752 M. M. FEN NEMAN 



reasoning above is not dependent on its application to the St. Louis 

 region. This illustration is not brought forward to prove the argu- 

 ment but to show the nature of the problem in which the question 

 occurs. 



It should also be noted that the case of uniform down-cutting over 

 a wide area implies a mature drainage system throughout. In the 

 first dissection of a plateau this condition is reached first near the 

 edge, hence the whole plateau cannot be simultaneously lowered. 

 The erosion of an uplifted peneplain may, however, begin with a 

 ready-made drainage system, the whole of which soon becomes 

 incised. The whole area has, therefore, something of an even start 

 in down-cutting. 



Bearing of these principles on profile, and cross-section of valleys. — 

 Returning to the principles, let us examine their bearing on the 

 profile and cross-section of the young valley. Whether there is or is 

 not any progressive union of overloaded rills, the power of such wash 

 increases as the slope is descended, though it is not necessary to 

 assume that the rills become less overloaded, for their load is like- 

 wise increasing. The reason for the increase of power lies in the 

 increased amount of water. It is to be assumed that the fall of rain is 

 equal throughout the slope, but since some of the descending water 

 fails to percolate, the amount which joins the run-off is cumulative 

 as the slope is descended. The effect of increasing power in this 

 case is increasing slope. 



Increase of slope as the result of increase of power is contrary to 

 our customary conceptions gained from constant attention to rivers 

 in which the reverse is usually true. A brief statement of the geomet- 

 rical principle involved may therefore be necessary. Where a current 

 is limited in its down-cutting by a certain level beneath which it 

 cannot cut, additional power results in cutting nearer to that level 

 and flattening the profile. Where the opportunity for down-cutting 

 is not thus hmited, the current is free to cut downward in proportion 

 to its power and the effect of progressively increasing power is a pro- 

 gressively steepening profile. This is the case where a cutting stream 

 encounters a fall. Both the principle and the form are illustrated 

 in the rapids above all falls. However low the fall may be, it removes 

 the upper stream entirely from the influence of the limiting level 



