| Fig. 155—Steep walls in the Majes Valley below Cantas and the abrupt termination against them of a deep alluvial fill. | Fig. 156—Canyon of the Majes River through the Coast Range north of Camaná. The rock is a granite-gneiss capped by rather flat-lying sedimentaries. |
It is known that the Pleistocene was a time of augmented waste delivery. At the head of the broadly opened Majes Valley there was deposited a huge mass of extremely coarse waste several hundred feet deep and several miles long. Forward from it, interstratified with its outer margin, and continuing the same alluvial grade, is a still greater mass of finer material which descends to lower levels. The fine material is deposited on the floor 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 precipitation 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 evidence 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 valley 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 gradients in the direction of the oldest deposits, shows very clearly a corresponding progression in the growth of the Andes at intervals throughout the Tertiary.
Thus we have aggradation in the Tertiary at the foot of the growing Andes; aggradation in the Pliocene or early Pleistocene on the floor of a deep valley cut in earlier deposits; aggradation in the glacial epoch; and aggradation now in progress. Basin deposits within the borders of the Peruvian Andes are relatively rare. The profound erosion implied by the development, first of a mature topography across this great Cordillera, and second of many deep canyons, calls for deposition on an equally great scale on the mountain borders. The deposits of the western border are a mile thick, but they are confined to a narrow zone between the Coast Range and the Cordillera. Whatever material is swept beyond the immediate coast is deposited in deep ocean water, for the bottom falls off rapidly. The deposits of the eastern border of the Andes are carried far out over the Amazon lowland. Those of earlier geologic periods were largely confined to the mountain border, where they are now upturned to form the front range of the Andes. The Tertiary deposits of the eastern border are less restricted, though they appear to have gathered chiefly in a belt from fifty to one hundred miles wide.
The deposits of the western border were laid down by short streams rising on a divide only 100 to 200 miles from the Pacific. Furthermore, they drain the dry leeward slopes of the Andes. The deposits of the wet eastern border were made by far larger streams that carry the waste of nearly the whole Cordillera. Their shoaling effect upon the Amazon depression must have been a large factor in its steady growth from an inland sea to a river lowland.
CHAPTER XV
PHYSIOGRAPHIC AND GEOLOGIC DEVELOPMENT
GENERAL FEATURES
In the preceding chapter we employed geologic facts in the determination of the age of the principal topographic forms. These facts require further discussion in connection with their closest physiographic allies if we wish to show how the topography of today originated. There are many topographic details that have a fundamental relation to structure; indeed, without a somewhat detailed knowledge of geology only the broader and more general features of the landscape can be interpreted. In this chapter we shall therefore refer not to the scenic features as in a purely topographic description, but to the rock structure and the fossils. A complete and technical geologic discussion is not desirable, first, because it should be based upon much more detailed geologic field work, and second because after all our main purpose is not to discuss the geologic features per se, but the physiographic background which the geologic facts afford. I make this preliminary observation partly to indicate the point of view and partly to emphasize the necessity, in a broad, geographic study, for the reconstruction of the landscapes of the past.