls and blotches—and therein lies a strange tale of geology, climate change and the history of life on this planet.

Those blemishes are what make the Isle La Motte stone priceless today, so much so that the quarry is no longer available to stonecutters and instead has been preserved as an outdoor science laboratory. The "flaws" in the stone are fossils, evidence of sea creatures of stunning antiquity—some dating back nearly half a billion years, when the only existing animals lived in oceans. And what incredible animals they were! There was coral, of course, but also large, tentacled ancestors of squid; trilobites, arthropods related to horseshoe crabs; and spongy, cabbage-shaped animals called stromatoporoids. Peculiar as it may sound, Isle La Motte, which is some 175 miles from the Atlantic Coast, is the best place to see one of the oldest reefs on earth.

Seven miles long and three miles wide, the island was the site of the first European settlement in Vermont, in 1666. Today it is home to about 500 year-round residents. The fossil reef, called the Chazy Reef after a town in upstate New York where this type of rock was first studied, covers the southern third of the island. What is it doing here? When the reef began to form, 450 million years ago, it lay in warm waters in the Southern Hemisphere. It thrived there for about five million years. Some 250 million years later, rotating tectonic plates deposited the fossilized reef where it is today. Other parts of the reef, which originally stretched a thousand miles, can be found all the way from Newfoundland to Tennessee. But it is in Isle La Motte where the reef best opens itself to scientific study.

Charlotte Mehrtens, a geologist at the University of Vermont in Burlington, says that in Tennessee, for example, the reef lies flatter and has fewer layers to examine. But in Vermont, it has tilted so its layers of prehistory—fossils piled upon earlier fossils—are visible horizontally; the reef does not have to be drilled or blasted to examine its different "horizons," as the layers are called. The reef's timeline is clearest in an area called Goodsell Ridge, just northeast of the Fisk Quarry.

Not long ago, Linda Fitch, president of the Isle La Motte Preservation Trust, which recently purchased the Goodsell Ridge, gave me a tour of it. As we walked across pastures from south to north, we traversed millions of years of the reef's lifetime. Fitch lifted turf from the rock in different spots, exposing fossils galore from what she describes as "the first great reef city in the world."

Construction workers often lose their lives when erecting great structures. Eleven men died building the Golden Gate Bridge. Hoover Dam claimed more than a hundred. In reefs, all the builders die: the bricks are calcium carbonate shells.

The Chazy Reef is the oldest reef in the world built by a community of organisms (a few older reefs are made up of one species only). Its foundation was built by Bryozoa, animals that preceded coral by millions of years but exist in similar forms today. The soft-bodied animals, a fraction of an inch long, resemble twigs and gumdrops in shape.

In the next horizon we find the stromatoporoids, extinct relatives of sponges. Then comes an extinct type of algae, followed by actual sponges, more algae and the oldest-known reef-building coral. The coral species found in the Chazy Reef are also extinct. Some looked like flowers, others like organ pipes or honeycombs.

In its heyday, the reef was also home to a bizarre menagerie of other marine life. Large tentacled cephalopods, ancestors of the squid and nautilus, scarfed up trilobites. Crinoids, delicate animals related to starfish that looked like flowers atop a long stem, waved back and forth in the currents. Gastropods, or large snails, proliferated—some of the fossil swirls that "mar" radio black limestone.

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