| Fig. 144—Cliffed canyon wall in the Urubamba Valley between Huadquiña and Torontoy. There is a descent of nearly 2,000 feet shown in the photograph and it is developed almost wholly along successive joint planes. | Fig. 145—Another aspect of the canyon wall of [144] . The almost sheer descents are in contrast with the cliff and platform type of topography characteristic of the Grand Canyon of Colorado. |
If we apply the foregoing considerations to the Cordillera Vilcapampa, we shall find some striking illustrations of the principles involved. The invasion of the granite was accompanied by moderate absorption of the displaced rock, and more especially by the marginal pushing aside of the sedimentary rim. The immediate effect must have been to give both intruded rock and country rock greater height and marked ruggedness. There followed a period of regional compression and torsion, and the development of widespread joint systems with strikingly regular features. In the Silurian shales and slates these joints are closely spaced; in the granites they are in many places twenty to thirty feet apart. The shales, therefore, offer many more points of attack and have weathered down into a smooth-contoured topography boldly overlooked along the contact by walls and peaks of granite. In some cases a canyon wall a mile high is developed entirely on two or three joint planes inclined at an angle no greater than 15°. The effect in the granite is to give a marked boldness of relief, nowhere more strikingly exhibited than at Huadquiña, below Colpani, where the foot-hill slopes developed on shales and slates suddenly become moderate. The river flows from a steep and all but uninhabited canyon into a broad valley whose slopes are dotted with the terraced chacras, or farms, of the mountain Indians.
The Torontoy granite is also homogeneous while the shales and slates together with their more arenaceous associates occur in alternating belts, a diversity which increases the points of attack and the complexity of the forms. Tending toward the same result is the greater hardness of the granite. The tendency of the granite to develop bold forms is accelerated in lofty valleys disposed about snow-clad peaks, where glaciers of great size once existed, and where small glaciers still linger. The plucking action of ice has an excellent chance for expression, since the granite may be quarried cleanly without the production of a large amount of spoil which would load the ice and diminish the intensity of its plucking action.
As a whole the Central Andes passed through a cycle of erosion in late Tertiary time which was interrupted by uplift after the general surface had been reduced to a condition of topographic maturity. Upon the granites mature slopes are not developed except under special conditions (1) of elevation as in the small batholith above Chuquibambilla, and (2) where the granite is itself bordered by resistant schists which have upheld the surface over a broad transitional belt. Elsewhere the granite is marked by exceedingly rugged forms: deep steep-walled canyons, precipitous cirques, matterhorns, and bold and extended escarpments of erosion. In the shale belt the trails run from valley to valley in every direction without special difficulties, but in the granite they follow the rivers closely or cross the axis of the range by carefully selected routes which generally reach the limit of perpetual snow. Added interest attaches to these bold topographic forms because of the ruins now found along the canyon walls, as at Torontoy, or high up on the summit of a precipitous spur, as at Machu Picchu near the bridge of San Miguel.
The Vilcapampa batholith is bordered on the southwest by a series of ancient schists with which the granite sustains quite different relations. No sharp dividing line is visible, the granite extending along the planes of foliation for such long distances as in places to appear almost interbedded with the schists. The relation is all the more striking in view of the trifling intrusions effected in the case of the seemingly much weaker shales on the opposite contact. Nor is the metamorphism of the invaded rock limited to simple intrusion. For several miles beyond the zone of intenser effects the schists have been enriched with quartz to such an extent that their original darker color has been changed to light gray or dull white. At a distance they may even appear as homogeneous and light-colored as the granite. At distant points the schists assume a darker hue and take on the characters of a rather typical mica schist.
It is probable that the Vilcapampa intrusion is one of a family of batholiths which further study may show to extend over a much larger territory. The trail west of Abancay was followed quite closely and accidentally crosses two small batholiths of peculiar interest. Their limits were not closely followed out, but were accurately determined at a number of points and the remaining portion of the contact inferred from the topography. In the case of the larger area there may indeed be a connection westward with a larger mass which probably constitutes the ranges distant some five to ten miles from the line of traverse.