The deposits on the western border of the Andes are excellently exposed in the Majes Valley, one of the most famous in Peru, though its fame rests rather upon the excellence and abundance of its vineyards and wines than its splendid geologic sections. Its head lies near the base of the snow-capped peaks of Coropuna; its mouth is at Camaná on the Pacific, a hundred miles north of Mollendo. It is both narrow and deep; one may ride across its floor anywhere in a half hour. In places it is a narrow canyon. Above Cantas it is sunk nearly a mile below the level of the desert upland through which it flows. Along its borders are exposed basal granites, old sedimentaries, and lavas; inter-bedded with it are other lavas that lie near the base of the great volcanic series; through it still project the old granites of the Coast Range; and upon it have been accumulated additional volcanic rocks, wind-blown deposits, and, finally, coarse wash formed during the glacial period. From both the variety of the formations, the small amount of marginal dissection, and the excellent exposures made possible by the deep erosion and desert climate, the Majes Valley is one of the most profitable places in Peru for physiographic and geologic study.
Fig. 171—Generalized sketch section to show the structural relations of the Maritime Cordillera, the desert pampas, and the Coast Range.
The most complete succession of strata (Tertiary) occurs just below Cantas on the trail to Jaguey ([Fig. 171]). Upon a floor of granite-gneiss, and alternating beds of quartzite and shale belonging to an older series, are deposited heavy beds of red sandstone with many conglomerate lenses. The sandstone strata are measurably deformed and their upper surfaces moderately dissected. Upon them have been deposited unconformably a thicker series of deposits, conglomerates, sandstones, and finer wind-blown material. The basal conglomerate is very coarse—much like beach material in both structure and composition, and similar to that along and south of the present coast at Camaná. Higher in the section the material is prevailingly sandy and is deposited in regular beds from a few inches to a few feet in thickness. Near the top of the section are a few hundred feet of strata chiefly wind deposited. Unconformably overlying the whole series and in sharp contrast to the fine wind-blown stuff below it, is a third series of coarse deposits about five hundred feet thick. The topmost material, that forming the surface of the desert upland, consists of wind-blown sand now shifted by the wind and gathered into sand dunes or irregular drifts, banks of white earth, “tierra blanca,” and a pebble pavement a few inches thick.
If the main facts of the above section are now summarized they will facilitate an understanding of other sections about to be described, inasmuch as the summary will in a measure anticipate our conclusions concerning the origin of the deposits and their subsequent history. The sediments in the Majes Valley between Cantas and Jaguey consist of three series separated by two unconformities. The lowermost series is evenly bedded and rather uniform in composition and topographic expression, standing forth in huge cliffs several hundred feet high on the eastern side of the valley. This lower series is overlain by a second series, which consists of coarse conglomerate grading into sand and ultimately into very fine fluffy wind-deposited sands and silts. The lower series is much more deformed than the upper, showing that the deforming movements of later geologic times have been much less intense than the earlier, as if there had been a fading out or weakening of the deforming agents. Finally there is a third series several hundred feet thick which forms the top of the section.