Geoscience Reference
In-Depth Information
line. Prince Edward Island was cut off from the mainland seven thousand years ago, and waters in the Gulf of
St. Lawrence transformed the highlands into the island of Anticosti and the Magdalen Islands.
As the glaciers retreated, however, the land rebounded in response to removal of the glaciers' weight. The
glaciers had depressed the land by a few hundred meters, pushing down on the crust overlying the relatively
plastic asthenosphere, which was squeezed outward at the edge of the ice sheet, causing the land or seafloor to
bulge upwards. As the weight of the glacier was lifted, however, the land as a whole rebounded, just as a tram-
poline rebounds when weight is removed. Such changes in sea level due to the ups and downs of the litho-
sphere, or Earth's crust, are known as “isostatic” changes. The land reached its maximum height about six
thousand years ago and then slowly began to subside. At that time, Georges Bank was an island, but waters
now began to flood over it, reaching into the Bay of Fundy, where they would eventually propagate the great
tides there. Today, the only place in the Atlantic region of North America where the land is still rebounding
from the removal of the ice burden is northern Newfoundland and Labrador. Elsewhere the land is still subsid-
ing, resulting in a steady, if gradual, rise in sea level at an average rate of 8 centimeters (3 inches) per year.
By two thousand years ago, Cape Cod and its islands had assumed the shapes that they have today, even
though the shoreline was still several kilometers from its present-day position. Great sea cliffs faced the At-
lantic, with narrow beaches at their base. Barrier spits and islands formed by longshore currents protected the
bays and inlets, and the salt marshes sheltered in their lee. Essentially, the coastline had assumed its contem-
porary character.
The Atlantic Coastal Plain
Today, the Appalachians serve as a rugged backbone of eastern North America, with the land sloping away on
either side—in the west to the Mississippi and in the east toward the Atlantic Ocean. Since the opening of the
present-day Atlantic in the Triassic period, sediments have been washing down from these once-towering
mountains and pouring into the deep rift basins that formed along the eastern coast of North America.
As we have seen, a geological divide in the character of the Atlantic coast runs in an east-west direction
through Long Island, New York, which can be traced to the more recent glacial history. North of Long Island,
the bedrock of the Appalachians extends to the shoreline. The coastline is irregular where it has been gouged
out by the action of the glaciers, and the rocks are resistant to erosion. The rugged coast is distinguished by
rocky headlands and numerous small islands and shoals. South of Long Island, all the way to Florida, the At-
lantic coast is markedly different, being more linear and flanked by chains of barrier islands that run parallel to
the coast. The unconsolidated sediments of the Atlantic Coastal Plain are the building materials for this nearly
unbroken chain. Barrier islands usually form on trailing edge coasts, such as the eastern side of the North
American continent. The broad, low relief of the coastal plain and shallow continental shelf, together with the
abundant supply of sediments, provides the ideal conditions for the formation of barrier islands.
Three fundamental ideas about how barrier islands come into being have been proposed and continue to be
debated and elaborated on. According to a theory put forward in the 1960s by the late John Hoyt, of the
University of Georgia, barrier islands are formed from the drowning of coastal ridges, and this theory seems to
apply to the majority of such islands. A second theory holds that they begin as spits created by longshore drift,
which are later breached, disconnecting them from the mainland. The third theory posits that barrier islands
may form as a result of the emergence and upward-shoaling of offshore sandbars. Each of these mechanisms
applies in different cases but all, to varying degrees, are tied to the changes in sea level caused by the waxing
and waning of the glaciers.
The sea level rose very rapidly between eighteen thousand and fifteen thousand years ago as the icecaps
gave up their water—a rate of 1 meter (3 feet) per century, or over 100 meters (300 feet) during this initial
period of glacial melting. As the sea level rose, the ridges of mainland beach dunes were breached, the low-
lands behind these ridges were flooded to form lagoons, and the ridges became barrier islands. The Hoyt the-
