Distinguishing between Beach and Dune Sands

Abstract

ABSTRACT The comparison of adjacent pairs of beach (largely foreshore) and dune sands from 74 localities of world-wide distribution has shown that it is generally possible to distinguish between the two, particularly if there are predominant onshore winds. The dune sands are usually rounder than the adjacent beach sands, have a larger silt content, and this silt has a higher content of heavy minerals. The beach foreshores, on the other hand, usually contain more shells and other calcareous organisms than the dunes. There is also some reason to believe that mica content is greater in beach foreshores than in adjacent dunes. The sand of beach berms appears to be intermediate in most respects between that of the foreshores and dunes. The areas where beaches and dunes are most difficult to distinguish are those where there are longshore, extremely variable, or offshore winds. In such localities it is thought that the dune sands are blown back onto the beaches causing intermixture and that in some cases the dunes are derived from sources other than from the adjacent beach. The best results in separating the two environments have come from beaches of the west coast of the United States and Baja California where onshore winds predominate. It is suggested that the chief reason for the differences between dune and beach sands is that the wind picks up from the beaches more of the rounder grains than of the flat and angular grains. There is no reason to believe that the grains are rounded appreciably by the wind in transit to the dunes. Nor does there appear to be any evidence from these comparisons that the dune grains become frosted. The greater quantity of heavy minerals in the dune silt fraction may be caused by removal by the wind of the light minerals.