The silex pebbles that abound on the beaches of Normandy come from the high cliffs that border them. Encompassed in the mass of chalk composed of skeletal debris of planktonic algae, which crumbles, they will then be rid of their chalk gangue by the swell. Their shapes evolve through abrasion or attrition, and shock waves can break the rock (traces of conchoidal breaks). They are often moderated by the waves of water that leave percussion marks in the form of dihedrals or traces of nails.