A coastline represents the dynamic zone where the land meets the sea or ocean, a boundary constantly being reshaped by powerful natural forces like waves, tides, and currents. This interface is not a static line but rather a broad, active region that reflects the ongoing interaction between marine processes and terrestrial resistance. The character of the coastline—whether sandy, rocky, or marshy—is determined by the geological materials present and the intensity of the water’s energy. This interaction sculpts a variety of distinctive landforms, creating some of the most diverse environments on Earth.
Defining the Coastline’s Extent
The physical extent of the coastline is complex, encompassing both a precise, moving boundary and a broader geographical zone. The “shoreline” is the immediate, fluid line of contact between the water and the land, which shifts continuously with the daily rise and fall of the tides. In contrast, the “coastline” refers to the wider area of land that is influenced by the sea, extending both inland and seaward.
To establish a measurable, fixed boundary for mapping and legal purposes, authorities often reference specific tidal marks. The most common reference point is the Mean High Water Mark (MHWM), which represents the average height of all high tides recorded over a long period. This legal coastline separates the permanently dry land from the intertidal zone, the area covered by water during high tide and exposed during low tide. The vertical limit stretches from the lowest point influenced by wave action, which can be far offshore, up to the highest point reached by storm surges and sea spray. Horizontally, the coastal zone extends inland until the features of the landscape are no longer influenced by marine processes, a distance that can range from a few hundred meters to several kilometers.
Major Erosional and Depositional Features
The appearance of a coastline is determined by whether the dominant process is the removal of material (erosion) or the accumulation of sediment (deposition).
Erosional Features
Erosional coastlines are typically rugged, forming in areas where the land is composed of resistant rock and wave energy is high. These forces create steep sea cliffs, which retreat inland as waves undercut their base, forming a notch that eventually collapses. As the cliff line retreats, a gently sloping, rocky surface called a wave-cut platform is left behind at the base, visible at low tide. On coastlines where bands of hard and soft rock are perpendicular to the shore, differential erosion occurs, carving out bays in the softer rock and leaving headlands of harder rock jutting into the sea. Continued wave attack on these headlands can exploit cracks, leading to a sequence of features including sea caves, arches, and isolated rock columns known as stacks.
Depositional Features
Depositional coastlines are characterized by the accumulation of sand, gravel, and mud, often occurring in areas with lower wave energy and abundant sediment supply. Beaches are the most common depositional landform, consisting of loose material transported and sorted by waves. The material on a beach is constantly moved by “longshore drift,” a process where waves approach the shore at an angle and the backwash pulls sediment straight down, resulting in a net movement of material along the coast. This lateral movement of sediment can build elongated ridges of sand or shingle called spits, which extend from the mainland into a body of water. Where these spits fully enclose a bay, they form a bay-mouth bar, often creating a lagoon on the landward side. Further offshore, large, detached sand ridges known as barrier islands run parallel to the coast, shielding the mainland from the full force of ocean waves and storms.
Geological Classification of Coastlines
Geologists classify coastlines based on their origin and historical development, reflecting the dominant forces that shaped them over geological timescales.
Primary and Secondary Coasts
One distinction is made between Primary and Secondary coastlines, based on whether the shape was initially determined by non-marine or marine forces. Primary coastlines are young features shaped mainly by terrestrial processes that occurred before the sea level stabilized. Examples include fiords, which are deep, U-shaped valleys carved by glaciers and later flooded, and deltaic coasts, formed by large-scale deposition of river sediment. Secondary coastlines are those whose forms have been significantly modified by the ongoing action of marine processes like wave erosion and sediment transport. Most erosional and depositional features, such as cliffs and barrier islands, are characteristic of secondary coasts.
Emergent and Submergent Coasts
Another classification system groups coastlines into emergent or submergent categories based on relative sea level change. Emergent coasts have experienced a fall in sea level or a rise in the land, exposing landforms that were once underwater, such as raised beaches. Submergent coasts occur where the land has sunk or the sea level has risen, resulting in “drowned” features like rias, which are submerged river valleys, exemplified by the Chesapeake Bay.