The
coast is the strip of land near the ocean that includes the beach and the immediate inland area beside it. Coasts can be rocky and rugged or gently sloped.
Paleocoasts are generally older coasts that were submerged under later marine transgressions and then lifted tectonically above sea level, exposing their sea floor features.
The constant impact of waves can dramatically alter even the most rugged, rocky coastline. Soluble rocks like limestones are dissolved, softer rocks are easily eroded, and even harder rocks like granite are fractured by the impact of waves. An irregular coast has many coves separated by irregular rocky points called headlands.
Wave action eventually straightens and smoothes the coastline. The headlands receive the greatest force of the waves, and the bays are the most sheltered. The headlands are broken down more quickly than the bays, and the eroded material is deposited in the coves. This process of the headlands being cut back and the flanking beaches being widened is called coastal straightening (Figure 1).
Figure 1
Coastal Straightening
Constant wave action along a rocky shore creates prominent sea cliffs. Sea cliffs retreat by mass wasting as waves undercut the cliffs in the wave zone, creating rock falls into the sea. Sea caves are cavities that are eroded into rock in the wave zone. As sea cliffs retreat, they leave behind, beneath the surf, a flat‐lying bench of eroded rock called the wave‐cut platform. Stacks are erosional remnants of sea cliffs that are rooted to the wave‐cut platform and stand above the surface of the water. Sea arches are stacks whose centers have been eroded through because the rock is softer or more fractured, resulting in a bridgelike shape.
Depositional coasts are those gently sloped coasts that have been built up by sediments deposited from longshore drift. Barrier islands are large, elongate masses of sand that parallel the coast and form islands. These islands are separated from the coast by protected lagoons. Tidal currents may deposit tidal deltas in gaps between the barrier islands. Depositional coasts are also built outward by sedimentation in river deltas. Glacial sediments can also contribute to the growth of a coastline.
Biological activity is also important in stabilizing a coast. The development of offshore coral and algal reefs helps protect the coastline from erosion by being a barrier to strong wave action. Heavily vegetated coastlines with dense root systems, such as those of mangrove trees in Florida, anchor the beach, reduce erosion, and help trap sediments.
Estuaries are parts of old river channels that now extend inland from the coast. The shallow water in estuaries is typically brackish from the mixing of ocean water and fresh water.
Uplifted coasts are former coasts (paleocoasts) that have been lifted above the present coastline by tectonic activity. They are often identified by uplifted marine terraces that were formed below the older surf zone and are generally found along tectonically unstable coastlines, such as the Pacific coast in the United States and Canada.