According to the theory of river development, as it has been worked out in its full modern form by a number of eminent American morphologists, the occurrence of narrow v-shaped valleys in a region indicates that the region in question has passed only the very first stages of the cycle of erosion. This type of valleys, considered by older European geologists as the most important feature of river-action, is, according to the modern American school, only an immature stage in the development of river valleys. In the course of time the blocks of land left between the principal valleys will be dissected by secondary streamlets, the valleys will be broadened, and their slopes will get more and more gentle. At last, if the subaerial denudation has time enough to fulfil its work under unaltered conditions, the hills are wholly consumed, the region is base-leveled, and the land once deeply dissected is turned into a slightly undulating plain with broad and shallow, faintly insected river valleys. Not till then has the process reached the final stage of its cycle, the "peneplain." This theory of subaerial denudation is based upon the premise that river-erosion works all over the areas, and not only linearly, as was supposed by an older school of geologists. This assumption looks at first somewhat startling. Evidently the main stream and the principal tributaries of a river form only a branching system of erosive lines. Only when the innumerable rivulets and feeders, all the transient small waters washing hillsides and slopes at rainfalls and snow-meltings, are taken into consideration, does the erosive system present a network so close that it can be said to cover all the area.