A coastal plain is a broad, flat stretch of low-lying land that borders an ocean or sea. It forms over millions of years as rivers carry sediment from inland mountains toward the coast and as rising and falling sea levels deposit layers of sand, clay, and gravel. The most familiar example in the United States stretches over 3,540 kilometers (2,200 miles) from Cape Cod, Massachusetts, to the Mexican border, making it the flattest geographical province in the country.
How Coastal Plains Form
Coastal plains are built layer by layer through two main processes working together over geological time. First, rivers and streams carry eroded material from mountain ranges and deposit it as they slow down near the coast. Along the U.S. Atlantic seaboard, streams have been washing clays and sands from the Appalachian Mountains toward the ocean since the Cretaceous period, roughly 100 million years ago. Second, the ocean itself advances and retreats over the land as global sea levels rise and fall. When sea levels are high, marine sediments settle across the plain. When they drop, those former sea floors become dry land.
This cycle repeated many times during the Cenozoic era (the last 65 million years) and accelerated during the Pleistocene ice ages, when warm periods pushed the ocean inland and glacial periods pulled it back. Each cycle left behind a new layer of sediment. The result is a wedge-shaped stack of material that’s thin at its inland edge and thickens dramatically toward the ocean. In Virginia, for example, this sedimentary wedge starts as a razor-thin layer near the inland boundary and reaches more than 4,000 meters thick under the continental shelf.
The Fall Line: Where the Plain Begins
If you travel inland from the coast, the coastal plain doesn’t gradually blend into hilly terrain. Instead, there’s a relatively sharp boundary called the fall line (or fall zone), where the flat, soft sedimentary rocks of the plain meet the harder crystalline rocks of the elevated interior. In the eastern U.S., this line separates the coastal plain from the Piedmont region.
The name comes from what rivers do when they cross it. As streams flow from the resistant rocks of the Piedmont onto the softer, younger rocks of the coastal plain, they drop in elevation, creating waterfalls and rapids. In Georgia, the fall line runs about twenty miles wide from Columbus to Augusta. Cities like Richmond, Virginia, and Columbia, South Carolina, also sit along this boundary. The geological differences on either side of the fall line produce distinct soil types, water cycles, and stream behavior.
What the Landscape Looks Like
Coastal plains aren’t perfectly smooth. The terrain is a terraced landscape that stair-steps downward toward the coast and major rivers. The “steps” are low ridges, or scarps, that mark ancient shorelines from times when the ocean stood at a higher level. The flat “treads” between them are former bay and river bottoms that emerged as the sea retreated. Closer to the inland boundary, these surfaces are older and more carved up by stream erosion. Nearer the coast, the terraces are younger, lower, and flatter.
This terraced structure also means the coastal plain isn’t one uniform surface. Geologists often divide it into upper and lower sections. The upper coastal plain sits farther inland at slightly higher elevation, with more rolling terrain. The lower coastal plain hugs the coast and is extremely flat, often just a few feet above sea level. This is where you find extensive wetlands, marshes, and barrier islands.
Soil and Agriculture
Coastal plain soils are typically sandy and well-drained, which makes them easy to work but naturally low in organic matter and fertility. A common soil type in the southern coastal plain is loamy sand on gentle slopes of just 0 to 2 percent. These soils respond well to improved management but often need irrigation because they don’t hold water efficiently.
Despite those limitations, coastal plains are heavily farmed. The dominant crops across the southern U.S. coastal plain include cotton (often grown in continuous monoculture), soybeans, peanuts, corn, and wheat. Cover crops like rye help protect the sandy surface from erosion between growing seasons. The flat terrain, long growing season, and manageable soils make these regions some of the most productive agricultural land in the Southeast, provided farmers supplement fertility and water.
Groundwater and Aquifers
Beneath the surface, the same layered sediments that built the coastal plain also create important aquifer systems. The Northern Atlantic Coastal Plain aquifer, for instance, occupies more than 50,000 square miles along the eastern seaboard from Long Island, New York, to northeastern North Carolina. It consists of alternating layers of sand and gravel (which hold water) separated by layers of clay and silt (which act as barriers). Ten distinct regional aquifers have been identified within this single system, stacked on top of one another.
These aquifers serve as a critical freshwater source for drinking water, agriculture, and industry across the region. The water slowly filters through the unconsolidated sediments, which range in age from the Early Cretaceous to the present. Because coastal plain aquifers sit close to the ocean, they’re vulnerable to saltwater intrusion when too much freshwater is pumped out or when sea levels rise.
Wildlife and Ecosystems
The North American Coastal Plain is recognized as a global biodiversity hotspot, home to a remarkable number of species found nowhere else on Earth. Its mix of wetlands, pine savannas, flatwoods, and pine barrens supports an unusual range of life.
The longleaf pine, now endangered, once dominated millions of acres across the southern coastal plain and remains a keystone species. The red-cockaded woodpecker depends entirely on longleaf pine habitat. One of the most dramatic examples of restricted range is the Florida yew, a small evergreen tree that grows wild along just a single 15-mile stretch of the Apalachicola River bank and is critically endangered.
Among animals, the coastal plain hosts the gopher tortoise, the oak toad (the smallest toad in North America), and the two-toed amphiuma, one of the world’s largest amphibians at up to 115 centimeters long. The Florida scrub jay is the only bird species found exclusively in one U.S. state. Freshwater systems support critically endangered species like the Alabama sturgeon. Many of these species are vulnerable because coastal plain habitats face pressure from development, fire suppression, and habitat conversion.
Threats From Sea Level Rise
Because coastal plains are low and flat, even small increases in sea level can affect large areas. Rising water levels worsen seasonal tidal flooding, creating “sunny day” flooding events that put roads, homes, and marinas underwater without any storm. Wetlands, seagrass beds, and mangroves are particularly sensitive to these changes.
Erosion is accelerating as well. Many beaches along the U.S. coastline have lost more than six feet of shoreline per year over the last century, and that rate is expected to increase. Stronger storms paired with higher baseline sea levels generate more powerful waves and storm surge, compounding the erosion problem. For the millions of people living on coastal plains, and for the aquifers and ecosystems beneath them, these changes represent some of the most direct consequences of a warming climate. Sea level has been rising for the last 12,000 years, advancing the shoreline landward, but the pace of recent change is what makes current conditions different from the geological cycles that built these plains in the first place.