_mature_[163] so far as wave-erosion is concerned. Since coastal lands are, in general, both heterogeneous and unequally exposed, a mature coast-line is somewhat irregular. Its maturity is attained when the lesser exposure in the reëntrants developed in the less resistant parts, balances the superior exposure of the projections of the more resistant portions. Since the conditions of erosion along coasts are constantly, even if slowly, changing, maturity is constantly being approached, but rarely reached. Other forces and processes, such as those of aggradation, vulcanism, and diastrophism, are in operation along coasts, and their results are sometimes antagonistic to those of the waves. The horizontal configuration of coasts is, therefore, the result of many coöperating forces, of which waves are but one. It is, nevertheless, important to note the goal to which the waves are working, even though they are continually defeated in their attempt to reach it. Their immediate goal is an equilibrium of erosion-rate and maturity of configuration; their final goal is the destruction of the land and the deposition of its substance in the sea, that is, in a position nearer the center of gravity of the earth. _Transportation by Waves._ The material eroded from the shore by the waves in the shaping of the cliff and terrace is carried away by the joint action of the waves, undertow, and shore-currents. The in-coming wave begins to shift material where it begins to drag bottom, that is, a little outside the line of breakers. From the line where transportation begins, to the line of breakers, bottom detritus is shifted shoreward by the waves, while the undertow tends to carry it back again. Between the breakers and the shore there is also a tendency for the on-shore movement to carry débris to the water’s edge, and for the ebbing wave to carry it back again. The result of these opposed tendencies is to keep sediment in transit between the shore and the line of breakers. If the in-coming waves have a direction normal to the shore, the advance and recoil of the water move particles toward and from the shore, but effect no transfer along the shore; but the results which waves normal to the shore would achieve are always modified by other waves and by littoral currents.