Waves


WAVES, another agent of erosion, shape shoreline topography through the process of WAVE ACTION. Erosional shoreline are distinctly different from depositional shorelines. On irregular shorelines, HEADLANDS are sites of erosion while the debris is carried away and deposited inside the recessed BAYS or COVES (coves are usually smaller than bays, although the distinctions are unclear). On these headlands, erosion will steepen the slope and is concentrated on a narrow area at the water level. The steepened slope becomes a WAVE CUT CLIFF, and where the water is actively undercutting the slope forming a NOTCH. The wave cut cliff wears back leaving a WAVE CUT TERRACE beneath the water. Further offshore, debris is deposited on top of this erosion surface creating a WAVE BUILT TERRACE. Weak areas on this wave cut cliff may be worn away to form a SEA CAVE. Two sea caves may wear back and intersect to form a SEA ARCH with water surrounding the column of rock. If the overhead connection collapses leaving part of the column remaining, the feature is a STACK. Uplift of the coastal area can result in the elevation of these features above sea level. When wave cut terraces or wave built terraces have been elevated, they become known as MARINE TERRACES.

When waves strike the shore at an angle, they move material along the beach by BEACH DRIFTING, and force water along parallel to the shore by the LONGSHORE CURRENT. Where deposition is occurring, sand is the most common deposit. The area between high tide and low tide is the BEACH. Deposits of sand are generally called sand bars, and are named by their shape and location. OFFSHORE BARS are unattached to the mainland and parallel to it. A SPIT is a sand bar attached to the mainland at one end, usually formed by the longshore current pushing material on past the end of the shore. Sometimes the end of a spit becomes curved. When this happens, it is called a HOOK. A hook is a type of spit. If the spit continues to grow across the bay until it reaches the opposite side of the inlet, it forms a BAYMOUTH BAR. If a sand bar particularly protects a body of water, that body of water may be called a LAGOON. A sandbar that connects an island to the mainland is called a TOMBOLO.

Classification of coastlines helps to understand the types of features that may be found on them. One classification system distinguishes four types of coasts. An EMERGENT shoreline is one where the land has risen relative to the water level. The southeastern U.S. coast is an emergent shoreline. This is what is responsible for the parallel drainage pattern that was mentioned in an earlier lesson. The opposite type of shoreline is the SUBMERGENT shoreline, where the land has sunk relative to the water level. A submergent shoreline is characterized by flooded stream mouths that have tides called ESTUARIES. The northeast Atlantic coast is a submergent coast. A NEUTRAL shoreline is one that is neither emergent nor submergent. Something else has created this shoreline. Examples include lava flows, volcanoes, coral reefs, deltas, and glaciation. The fourth type of shoreline is a COMPOUND shoreline that results from a combination of the above situations.


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