Where waves approach the coastline at an angle, when they break
their swash pushes beach material up the beach at the same angle. The backwash
then drags the material down the beach perpendicular (at a 90º angle) to the
shore, following the line of the steepest gradient. This produces a zig-zag
movement of sediment along the beach known as longshore drift. Largest
beach sediment is found updrift, and the smallest material, which is more
easily moved, downdrift.