After the development of a base-level plain, its surface would suffer little change (except that effected by underground water) so long as it maintained its position. But if, after its development, a base-level plain were elevated, the old surface in a new position would be subject to a new series of changes identical in kind with those which had gone before. The elevation would give the established streams greater fall, and they would reassume the characteristics of youth. The greater fall would accelerate their velocities; the increased velocities would entail increased erosion; increased erosion would result in the deepening of the valleys, and the deepening of the valleys would lead to the roughening of the surface. But in the course of time, the _rejuvenated_ streams would have cut their valleys as low as the new altitude of the land permitted, that is, to a new base-level. The process of deepening would then stop, and the limit of vertical relief which the streams were capable of developing, would be attained. But the valleys would not stop widening when they stopped deepening, and as they widened, the intervening divides would become narrower, and ultimately lower. In the course of time they would be destroyed, giving rise to a new level surface much below the old one, but developed in the same position which the old one occupied when it originated; that is, a position but little above sea level. If at some intermediate stage in the development of a second base-level plain, say at a time when the streams, rejuvenated by uplift, had brought half the elevated surface down to a new base-level, another uplift were to occur, the half completed cycle would be brought to an end, and a new one begun. The streams would again be quickened, and as a result they would promptly cut new and deeper channels in the bottoms of the great valleys which had already been developed. The topography which would result is suggested by the following diagram (Fig. 22) which illustrates the cross-section which would be found after the following sequence of events: (1) The development of a base-level, A A; (2) uplift, rejuvenation of the streams, and a new cycle of erosion half completed, the new base-level being at B B; (3) a second uplift, bringing the second (incomplete) cycle of erosion to a close, and by rejuvenating the streams, inaugurating the third cycle. As represented in the diagram, the third cycle has not progressed far, being represented only by the narrow valley C. The base-level is now 2-2, and the valley represented in the diagram has not yet reached it. [Illustration: Fig. 22.--Diagram (cross-section), illustrating the topographic effect of rejuvenation by uplift.] [Illustration: Fig. 23.--Normal profile of a valley bottom in a non-mountainous region.]