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. The following swash tends to re-enforce this sorting. Further observations on the beach confirm that opposing valves behave as predicted. The sense of sorting is correlated to the direction of wave approach and hence with the direction of longshore drift. Interpretations of paleodrift directions from R-L sorting may be possible, but the narrowness of the inter-tidal zone in which the sorting takes place, and the rapidity of resorting, plus uncertainties in the hydro-dynamic behavior of fossilized material, dictate extreme care in attempting such analyses. |
