Features of Coasts

Ocean forces create a variety of features on secondary coasts. The constant erosion caused by waves pounding on the shore carves out sea cliffs and caves. Just off the coast, the same wave action sculpts natural arches or flat platforms. In places where the underwater slope of the seafloor is not steep, waves and tides can deposit sediment and build an area of loose particles called a beach. In the United States, about 30 percent of the coastlines have beaches. 

Beaches are subject to seasonal changes. The low, gentle waves of summer bring sand to a beach. During the winter, storms create higher, stronger waves that carry away sand. In the Northern Hemisphere, the most severe wave action starts in December but slows significantly by April. 

Some of the features of beaches are created by wave action. An accumulation of sediment that is deposited parallel to shore forms a section called the berm. The berm marks the upper limit of sand deposition by waves. The top of the berm, the berm crest, is usually the highest place on the beach. The backshore, an area made up of sand that is deposited in 

The sand on a beach displays several interesting features, as seen in the upper color insert on page C-l. Ripples are the marks made in sand by waves that rush onto the shore. Rills are small, branched depressions in the sand that drain water back toward the ocean. Diamond-shaped deposits of silt are backwash marks, places where the shells of animals interfere with the normal backwash of water. Regularly spaced, crescent-shaped depressions along the sand are called cusps. No one knows for sure how cusps form, but many believe them to be due to irregularities along the beach that are enlarged by swash, the water that runs off the beach after a wave breaks. 

Some shores are bordered by barrier islands, exposed sandbars that run parallel to the shore. Worldwide, about 13 percent of the coasts have barrier islands. These protective, sandy walls form in one of three ways. Some are the result of sediment deposits just offshore, like the islands off the coasts of Alabama and Mississippi. Others were ancient sand dunes that formed on the extended beaches of the last ice age. When glaciers melted and sea levels rose, these dunes were surrounded by water. Most of the islands off the coast of the southeastern United States, including the Outer Banks of North Carolina and Georgia s Tybee Island, formed in this way. Another type, called a sea island, was actually part of the mainland that remained exposed when the sea level rose. Sea islands, like Cumberland and Hilton Head off the coast of Georgia, are not as sandy as dunelike barrier islands. 

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