Ripples and Dunes

When a wind or water current flows across loose sand, the sand is dragged along the bottom and frequently is piled up to form ripples and dunes. The main difference between a ripple and dune is size, with dunes being taller than about 10 cm. You have no doubt seen ripples and dunes at the beach (lake or ocean), along a sandy river bank, or in the desert. Sometimes tiny ripples form on silty sediment, and larger dunes form where very fast water currents flow over gravel. Ripples and dunes are useful because they are fairly commonly preserved in the sedimentary record. Assymetrical ripples and dunes form when current flows in a single direction, as along a river bottom. Symmetrical ripples and dunes form where currents flow in two directions, as where waves wash back and forth. Preserved ripples and dunes thus provide clues as to whether a given sandstone was influenced by waves or unidirectional currents and, in the later case, they indicae which way the water flowed. See your text book for illustrations and more explanation.

These are slightly assymetrical ripples from a beach in North Carolina. Because sediment is carried up the gentler slope and dumped down the steeper side, the current flowed from the upper left to the lower right. The ripples formed at high tide as a tidal current flowed out along a tidal inlet in a barrier island beach.
These are molds of ripples preserved on the underside of a tilted sedimentary layer in Death Valley, CA. The ripples themselves are preserved in the thick underlying sandstone layer. Since these ripples appear to be symmetrical, they probably formed as the result of wave action.
The two examples above show ripples with fairly straight crests (tops). Ripples and dunes come in a variety of shapes and sizes, so the next time you are at a sandy beach, keep your eyes open!
This is another example of ripples preserved as molds on the underside of tilted beds. Note how the ripple crests are straight in the lower layer and along the right of the upper layer, but show an interference pattern in the center left part of the upper layer. This interference pattern resulted from waves coming from two different directions meeting at this one spot. Death Valley, CA

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