Fig. 170—The inclined lower and horizontal upper sandstone on the southeastern wall of the Majes Valley at Hacienda Cantas. The section is a half-mile high.
The structural relation of the red sandstone series to the older rocks is well displayed about half-way between Urubamba and Ollantaytambo in the deep Urubamba Valley. The basal rocks are slaty schist and granite succeeded by agglomerates and basalt porphyries upon whose eroded surfaces ([Fig. 169]) are gray to yellow cross-bedded sandstones. Within a few hundred feet of the unconformity gypsum deposits begin to appear and increase in number to such an extent that the resulting soil is in places rendered worthless. Copper-stained bands are also common near the bottom of the series, but these are confined to the lower beds. Higher up in the section, for example, just above the gorge between Urubamba and Ollantaytambo, even-bedded sandstones occur whose most prominent characteristic is the regular succession of coarse and fine sandstone beds. Such alternations of character in sedimentary rocks are commonly marked by alternating shales and sandstones, but in this locality shales are practically absent. Toward the top of the section gypsum deposits again appear first as beds and later, as in the case of the hill-slope on the southern shore of Lake Huaipo, as veins and irregular masses of gypsum. The top of the deformed Cretaceous (?) is eroded and again covered unconformably by practically flat-lying Tertiary deposits.
TERTIARY
The Tertiary deposits of the region under discussion are limited to three regions: (1) the extreme eastern border of the main Cordillera, (2) intermontane basins, the largest and most important of which are (a) the Cuzco basin and (b) the Titicaca-Poopó basin on the Peruvian-Bolivian frontier, and (3) in the west-coast desert and in places upon the huge terraces that form a striking feature of the topography of the coast of Peru.
It has already been pointed out that the eastern border of the Cordillera is marked by a fault of great but undetermined throw, whose topographic importance may be estimated from the fact that even after prolonged erosion it stands nearly four thousand feet high. Cross-bedded and ripple-marked features and small lenses of conglomerate are common. The beds now dip at an angle approximately 20° to 50° northward at the base of the scarp, but have decreasing dip as they extend farther north and east. It is noteworthy that the deposits become distinctly conglomeratic as flatter dips are attained, and that there seems to have been a steady accumulation of detrital material from the mountains for a long period, since the deposits pass in unbroken succession from the highly indurated and massive beds of the mountain base to loose conglomerates that now weather down much like an ordinary gravel bank. In a few places just below the mouth of the Ticumpinea, logs about six inches in diameter were observed embeded in the deposits, but these belong distinctly to the upper horizons.
The border deposits, though they vary in dip from nearly flat to 50°, are everywhere somewhat inclined and now lie up to several hundred feet above the level of the Urubamba River. Their upper surface is moderately dissected, the degree of dissection being most pronounced where the dips are steepest and the height greatest. In fact, the attitude of the deposits and their progressive change in character point toward, if they do not actually prove, the steady and progressive character of the beds first deposited and their erosion and redeposition in beds now higher in the series.
Upon the eroded upper surfaces of the inclined border deposits, gravel beds have been laid which, from evidence discussed in a later paragraph, are without doubt referable to the Pleistocene. These in turn are now dissected. They do not extend to the highest summits of the deformed beds but are confined, so far as observations have gone, to elevations about one hundred feet above the river. From the evidence that the overlying horizontal beds are Pleistocene, the thick, inclined beds are referred to Tertiary age, though they are nowhere fossiliferous.
Observations along the Urubamba River were extended as far northward as the mouth of the Timpia, one of the larger tributaries. Upon returning from this point by land a wide view of the country was gained from the four-thousand-foot ridge of vertical Carboniferous limestone, in which it appeared that low and irregular strike ridges continue the features of the Tertiary displayed along the mountain front far northward as well as eastward, to a point where the higher ridges and low mountains of older rock again appear—the last outliers of the Andean system in Peru. Unfortunately time enough was not available for an extension of the trip to these localities whose geologic characters still remain entirely unknown. From the topographic aspects of the country, it is, however, reasonably certain that the whole intervening depression between these outlying ranges and the border of the main Cordillera, is filled with inclined and now dissected and partly covered Tertiary strata. The elevation of the upper surface does not, however, remain the same; it appears to decrease steadily and the youngest Tertiary strata disappear from view below the sediments of either the Pleistocene or the present river gravels. In the more central parts of the depression occupied by the Urubamba Valley, only knobs or ridges project here and there above the general level.
The Coastal Tertiary
The Tertiary deposits of the Peruvian desert region southwest of the Andes have many special features related to coastal deformation, changes of climate, and great Andean uplifts. They lie between the west coast of Peru at Camaná and the high, lava-covered country that forms the western border of the Andes and in places are over a mile thick. They are non-fossiliferous, cross-bedded, ripple-marked, and have abundant lenses of conglomerate of all sizes. The beds rest upon an irregular floor developed upon a varied mass of rocks. In some places the basement consists of old strata, strongly deformed and eroded. In other places it consists of a granite allied in character and probably in origin with the old granite-gneiss of the Coast Range toward the west. Elsewhere the rock is lava, evidently the earliest in the great series of volcanic flows that form this portion of the Andes.