Rate of recession.  The rate at which a shore recedes depends on several factors.  In soft or incoherent rocks exposed to violent storms the retreat is so rapid as to be easily measured.  The coast of Yorkshire, England, whose cliffs are cut in glacial drift, loses seven feet a year on the average, and since the Norman conquest a strip a mile wide, with farmsteads and villages and historic seaports, has been devoured by the sea.  The sandy south shore of Martha’s Vineyard wears back three feet a year.  But hard rocks retreat so slowly that their recession has seldom been measured by the records of history.

SHORE DRIFT

Bowlder and pebble beaches.  About as fast as formed the waste of the sea cliff is swept both along the shore and out to sea.  The road of waste along shore is the beach.  We may also define the beach as the exposed edge of the sheet of sediment formed by the carriage of land waste out to sea.  At the foot of sea cliffs, where the waves are pounding hardest, one commonly finds the rock bench strewn on its inner margin with large stones, dislodged by the waves and by the weather and some-what worn on their corners and edges.  From this bowlder beach the smaller fragments of waste from the cliff and the fragments into which the bowlders are at last broken drift on to more sheltered places and there accumulate in a pebble beach, made of pebbles well rounded by the wear which they have suffered.  Such beaches form a mill whose raw material is constantly supplied by the cliff.  The breakers of storms set it in motion to a depth of several feet, grinding the pebbles together with a clatter to be heard above the roar of the surf.  In such a rock crusher the life of a pebble is short.  Where ships have stranded on our Atlantic coast with cargoes of hard-burned brick or of coal, a year of time and a drift of five miles along the shore have proved enough to wear brick and coal to powder.  At no great distance from their source, therefore, pebble beaches give place to beaches of sand, which occupy the more sheltered reaches of the shore.

Sand beaches.  The angular sand grains of various minerals into which pebbles are broken by the waves are ground together under the beating surf and rounded, and those of the softer minerals are crushed to powder.  The process, however, is a slow one, and if we study these sand grains under a lens we may be surprised to see that, though their corners and edges have been blunted, they are yet far from the spherical form of the pebbles from which they were derived.  The grains are small, and in water they have lost about half their weiglit in air; the blows which they strike one another are therefore weak.  Besides, each grain of sand of the wet beach is protected by a cushion of water from the blows of its neighbors.