Coastal Sediments—Sorting

beach pebbles
Detail of beach sediment (pebbles) and organic debris at wrack line. Bering Land Bridge National Preserve, Alaska.

Wind, waves, and currents constantly move and redistribute coastal sediments along shorelines. Beach sediments that have been transported great distances will be sorted according to grain size. For example, Gulf coast sediments may have traveled the length of the Mississippi River. Winnowing of more uniform sediment shapes and sizes results in a beach with well-sorted sediments. In contrast, poorly-sorted sediments are composed of a variety of sediment shapes and sizes. Low-energy conditions that allow fine sediments to settle from suspension must exist for the formation of muddy or sandy beaches. High-energy conditions are typical of rocky beaches that allow for the transportation of coarser sediments. Extreme storm events are necessary to move the largest sediments (e.g., boulders).

sand dunes
Dunes on Padre Island National Seashore, Texas. The park was established to preserve one of the last undeveloped barrier islands in the United States.

Well-Sorted Sediments

Well-sorted beach sands are visible in this photograph of Padre Island National Seashore in Texas. Note that the sand grains are all of similar size and color. The sand dunes found at Padre Island support a wide variety of coastal vegetation, such as sea purslane (Sesuvium portulacastrum), which serves to stabilize dunes and decrease overwash.

rocky shore
Boston Harbor Islands National Recreation Area, Massachusetts.

Poorly-Sorted Sediments

Poorly-sorted sediments display a variety of sizes including cobbles, pebbles, and sand. The sediments come in many shapes, from flat to round, due to the nature of the source rock and abrasion experienced during sediment transport. Rounded cobbles are the most predominant shape and size along this coastline at Boston Harbor Islands.

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Part of a series of articles titled Coastal Geology—Beach Materials.

Last updated: September 10, 2018