So high is the elevation of the lava plateau, so porous its soil, so dry the climate, that a few through-flowing streams gather the drainage of a vast territory and, as in the Grand Canyon country of our West, they have at long intervals cut profound canyons. The Arma has cut a deep gorge at Salamanca; the Cotahuasi runs in a canyon in places 7,000 feet deep; the Majes heads at the edge of the volcanic field in a steep amphitheatre of majestic proportions.
Finally, we have the plateaus of the coastal zone. These are plains with surfaces several thousand feet in elevation separated by gorges several thousand feet deep. The Pampa de Sihuas is an illustration. The post-maturely dissected Coast Range separates it from the sea. The pampas are in general an aggradational product formed in a past age before uplift initiated the present canyon cycle of erosion. Other plateaus of the coastal zone are erosion surfaces. The Tablazo de Ica appears to be of this type. That at Arica, Chile, near the southern boundary of Peru, is demonstrably of this type with a border on which marine planation has in places given rise to a broad terrace effect.[43]
CHAPTER XII
THE WESTERN ANDES: THE MARITIME CORDILLERA OR CORDILLERA OCCIDENTAL
THE Western or Maritime Cordillera of Peru forms part of the great volcanic field of South America which extends from Argentina to Ecuador. On the walls of the Cotahuasi Canyon ([Fig. 131]), there are exposed over one hundred separate lava flows piled 7,000 feet deep. They overflowed a mountainous relief, completely burying a limestone range from 2,000 to 4,000 feet high. Finally, upon the surface of the lava plateau new mountains were formed, a belt of volcanoes 5,000 feet (1,520 m.) high and from 15,000 to 20,000 feet (4,570 to 6,100 m.) above the sea. There were vast mud flows, great showers of lapilli, dust, and ashes, and with these violent disturbances also came many changes in the drainage. Sixty miles northeast of Cotahuasi the outlet of an unnamed deep valley was blocked, a lake was formed, and several hundred feet of sediments were deposited. They are now wasting rapidly, for they lie in the zone of alternate freezing and thawing, a thousand feet and more below the snowline. Some of their bad-land forms look like the solid bastions of an ancient fortress, while others have the delicate beauty of a Japanese temple.
Not all the striking effects of vulcanism belong to the remote geologic past. A day’s journey northeast of Huaynacotas are a group of lakes only recently hemmed in by flows from the small craters thereabouts. The fires in some volcanic craters of the Peruvian Andes are still active, and there is no assurance that devastating flows may not again inundate the valleys. In the great Pacific zone or girdle of volcanoes the earth’s crust is yet so unstable that earthquakes occur every year, and at intervals of a few years they have destructive force. Cotahuasi was greatly damaged in 1912; Abancay is shaken every few years; and the violent earthquakes of Cuzco and Arequipa are historic.
On the eastern margin of the volcanic country the flows thin out and terminate on the summit of a limestone (Cretaceous) plateau. On the western margin they descend steeply to the narrow west-coast desert. The greater part of the lava dips beneath the desert deposits; there are a few intercalated flows in the deposits themselves, and the youngest flows—limited in number—have extended down over the inner edge of the desert.
The immediate coast of southern Peru is not volcanic. It is composed of a very hard and ancient granite-gneiss which forms a narrow coastal range ([Fig. 171]). It has been subjected to very long and continued erosion and now exhibits mature erosion forms of great uniformity of profile and declivity.
From the outcrops of older rocks beneath the lavas it is possible to restore in a measure the pre-volcanic topography of the Maritime Cordillera, In its present altitude it ranges from several thousand to 15,000 feet above sea level. The unburied topography has been smoothed out; the buried topography is rough (Figs. 29 and 166). The contact lines between lavas and buried surfaces in the deep Majes and Cotahuasi valleys are in places excessively serrate. From this, it seems safe to conclude that the period of vulcanism was so prolonged that great changes in the unburied relief were effected by the agents of erosion. Thus, while the dominant process of volcanic upbuilding smoothed the former rough topography of the Maritime Cordillera, erosion likewise measurably smoothed the present high extra-volcanic relief in the central and eastern sections. The effect has been to develop a broad and sufficiently smooth aspect to the summit topography of the entire Andes to give them a plateau character. Afterward the whole mountain region was uplifted about a mile above its former level so that at present it is also continuously lofty.
The zone of most intense volcanic action does not coincide with the highest part of the pre-volcanic topography. If the pre-volcanic relief were even in a very general way like that which would be exhibited if the lavas were now removed, we should have to say that the chief volcanic outbursts took place on the western flank of an old and deeply dissected limestone range.