The Atlantic Coastal Plain owes its origin to the continuous rise and fall of oceans and the erosive power of rivers.
Evidence of these events is visible in sediments throughout the Coastal Plain, including in the parks of the National Capital Region (NCR) of the National Park Service (NPS).
These sediments were deposited in layers over time, so that younger layers of sediment were deposited on top of older layers. Geologists can look at the sediments and determine whether they were deposited underwater in a marine setting or on land through river processes. One of the key clues to differentiate the two settings is the presence or absence of marine organisms.
Within the Atlantic Coastal Plain, the presence of silty sand with marine shells and diatoms indicates the layer was laid down under seawater. Some layers even contain the fossilized bones of whales, a great indicator that the sediment was of marine origin. Deposits containing only sand, gravel, and silt were laid down from rivers and streams carrying eroded fragments of rock.
The Coastal Plain was formed through multiple increases and decreases in ocean level during the past 150 million years. The layers deposited by these ancient oceans and rivers are thinner towards the western edge of the plain, and thicken towards the eastern edge to depths of several thousand feet.
The source of much of the sediment was the eroding Appalachian Mountains, which at one time towered over the region. Only the core rocks, those that were once buried deep beneath the surface of the Appalachian Mountains are visible today. Ancestral rivers transported the overlying material and deposited it as sediment over the Coastal Plain.
Sediments became exposed at the surface following decreases in ocean level, which left them vulnerable to the erosive power of rivers. These rivers carved through the sediments of the Coastal Plain, forming new channels and leaving behind river bluffs like those along the Potomac River. These bluffs are especially visible in parks of National Capital Parks–East.