The Atlantic Coastal Plain is a broad, low-lying physiographic province situated along the eastern coast of the United States. This vast region represents the landward portion of a much larger geological feature known as the Atlantic Plain, which continues eastward beneath the ocean as the continental shelf. The entire province slopes gently toward the sea, forming a relatively flat expanse that stretches parallel to the Atlantic Ocean. This extensive coastal region holds significant importance for the ecology, hydrology, and historical development of the Eastern Seaboard.
Geographical Extent of the Coastal Plain
The Atlantic Coastal Plain spans approximately 2,200 miles from its northern limit near Cape Cod in Massachusetts, extending southward along the coastline until it merges with the Gulf Coastal Plain. It encompasses all or parts of numerous states. This low-relief region includes the entirety of Delaware and Florida, and substantial portions of New Jersey, Maryland, Virginia, North Carolina, South Carolina, and Georgia. The plain gradually widens as it progresses toward the south, reaching its maximum breadth in the Carolinas and Georgia before forming the Florida peninsula.
Defining Physical Characteristics
The topography of the Atlantic Coastal Plain is defined by its subtle, almost imperceptible seaward slope and its notably low elevation. Most of the region lies less than 100 feet above sea level. The soils are primarily composed of unconsolidated marine sediments, including sand, silt, and clay that were deposited over millennia. The plain is characterized by vast stretches of wetlands, marshes, and swamps, which are fed by numerous rivers that flow sluggishly across the flat terrain toward the ocean. The coastal edge features extensive systems of barrier islands and sandy beaches that buffer the mainland from the energy of the open Atlantic.
Geological Origin and Formation
The Atlantic Coastal Plain originated as a passive continental margin following the breakup of the supercontinent Pangea, which began roughly 200 million years ago. As the North American plate drifted westward, the continental edge cooled and subsided. This subsidence created a basin for the accumulation of sediments eroded from the Appalachian Mountains. The plain’s formation is the result of continuous deposition from rivers and the fluctuating sea levels over the last 100 million years. During periods when global sea levels were high, known as marine transgressions, the ocean covered the continental shelf, depositing layers of sand, clay, and limestone. When sea levels dropped during glacial periods, the exposed continental shelf material formed the landmass we recognize today as the Coastal Plain. The present-day plain is essentially a wedge of relatively young, unconsolidated sediment resting upon a much older, hard bedrock base. This sedimentary wedge thickens dramatically toward the ocean, reaching depths of thousands of feet near the coastline.
Subdivisions and the Fall Line Boundary
Subdivisions
The Atlantic Coastal Plain is typically divided into an Inner Coastal Plain and an Outer Coastal Plain, reflecting differences in elevation, age, and proximity to the sea. The Inner Coastal Plain is located closer to the mainland, features older sediments, and is slightly higher and more dissected by rivers. The Outer Coastal Plain is characterized by younger sediments, extremely low elevations, and includes the modern coast with its dynamic barrier islands and marsh ecosystems.
The Fall Line Boundary
The western boundary of the entire physiographic province is marked by the Fall Line, a geological transition zone that separates the soft, sedimentary rocks of the Coastal Plain from the hard, crystalline rocks of the adjacent Piedmont region. Rivers flowing from the Piedmont’s higher elevations encounter this boundary and drop rapidly onto the softer plain, creating a series of rapids or small waterfalls. This change in elevation historically served as the upstream limit of navigation for commercial vessels, leading to the establishment of major cities like Philadelphia, Baltimore, and Richmond along this linear feature. The Fall Line is a geologically significant boundary, representing the maximum westward reach of marine sediment deposition onto the continent’s ancient bedrock base.