Lateral Grading of Beach Sediments: A Commentary

Lateral grading of beach sediments can be achieved by downdrift attrition and/or longshore drifting. Chesil Beach (England) is graded from small to large pebbles and cobbles south-eastward in the direction of increasing wave energy. Attrition is very slow, and grading seems to have been achieved by alternations of longshore drifting, with selection of coarser from finer particles by the stronger south-eastward movement. Wave energy gradients may have contributed by moving smaller particles westward, and by conserving the lateral gradation. Hawke Bay beach (New Zealand) is graded in the reverse direction. It is fed with sand and gravel by the Mohaka River, near the western end, and lateral grading has been attributed to downdrift attrition eastward. Longshore sorting has probably also contributed, and a method of separating attrition from sorting is proposed. The two systems are compared as a basis for elucidating the causes of lateral drifting, which can evidently develop in different ways.