482 JOHN LYON RICH 



heavy load of coarse debris prevents rapid down-cutting. Sweep 

 therefore dominates. 



Conditions unfavorable for rapid sweep. — The following con- 

 ditions are unfavorable for rapid sweep : low gradient, as explained 

 above; relatively rapid down-cutting; and an original meander- 

 ing course sunk into bed-rock (the latter not, of course, affecting 

 the tendency to sweep, but only its absolute rapidity in a given 

 case). 



Under the second of these, relatively rapid down-cutting, the 

 stream is continually cutting deeper into bed-rock and thus con- 

 stantly increasing the amount of material to be moved by a meander 

 in sweeping down-valley; not only so, but in the interval between 

 the passage of successive meanders, the stream, at a given point 

 has deepened its bed and there is stiU rock, not flood-plain material, 

 to be removed by each succeeding meander as it sweeps down the 

 valley. 



In cases where down-cutting is long continued, but slow enough 

 so that lateral cutting is conspicuous, there may result a peculiar 

 type of valley with its bottom section exhibiting fine specimens 

 of in-grown meanders, but with the upper part of its walls rela- 

 tively straight. Such a condition would be the necessary result of 

 a passage down- valley of a series of meanders during a slow uplift 

 or its equivalent. What may be a valley of this sort is illustrated 

 by the Chaquaqua Canyon as depicted on the Mesa de Maya 

 (Colo.) sheet (Fig. 9). 



The influence of load. — -A full load of sediment, by occupying 

 all the stream's available energy in its transportation, is an effec- 

 tive check to down-cutting. This does not, however, interfere 

 with the process of sweep on the flood plain, for the sediment 

 derived from cutting on a concave bank may be redeposited on 

 the convex bank next below without in any way increasing the 

 permanent load of the stream. It is a question also if it entirely 

 prevents lateral cutting on the valley walls at the point of 

 impingement of the meander bends. In a fully loaded stream, 

 at any rate, sweep is dominant and the straight-walled valleys 

 are to be expected. 



