Photo by Clay Banks on Unsplash

Geology of Beaches and Barrier Islands

Patrick J. Lynch—

The outstanding feature of the Middle Atlantic Coast is a segment of the world’s longest string of barrier islands, with the sounds and bays that separate these islands from the mainland Atlantic coast. The barrier islands of the Mid-Atlantic Coast are part of a series of sand and coral coastal islands that stretch south through the Florida Keys and around the Gulf Coast to the Mexican border. This article looks at how these sandy barrier islands of the Mid-Atlantic Coast came to be, concentrating on the geologic processes that created this unique coastline.

Sea level rise and wave erosion on the sound side of the Outer Banks at Nags Head Woods Preserve are gradually claiming the western edges of the maritime forest.

Broad, shallow lagoons lie behind the barrier islands, sheltering extensive areas of salt marsh and biologically rich estuaries—places where saltwater and freshwater meet—that are the nurseries for most coastal and many marine creatures. This happy combination of protective barrier islands harboring enormously productive marshes and estuaries has made the Mid-Atlantic Coast one of the richest coasts in the world: rich in beautiful beaches, rich in seafood production, and rich in natural sounds, bays, and harbors to support wildlife, fishing, and commerce.

Barrier islands are almost invariably described as fragile ecosystems, always vulnerable to storm waves and ocean and wind currents. But seeing barrier islands as fragile ignores the inherent toughness and flexibility of natural coastal ecosystems, which have withstood the worst of what the Atlantic Ocean could throw at them for more than 5,000 years in their current configuration. Sand islands are almost as fluidly changeable as the seas around them. Only when we build fixed structures on these dynamic, variable strands do the islands begin to seem frail. In reality it is our buildings, roads, and bridges that are fragile when we construct them on shifting sand.

Although the ancient North American East Coast had a violent early geologic history, for the past 100 million years or so the East Coast and the Appalachian Mountains have been geologically quiet. The Atlantic Ocean continues very gradually to widen as the African plate and North American plates drift apart. The East Coast of North America has become what geologists call a passive or trailing edge coastline, dominated by the slow forces of weathering and erosion. The once mighty Appalachians have steadily worn down from high, rugged peaks to low, eroded stumps, remnants of their former selves.

Today’s low, rounded Appalachian Mountains are the heavily eroded remnants of taller, more rugged peaks that once probably looked like these mountains in the modern Sierra Nevada. Much of today’s Atlantic Coastal Plain and the continental shelf off the Mid-Atlantic Coast originated as sediments eroded from those ancient Appalachians.

As erosion and weathering attacked the Appalachians, gravel, sand, and mud flowed down their eastern slopes to blanket and bury the bedrock topography of the East Coast, creating a broad apron of deep soils and sediments called the Atlantic Coastal Plain. The coastal plain sediments continue below today’s sea level as the broad continental shelf off the coastline. Most of the coastal plain sediments were laid down in the Cretaceous Period (140–66 million years ago), late in the age of dinosaurs. So much eroded material has accumulated along the coastal plain that the Appalachian Mountains are now situated more than 100 miles inland, and the blanket of coastal plain sediments is as much as 15,000 feet thick in some areas of the Atlantic Coastal Plain and continental shelf.

The continental shelf offshore of the central Atlantic Coast is a thick wedge of sand, silt, and clay sediments, thinner near the coast and thickening near where the continental shelf drops off sharply into the Atlantic abyssal plain. Heavier sand and gravel sediments were generally deposited near the coastline, while lighter silt and clay sediments drifted farther offshore toward the edge of the continental shelf. Studies on the sediments of the continental shelf show at least seven distinct cycles of sea level rise and fall, each associated with sand deposits from ancient beaches and barrier islands, now all submerged by today’s sea level. The continental shelf off the Mid-Atlantic Coast is large, encompassing roughly as much area as the Atlantic Coastal Plain. At its outer margins, the shelf dips sharply downward at the shelf break, and the bottom continues down as the continental slope until it bottoms out at the Atlantic abyssal plain.

The gentle slope of the continental shelf off the Mid-Atlantic Coast and the moderate tidal ranges were critical to the formation and later landward migration of barrier islands. Long barrier islands cannot form on steeply sloping bottoms or in areas where the daily tide range exceeds about 10 feet.

The character of the East Coast changes abruptly north of New York Harbor and Long Island, due to the effects of multiple glacial periods that reached their southernmost extent in the area. North of New York Harbor the Pleistocene glaciations scraped the ancient coastal plain sediments off the landscape, depositing much of the sediment as the great regional moraines that created the coastal surface geology of Long Island, Block Island, Martha’s Vineyard, Cape Cod, and Nantucket. The northern coastal plain sediments have not entirely disappeared in the glaciated coastline, but in places they are buried well below more recent surface glacial deposits. For example, New York’s Long Island is a complex layer cake of ancient coastal plain sediments, topped by a relatively thin veneer of glacial sediments from the Wisconsinan Glacial Episode, which ended about 11,000 years ago. Those ancient, sandy Cretaceous coastal plain sediments under Long Island are very important today, as they supply most of Long Island’s drinking water.

Adapted from A Field Guide to the Mid-Atlantic Coast by Patrick J. Lynch. Published by Yale University Press in 2021. Reproduced with permission.

Patrick J. Lynch is a former senior digital officer in Yale University’s Office of Public Affairs and Communications and an award-winning author, designer, illustrator, and photographer. He has authored or co-authored nine previous books, most recently A Field Guide to Cape Cod. He lives in North Haven, CT.

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