Fair- and foul-weather shell accumulations on a Georgia beach

Abstract

Wrack-line accumulations of beach shells are governed primarily by longshore drift and secondarily by local geomorphic variations along the strand. During low-energy conditions of summer on Sapelo Island beaches, most shells move in the direction of prevalent wave incidence and accumulate in beach re-entrants. Very few shells accumulate on beach protrusions, which are focal points for refracting waves, or on estuary-inlet beaches, where ebb-tidal currents predominate over longshore currents. During high-energy conditions associated with a minor tropical storm, virtually all trends in shell accumulation were reversed. Most shells were removed from the beach; yet more shells remained along relatively straight beach segments than within beach re-entrants, at a point midway between major northward and southward components of longshore drift. These contrasting assemblages thus help demonstrate the importance of comparative taphonomy as (1) a measure of fluctuating coastal conditions-especially the shift from background to episodic stratinomy-and (2) a prerequisite to paleoecological and paleoenvironmental reconstructions. Most beach shells, storm and nonstorm, were derived originally from the nearshore shelf. Major differences between the structure of nearshore populations and that of beach-shell assemblages, aside from physical stratinomic effects, are attributable to hermit crab utilization of select gastropod valves and to differential rates of productivity among standing crops of mollusks.