Sir: Regarding the shifting sands at Mawgan Porth (Letters, 5 August), T. G. Mercer's acute observation is in agreement with the known behaviour of sand in moving water. What he sees, as he walks along the dry beach, is the response of the sand to the last downslope movement of the water before it drained away altogether. A rippled surface forms as sand reacts with relatively slowly moving water, say in calm weather. At higher water velocities, associated with greater volumes of water during storms, a rippled interface between sand and water is no longer the most stable form. Instead the water and sand react to give a plane surface. It follows, of course, that the steeper the slope of the beach, the less the chance of finding ripples.
ROBIN G. C. BATHURST
Department of Earth Sciences