Nov., 1907.] Aggradation and Degradation of Valleys. 193 



valley than in the lower. In the latter the stream spread out 

 lazily in a shallow sheet covering nearly the entire width of the 

 valley floor. Here little if any erosion was taking place, because 

 the stream was essentially at base level. Sediments picked up 

 above were transported across this area without modifying the 

 lower valley, and into the lake to extend the delta. As often 

 as the delta was built out Vo the plug, it was artificially removed. 

 In order to have the lower part of the valley degraded, it was 

 found necessary gradually to lower the plug in the lake, just as, 

 reversing conditions, it was necessary while aggrading the valley 

 to increase its height. This done, terraces were formed by the 

 stream's meandering in the lower course, as well as in the upper. 



The rock spurs mentioned at first, jutting out into the valley, 

 resembling in perspective the entering spurs from alternate sides 

 of a vallev as seen in mature, dissected plateaus, were built to 

 test the theory of "defended cusps" as set forth in a paper by 

 Davis. ^ Our results supported his theory perfectly, for while 

 the stream swung back and forth across its meander belt, leaving 

 terraces at the limits of each meander, these terraces were very 

 often destroyed b}" the next migration until the cutting had 

 reached sufficient depth to discover the rock spurs, when subse- 

 quently all terraces, higher on the bluffs than the spur encoun- 

 tered, were preserved. Also in many cases terraces down 

 stream from the spurs were sufficiently defended for preservation. 



In Fig. 9, A, B, and C represent terrace tops, t and m, the 

 ledges encountered by the stream, and n, a continuation of the 

 effective spur seen above. While the stream was flowing on 

 terrace C the ledge t protected terraces B and A from being 

 destroyed. 



The experiment showed also that the succession of terraces 

 with narrowing strath on each lower level was not due to a 

 decrease in the volume of the stream, for we were careful to 

 maintain a constant stream; and, while there were temporary 

 variations, it is probable that every hour's work was done b}' 

 about the same quantity of water, and certain that no appre- 

 ciable decrease in the volume occurred during the degradation 

 process. The terraces were made by the persistent systematic 

 swinging of the stream back and forth across its flood plain. 

 Where the stream encountered the ledges it was restrained in its 

 lateral cutting and each lower level presented a narrower valley 

 than the one above, but where no ledges were met the stream 

 in the end usually undercut the older terraces and destroyed 

 them. The stream in this erosion stage of its work, however, 

 did not change its course nearly so often as it did during the 

 stage of sedimentation suggesting, as has been shown by Griggs,^ 



2 Bull. Mus. Comp. Zool. Harvard. 1-902. Geol. Series Vol. V. 



3 Bull. Amer. Geog. Soc. 1906. Vol. XXXVIII, pp. 168-177. 



