THE COASTAL TERRACES 231 



of a valley cut into Tertiary strata, hence it is younger than the 

 Tertiary. It is now, and has been for some time past, in process 

 of dissection, hence it was not formed under present conditions of 

 climate and relief. It is confidently assigned to the Pleistocene, 

 since this is definitely known to have been a time of greater pre- 

 cipitation and waste removal on the mountains, and deposition on 

 the plains and the floors of mountain valleys. Such a conclusion 

 appears, even on general grounds, to be but a shade less reliable 

 than if we were able to find in the upper Majes Valley, as in so 

 many other Andean valleys, similar alluvial deposits interlocked 

 with glacial moraines and valley trains. 



In regard to the second consideration — the upbowing of the 

 Cordillera — it may be noted that the valley and slope profiles of 

 the main Cordillera shown on p. 191, when extended toward the 

 margin of the mountain belt, lie nearly a mile above the level of 

 the sea on the west and the Amazon plains on the east. The evi- 

 dence of regional bowing thus afforded is checked by the depths of 

 the mountain valleys and the stream profiles in them. The 

 streams are now sunk from one to three thousand feet below their 

 former level. Even in the case of three thousand feet of erosion 

 the stream profiles are still ungraded, the streams themselves are 

 almost torrential, and from one thousand to three thousand feet 

 of vertical cutting must still be accomplished before the profiles 

 will be as gentle and regular as those of the preceding cycle of 

 erosion, in which were formed the mature slopes now lying high 

 above the valley floors. 



Further evidence of bowing is afforded by the attitude of the 

 Tertiary strata themselves, more highly inclined in the case of 

 the older Tertiary, less highly inclined in the case of the younger 

 Tertiary. It is noteworthy that the gradient of the present val- 

 ley floor is distinctly less than that of the least highly inclined 

 strata. This, is true even where aggradation is now just able to 

 continue, as near the nodal point of the valley, above Aplao, 

 where cutting ceases and aggradation begins. (See the Aplao 

 Quadrangle for change of function on the part of the stream 

 a half mile above Cosos). Such a progressive steepening of 



