Plate 84

Delicate scour marks on beach sand

These structures have a very shallow relief and a low preservation potential. Again they are found on the swash zone of a sandy beach, a hydrodynamically smooth surface that is permanently in upper flow regime. Part of the visible surface is smooth indeed (the shell has been placed on it for scale), and represents an area where the front of an incoming wave died. Another part is a little rougher, with shallow striations crossing at acute angles. The striations were made by previous waves, in particular by the interference of swash and backwash: when wave fronts are oblique to the shoreline, they produce oblique marks in climbing the beach face. The return flow is controlled by gravity and follows the maximum dip, which is normal to the shoreline. Under the repeated push of oblique waves, solid particles follow a sawtooth pathway and are drifted along shore.

The boundary between the smooth and the rough areas is marked by a narrow ribbon of sand, a swash mark.  The wave front dies there and abandons the sand it carried. Swash marks form a pattern of arcuate, intersecting lines whose concavity faces seaward in fair weather conditions. Coarser and heterogeneous debris, including flotsam and jetsam, is accumulated by storm waves in arc-shaped berms.

Delicate marks are imprinted on the sand by the wind, too. The sand is dry in this case, and the wind uses the ends of fixed plants (grass blades or twigs) as tools, producing arcuate and circular scratches (swing marks,  see color photo 22). When wind gusts slow down or stop, the flexible plants go back to their rest positions, and in so doing emphasize the grooves. The process can be repeated many times along the same tracks, as in record-playing by a pickup.

Photo: G. Piacentini 1970.