Page:The Journal of geology (IA journalofgeology21894univers).pdf/89
deepen their valleys beneath the constructional surface, it often happens that they discover structures of unequal hardness. If, in passing down stream, a weak structure succeeds a hard structure, the valley will be quickly deepened in the former and slowly in the latter; a local increase of slope appears and a fall or cascade is the result. This is a subsequent fall on a consequent stream. It endures until the harder structure is worn down or back so far that it overtakes the deepening of the stream bed below the fall. The extinction of falls is accomplished in adolescence on large streams and on tilted rocks; but it may not be reached until maturity on the smaller streams in regions of horizontal strata.
A further consequence of the discovery of the variable resistance of internal structure is the variable rate at which the narrow young consequent valley widens into the more mature open valley. If the consequent stream crosses a local transverse belt of hard rocks, the gorge-like form of the valley walls may there be retained into the maturity of the region as a whole. If it crosses a belt of weak rocks, the consequent valley may there widen so greatly as to develop other valleys on either side of its path. Thus many a transverse consequent stream, cutting its valley across belts of harder and softer structures, allows the development of longitudinal valleys on every belt of weak structure that it traverses, while the intermediate belts of harder structure stand up as longitudinal dividing ridges. The longitudinal streams and valleys are then called subsequent branches of the transverse consequent streams and valleys. Each of the subsequent streams deepens its valley only as fast as the down-stream deepening of the consequent valley permits.
It is extremely important to recognize the difference thus indicated between consequent and subsequent streams. The first control the drainage of a region in its early stages of development. The second are of increasing importance in the secondary and later stages of growth, when they share the drainage of the region with the surviving consequent streams. Subsequent falls