In the _old age_ of coasts the rock bench is cut back so far that the waves can no longer exert their full effect upon the shore. Their energy is dissipated in moving shore drift hither and thither and in abrading the bench when they drag bottom upon it. Little by little the bench is deepened by tidal currents and the drag of waves; but this process is so slow that meanwhile the sea cliffs melt down under the weather, and the bench becomes a broad shoal where waves and tides gradually work over the waste from the land to greater fineness and sweep it out to sea. [Illustration: Fig. 148. The South Shore of Martha's Vineyard The land is shaded. To what class of coasts does this belong? What stage has it reached, and by what process? What changes will take place in the future?] =Plains of marine abrasion.= While subaërial denudation reduces the land to baselevel, the sea is sawing its edges to _wave base_, i.e. the lowest limit of the wave's effective wear. The widened rock bench forms when uplifted a plain of marine abrasion, which like the peneplain bevels across strata regardless of their various inclinations and various degrees of hardness. How may a plain of marine abrasion be expected to differ from a peneplain in its mantle of waste? Compared with subaërial denudation, marine abrasion is a comparatively feeble agent. At the rate of five feet per century--a higher rate than obtains on the youthful rocky, coast of Britain--it would require more than ten million years to pare a strip one hundred miles wide from the margin of a continent, a time sufficient, at the rate at which the Mississippi valley is now being worn away, for subaërial denudation to lower the lands of the globe to the level of the sea. Slow submergence favors the cutting of a wide rock bench. The water continually deepens upon the bench; storm waves can therefore always ride in to the base of the cliffs and attack them with full force; shore waste cannot impede the onset of the waves, for it is continually washed out in deeper water below wave base. =Basal conglomerates.= As the sea marches across the land during a slow submergence, the platform is covered with sheets of sea-laid sediments. Lowest of these is a conglomerate,--the bowlder and pebble beach, widened indefinitely by the retreat of the cliffs at whose base it was formed, and preserved by the finer deposits laid upon it in the constantly deepening water as the land subsides. Such basal conglomerates are not uncommon among the ancient rocks of the land, and we may know them by their rounded pebbles and larger stones, composed of the same kind of rock as that of the abraded and evened surface on which they lie. CHAPTER VIII OFFSHORE AND DEEP-SEA DEPOSITS