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Beaches and bars have been formed during experiments conducted in a 45-foot wave tank at the Sedimentation Laboratory of the U. S. Geological Survey in Denver. By changing one variable at a time, factors responsible for major differences in primary structure and in shape of sand body have been determined. These factors are: depth of water, intensity of wave action, and supply of sand. Stages in the development of the bars and beaches were marked with dark layers of magnetite and cross sections were preserved on masonite boards coated with liquid rubber, thus making a record of cross-stratification patterns and sand-body shapes.
Offshore bars develop at the point of wave break. Where this occurs in very shallow water an emergent bar commonly forms; where it is in somewhat deeper water a submarine bar is built; where still deeper no bar develops. Increase in intensity of waves tends to build a bar forward toward, and even onto the beach. Weaker waves build upward to form barriers with lagoons to shoreward. Abundant sand furnished on the seaward side of a developing bar, simultating conditions developed by some longshore currents, causes gently sloping, seaward-dipping beds to form. In contrast, shoreward-dipping strata of steeper angle are characteristic of bars developed where the sand supply is limited.
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