Fig. 128—Composition of slopes and profiles in the Peruvian Andes. By superimposing the cross profiles of typical valleys as shown in [127] a restoration is possible of the longitudinal profiles of the earlier cycle of erosion. The difference in elevation of the two profiles gives less than the minimum amount of uplift that must have occurred. Case A represents a valley in which recent cutting has not yet reached the valley head. Below the point 1 the profile has been steepened and lowered by erosion in the current cycle. Above point 1 the profile is still in the stage it reached in the preceding cycle. In case B the renewed erosion of the current cycle has reached to the valley head. Case C represents conditions similar to those in the preceding cases save that the stream is typical of those that lie nearest the steep flexed or faulted margins of the Cordillera and discharge to the low levels of the desert pampa on the west or the tropical plains on the east.
Further proof of recent and great uplift is afforded by the deeply intrenched streams. After descending the long graded slopes one comes upon the cliffed canyons with a feeling of consternation. The effect of powerful erosion, incident upon uplift, is heightened by the ungraded character of the river bed. Falls and rapids abound, the river profiles suggest tumultuous descents, and much time will elapse before the river beds have the regular and moderate gradients of the streams draining the mature surface before uplift as shown in the profiles by the dotted lines representing the restored valley floors of the older cycle. Since the smooth-contoured landscape was formed great changes have taken place. The streams have changed from completely graded to almost completely ungraded profiles; in place of a subdued landscape we now have upland slopes intersected by mile-deep canyons; the high-level slopes could not have been formed under existing conditions, for they are being dissected by the present streams.
Since the slopes of the land in general undergo progressive changes in the direction of flatter gradients during a given geographical cycle, it follows that with the termination of one cycle and the beginning of another, two sets of slopes will exist and that the gradients of the two will be unlike. The result is a break in the descent of the slopes from high to low levels to which the name “topographic unconformity” is now applied. It will be a prominent feature of the landscape if the higher, older, and flatter gradients have but little declivity, and the gradients of the lower younger slopes are very steep. In those places where the relief of the first cycle was still great at the time of uplift, the erosion forms of the second cycle may not be differentiated from those of the first, since both are marked by steep gradients. In the Central Andes the change in gradient between the higher and lower slopes is generally well marked. It occurs at variable heights above the valley floors, though rarely more than 3,000 feet above them. In the more central tracts, far from the main streams and their associated canyons, dissection in the present erosion cycle has not yet been initiated, the mature slopes are still intact, and a topographic unconformity has not yet been developed. The higher slopes are faced with rock and topped with slowly moving waste. Ascent of the spur end is by steep zigzag trails; once the top is gained the trail runs along the gentler slopes without special difficulties.
It is worth noting at this point that the surface of erosion still older than the mature slopes herewith described appears not to have been developed along the seventy-third meridian of Peru, or if developed at one time, fragments of it no longer remain. The last well-developed remnant is southwest of Cuzco, [130] . I have elsewhere described the character and geographic distribution of this oldest recognizable surface of the Central Andes.[41] Southern Peru and Bolivia and northern Chile display its features in what seems an unmistakable manner. The best locality yet found is in the Desaguadero Valley between Ancoaqui and Concordia. There one may see thousands of feet of strongly inclined sediments of varying resistance beveled by a well-developed surface of erosion whose preserval is owing to a moderate rainfall and to location in an interior basin.[42]
The highest surface of a region, if formed during a prolonged period of erosion, becomes a surface of reference in the determination of the character and amount of later crustal deformations, having somewhat the same functions as a key bed in stratigraphic geology. Indeed, concrete physiographic facts may be the only basis for arguments as to both epeirogenic and orogenic movements. The following considerations may show in condensed form the relative value of physiographic evidence: