Later youth is characterized by a large accumulation of shore waste.  The rock bench has been cut back so that it now furnishes a good roadway for shore drift.  The stream of alongshore drift grows larger and larger, filling the heads of the smaller bays with beaches, building spits and hooks, and tying islands with sand bars to the mainland.  It bridges the larger bays with bay bars, while their length is being reduced as their inclosing promontories are cut back by the waves.  Thus there comes to be a straight, continuous, and easy road, no longer interrupted by headlands and bays, for the transportation of waste alongshore.  The Baltic coast of Germany is in this stage.

All this while streams have been busy filling with delta deposits the bays into which they empty.  By these steps a coast gradually advances to maturity, the stage when the irregularities due to depression have been effaced, when outlying islands formed by subsidence have been planed away, and when the shore line has been driven back behind the former bay heads.  The sea now attacks the land most effectively along a continuous and fairly straight line of cliffs.  Although the first effect of wave wear was to increase the irregularities of the shore, it sooner or later rectifies it, making it simple and smooth.  Northwestern France may be cited as an upland plain, dissected and depressed, whose coast has reached maturity.

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.

Plains of marine abrasion.  While subaerial 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 subaerial 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 subaerial denudation to lower the lands of the globe to the level of the sea.