Photographs

FIG. 50. Structure produced by a migrating nearshore bar with along-crest-migrating superimposed bedforms; Pliocene terrace deposits, Monterey Bay, California.

RECOGNITION: This photograph shows details of two scallop-shaped sets of cross-beds in a lateral sequence of approximately a dozen similar scallops (Rubin, 1987).  The main bedform that deposited this bed was composed of sand and migrated to the right and toward the viewer.  While that sandy bedform was migrating, superimposed gravel bedforms migrated along its trough (toward the left and toward the viewer). Scour pits that were formed by intersections of the troughs of the two sets of bedforms migrated directly out of the plane of the outcrop, toward the viewer.  Individual cross-beds within the scallops are curved in plan form and vary in composition.  Beds with a dip toward the left are composed primarily of gravel, because they were deposited on the lee slopes of the gravel bedforms migrating along the main trough.  Beds which were deposited on the lee slope of the main bedform dip toward the right and are composed mostly of sand. Migration of the main bedform, a nearshore bar, is inferred to have been caused by longshore currents, and simultaneous migration of the smaller bedforms is inferred to have been caused by a rip current.