Differential sorting of right and left valves of pelecypods was found to be common on Mustang and Padre Island beaches, Texas. Determination of stable orientations of valves in unidirectional flume currents enables one to predict their behavior on a beach washed by obliquely approaching waves. The valves become initially stable with respect to the oblique swash and then are sorted by the backwash. Further observations on the beach confirm that opposing valves behave as predicted. Sense of sorting is correlated to direction of wave approach and with direction of longshore drift. Interpretations of paleodrift directions from R-L sorting may be possible, but narrowness of the inter-tidal zone in which sorting takes place, and rapidity of resorting, plus uncertainties in hydrodynamic behavior of fossilized material, dictate extreme care in attempting analyses.

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