Geoscience Reference
In-Depth Information
coast is sinking under rising sea level and forming a wide continental shelf from the continuing erosion of the
Appalachian Mountains. Those mountains were the result of two separate orogenies, or mountain-building
episodes, during the Ordovician and Devonian periods, 450 to 350 million years ago.
To understand these events, we must go back in time to the Precambrian, when the continental plates were
welded together in one supercontinent known as Rodinia, which began to split into several large landmasses
750 million years ago. One of the largest was Laurentia, which included much of the North American contin-
ent, including the central United States, the Canadian Shield, and Greenland. A second large ancient continent
was called Gondwana and included most of Africa and South America, as well as Australia, Antarctica, and
India. Smaller landmasses included part of Europe (Baltica) and smaller subcontinents and island chains.
These disparate pieces were to collide as the old Iapetus Ocean closed and the North American and
European Plates drifted together, pushing up the Appalachian range perhaps to Himalayan height. For that
reason, the east coast of North America is known as a collision coast, and much of the geological complexity
found there is due to the folding and faulting that accompanied that collision of continents.
The Appalachians were actually built in two stages. The first event, the Taconic orogeny, is named for the
Taconic range of mountains of western New England and southeastern New York State. About 450 million
years ago, in the Late, or Upper, Ordovician period, the northern part of Europe drifted northwestward, closing
the ocean gap between it and the North American- Greenland plate of Laurentia. The suture or area of contact
can be traced from the eastern side of Norway, through southern Scotland, across Ireland to the central part of
Newfoundland. Eventually the sediments that had been deposited along the northern part of North America
were crushed against the margin and thrown up to form a 2,500-kilometer (1,500-mile) chain of mountains that
arced from western Newfoundland through eastern Quebec and southward through Vermont to eastern New
York State.
Then, for the next 75 million years, things went quiet on the margin of the North American Plate. The con-
tinents were still moving inexorably toward each other, however, resulting in volcanism that spewed out lava
flows and ash deposits during the Silurian Age (430-395 million years ago) and left a blanket of deposits
5,000 meters (16,000 feet) deep around Yarmouth, Nova Scotia—including at Cape Forchu, where I played as
a child—and northern Maine. During the Devonian period that followed, 390 million years ago, the continental
plates of North America and Europe squeezed closer together, further folding the continental sediments and
forming the Euramerica continent. This was the Acadian orogeny, the second phase of Appalachian mountain
building. As a result of this episode, the mountain chain was stretched from Newfoundland south to Alabama.
Parts of the range, like the Gaspé, were unaffected by this second cataclysm. But new parts were created, such
as the Nova Scotia uplands, and other parts, like the New Brunswick highland and uplands of Newfoundland,
were refolded by this second event.
As the Earth was being altered by these titanic events, so life on the planet was undergoing radical changes.
The fossil record of these evolutionary advances is truly remarkable along the Atlantic coast, where the ocean
itself has opened wide windows onto time's parade of organisms. At various places along the coast, we can
witness the development of the first vertebrates, the fishes, and the evolution of life onto land, with the rise
first of the amphibians and later of the reptiles, the first truly terrestrial group of animals.
