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.

The volume of the lavas is enormous. They are a mile and a half thick, nearly a hundred miles wide, and of indefinite extent north and south. Their addition to the Andes, therefore, _has greatly broadened the zone of lofty mountains_. Their passes are from 2,000 to 3,000 feet higher than the passes of the eastern Andes. They have a much smaller number of valleys sufficiently deep to enjoy a mild climate. Their soil is far more porous and dry. Their vegetation is more scanty. They more than double the difficulties of transportation. And, finally, their all but unpopulated loftier expanses are a great vacant barrier between farms in the warm valleys of eastern Peru and the ports on the west coast.

The upbuilding process was not, of course, continuous. There were at times intervals of quiet, and some of them were long enough to enable streams to become established. Buried valleys may be observed in a number of places on the canyon walls, where subsequently lava flows displaced the streams and initiated new drainage systems. In these quiet intervals the weathering agents attacked the rock surfaces and formed soil. There were at least three or four such prolonged periods of weathering and erosion wherein a land surface was exposed for many thousands of years, stream systems organized, and a cultivable soil formed. No evidence has been found, however, that man was there to cultivate the soil.

The older valleys cut in the quiet period are mere pygmies beside the giant canyons of today. The present is the time of dominant erosion. The forces of vulcanism are at last relatively quiet. Recent flows have occurred, but they are limited in extent and in effects. They alter only the minor details of topography and drainage. Were it not for the oases set in the now deep-cut canyon floors, the lava plateau of the Maritime Cordillera would probably be the greatest single tract of unoccupied volcanic country in the world.

The lava plateau has been dissected to a variable degree. Its high eastern margin is almost in its original condition. Its western margin is only a hundred miles from the sea, so that the streams have steep gradients. In addition, it is lofty enough to have a moderate rainfall. It is, therefore, deeply and generally dissected. Within the borders of the plateau the degree of dissection depends chiefly upon position with respect to the large streams. These were in turn located in an accidental manner. The repeated upbuilding of the surface by the extensive outflow of liquid rock obliterated all traces of the earlier drainage. In the Cotahuasi Canyon the existing stream, working down through a mile of lavas, at last uncovered and cut straight across a mountain spur 2,000 feet high. Its course is at right angles to that pursued by the stream that once drained the spur. It is noteworthy that the Cotahuasi and adjacent streams take northerly courses and join Atlantic rivers. The older drainage was directly west to the Pacific. Thus, vulcanism not only broadened the Andes and increased their height, but also moved the continental divide still nearer the west coast.

The glacial features of the western or Maritime Cordillera are of small extent, partly because vulcanism has added a considerable amount of material in post-glacial time, partly because the climate is so exceedingly dry that the snowline lies near the top of the country. The slopes of the volcanic cones are for the most part deeply recessed on the southern or shady sides. Above 17,500 feet (5,330 m.) the process of snow and ice excavation still continues, but the tracts that exceed this elevation are confined to the loftiest peaks or their immediate neighborhood. There is a distinct difference between the glacial forms of the eastern or moister and the western or dryer flanks of this Cordillera. Only peaks like Coropuna and Solimana near the western border now bear or ever bore snowfields and glaciers. By contrast the eastern aspect is heavily glaciated. On La Cumbre Quadrangle, there is a huge glacial trough at 16,000 feet (4,876 m.), and this extends with ramifications up into the snowfields that formerly included the highest country. Prolonged glacial erosion produced a full set of topographic forms characteristic of the work of Alpine glaciers. Thus, each of the main mountain chains that make up the Andean system has, like the system as a whole, a relatively more-dry and a relatively less-dry aspect. The snowline is, therefore, canted from west to east on each chain as well as on the system. However, this effect is combined with a solar effect in an unequal way. In the driest places the solar factor is the more efficient and the snowline is there canted from north to south.