=Differential movement. Warping.=—Where a land surface on which a river system is established suffers warping, some parts going up and others down, the opposite movements being either absolute or relative, various phenomena would result. This may be illustrated by the accompanying diagrams (Figs. 161 and 162), where the profiles of the streams are represented as warped from the positions represented by the dotted lines, to the positions shown by the full lines. The velocity will be accelerated below the points of differential elevation (between _a_ and _b_, Fig. 161, and between _a_ and _b_, and _c_ and _d_, Fig. 162), but checked above (above _a_, and between _b_ and _c_, Fig. 162). Above an elevation which notably checks its flow, a stream is _ponded_. If the ponding is slight, a marsh may develop above the obstruction; if more considerable, a lake is formed. Lakes of this class are likely to be short-lived, since the ponded waters are likely to soon overflow and lower their outlet so as to drain the lake. The elevation which ponds the stream may be great enough and rapid enough so that the resulting lake finds an outlet by some course other than that originally followed by the stream. Where a stream holds its course across an uplift athwart its valley, either with or without ponding, it becomes an antecedent stream (see p. 169), since it has a course assumed before the latest deformation of the crust and in apparent disregard of present surface configuration. Thus the Columbia River holds its antecedent course across areas which have been uplifted (differentially) hundreds and even thousands of feet.[65] Some of the striking scenic features of this noble valley are the result of these changes in the country through which it flows. A lesser stream would have been diverted, as many of its tributaries have been. Even its course across the Cascade ranges is believed to be antecedent.[66] [Illustration: +Figs.+ 163, 164.—Piracy stimulated by warping. Uplift along axis 1–2.] Another peculiarity of valleys and streams resulting from changes of level is illustrated in Fig. 2, Pl. XIV (southern California). The main valleys of this part of the coast were developed when the land stood considerably higher than now. Later the subsidence of the coast converted the lower ends of the valleys into bays or fiords. The bays were then transformed into lagoons by deposition. Subsequent rise of the land or depression of the sea allowed the drainage from the old lagoons to cut across the deposits which had converted the bays into lagoons. The result is an old, wide valley above, suggested by a young one below.