STREAM VALLEYS AND THEIR MEANING 483 



DEDUCTIVE STUDY OE THE EFFECTS OF THE RATE OF UPLIFT OF A 

 DRAINAGE BASIN ON THE FORM OF ITS VALLEYS 



We have already seen what a fundamental role in the determina- 

 tion of valley form is played by the rate of down-cutting. A deduct- 

 ive study of the effects of differing rates of uplift, with their 

 correlated differing rates of down-cutting, on streams of varying 

 characters should enable us to evaluate properly this important 

 factor. 



Rapid uplift of a relatively straight stream. — ^Assume a region of 

 perfectly homogeneous rocks, drained directly to the sea by a master 

 stream and its tributaries. Let the region have advanced to the 

 old-age stage in the erosion cycle, with all the streams thoroughly 

 graded and with the master stream swinging in long, open curves 

 on its way to the sea. Now, considering only the master stream, 

 assume an uplift of the land uniform in amount throughout the 

 drainage area, and so rapid that the stream in its down-cutting 

 cannot keep pace with it. What will be the result ? 



. In the first place, a wave of down-cutting, beginning at the 

 mouth of the stream, will progress up the valley. Since, by assump- 

 tion, the stream is a large one, down-cutting will be rapid and will 

 greatly outstrip lateral cutting and sweep. The stream will 

 quickly intrench itself in whatever course it happens to be following. 

 Until grade is reached on the cessation of the uplift, deepening of 

 the lower course of the stream will greatly exceed lateral cutting. 

 As soon, however, as grade is attained, lateral swinging and sweep 

 will increase in relative importance. The latter, as explained above, 

 will dominate and the constant downstream procession of more or 

 less open meander loops will widen the valley and at the same time 

 tend to produce relatively straight valley walls. Further widening 

 and further decrease in gradient will, as already explained, result in 

 the scalloping of the bluffs. Finally, as the flood plain comes to 

 exceed the width of the meander belt, the further widening will 

 be more irregular, and the older of the meander scaurs will lose 

 their original sharpness, though not their characteristic ground-plan. 



The valley resulting from such a series of events would corre- 

 spond to our "open valley" (type i) and the series of stages through 



