CHAPTER XI

THE PERUVIAN LANDSCAPE

From the west coast the great Andean Cordillera appears to have little of the regularity suggested by our relief maps. Steep and high cliffs in many places form the border of the land and obstruct the view; beyond them appear distant summits rising into the zone of clouds. Where the cliffs are absent or low, one may look across a sun-baked, yellow landscape, generally broken by irregular foothills that in turn merge into the massive outer spurs and ranges of the mountain zone. The plain is interrupted by widely separated valleys whose green lowland meadows form a brilliant contrast to the monotonous browns and yellows of the shimmering desert. In rare situations the valley trenches enable one to look far into the Cordillera and to catch memorable glimpses of lofty peaks capped with snow.

If the traveler come to the west-coast landscape from the well-molded English hills or the subdued mountains of Vermont and New Hampshire with their artistic blending of moderate profiles, he will at first see nothing but disorder. The scenery will be impressive and, in places, extraordinary, but it is apparently composed of elements of the greatest diversity. All the conceivable variations of form and color are expressed, with a predominance of bold rugged aspects that give a majestic appearance to the mountain-bordered shore. One looks in vain for some sign of a quiet view, for some uniformity of features, for some landscape that will remind him of the familiar hills of home. The Andes are aggressive mountains that front the sea in formidable spurs or desert ranges. Could we see in one view their entire elevation from depths of over 20,000 feet beneath sea level to snowy summits, a total altitude of 40,000 feet (12,200 m.), their excessive boldness would be more apparent. No other mountains in the world are at once so continuously lofty and so near a coast which drops off to abyssal depths.

The view from the shore is, however, but one of many which the Andes exhibit. Seen from the base the towering ranges display a stern aspect, but, like all mountains, their highest slopes and spurs must be crossed and re-crossed before the student is aware of other aspects of a quite different nature. The Andes must be observed from at least three situations: from the floors of the deep intermontane valleys, from the intermediate slopes and summits, and from the uppermost levels as along the range crests and the highest passes. Strangely enough it is in the summit views that one sees the softest forms. At elevations of 14,000 to 16,000 feet (4,270 to 4,880 m.), where one would expect rugged spurs, serrate chains, and sharp needles and horns, one comes frequently upon slopes as well graded as those of a city park--grass-covered, waste-cloaked, and with gentle declivity (Figs. 121-124).

The graded, waste-cloaked slopes of the higher levels are interpreted as the result of prolonged denudation in an erosion cycle which persisted through the greater part of the Tertiary period, and which was closed by uplifts aggregating at least several thousands of feet. Above the level of the mature slopes rise the ragged profiles and steep, naked declivities of the snow-capped mountains which bear residual relations to the softer forms at their bases. They are formed upon rock masses of greater original elevation and of higher resistance to denudation. Though they are dominating topographic features, they are much less extensive and significant than the tame landscape which they surmount.

Below the level of the mature slopes are topographic features of equal prominence: gorges and canyons up to 7,000 feet deep. The deeply intrenched streams are broken by waterfalls and almost continuous rapids, the valley walls are so abrupt that one may, in places, roll stones down a 4,000 foot incline to the river bed, and the tortuous trail now follows a stream in the depths of a profound abyss, now scales the walls of a labyrinthine canyon.

The most striking elements of scenery are not commonly the most important in physiography. The oldest and most significant surface may be at the top of the country, where it is not seen by the traveler or where it cannot impress him, except in contrast to features of greater height or color. The layman frequently seizes on a piece of bad-land erosion or an outcrop of bright-colored sandstone or a cliff of variegated clays or a snow-covered mountain as of most interest. All we can see of a beautiful snow-clad peak is mere entertainment compared with what subdued waste-cloaked hill-slopes may show. We do not wish to imply that everywhere the tops of the Andes are meadows, that there are no great scenic features in the Peruvian mountains, or that they are not worth while. But we do wish to say that the bold features are far less important in the interpretation of the landscape.

Amid all the variable forms of the Peruvian Cordillera certain strongly developed types recur persistently. That their importance and relation may be appreciated we shall at once name them categorically and represent them in the form of a block diagram (Fig. 126). The principal topographic types are as follows:

1. An extensive system of high-level, well-graded, mature slopes, below which are:

2. Deep canyons with steep, and in places, cliffed sides and narrow floors, and above which are:

3. Lofty residual mountains composed of resistant, highly deformed rock, now sculptured into a maze of serrate ridges and sharp commanding peaks.

4. Among the forms of high importance, yet causally unrelated to the other closely associated types, are the volcanic cones and plateaus of the western Cordillera.

5. At the valley heads are a full complement of glacial features, such as cirques, hanging valleys, reversed slopes, terminal moraines, and valley trains.

6. Finally there is in all the valley bottoms a deep alluvial fill formed during the glacial period and now in process of dissection.

Though there are in many places special features either remotely related or quite unrelated to the principal enumerated types, they belong to the class of minor forms to which relatively small attention will be paid, since they are in general of small extent and of purely local interest.

The block diagram represents all of these features, though of necessity somewhat more closely associated than they occur in nature. Reference to the photographs, Figs. 121-124, will make it clear that the diagram is somewhat ideal: on the other hand the photographs together include all the features which the diagram displays. In descending from any of the higher passes to the valley floor one passes in succession down a steep, well-like cirque at a glaciated valley head, across a rocky terminal moraine, then down a stair-like trail cut into the steep scarps which everywhere mark the descent to the main valley floors, over one after another of the confluent alluvial fans that together constitute a large part of the valley fill, and finally down the steep sides of the inner valley to the boulder-strewn bed of the ungraded river.

We shall now turn to each group of features for description and explanation, selecting for first consideration the forms of widest development and greatest significance--the high-level mature slopes lying between the lofty mountains which rise above them and the deep, steep-walled valleys sunk far below them. These are the great pasture lands of the Cordillera; their higher portions constitute the typical _puna_ of the Indian shepherds. In many sections it is possible to pasture the vagrant flocks almost anywhere upon the graded slopes, confident that the _ichu_, a tufted forage grass, will not fail and that scattered brooks and springs will supply the necessary water. At nightfall the flocks are driven down between the sheltering walls of a canyon or in the lee of a cliff near the base of a mountain, or, failing to reach either of these camps, the shepherd confines his charge within the stone walls of an isolated corral.

In those places where the graded soil-covered slopes lie within the zone of agriculture--below 14,000 feet--they are cultivated, and if the soil be deep and fertile they are very intensively cultivated. Between Anta and Urubamba, a day’s march north of Cuzco, the hill slopes are covered with wheat and barley fields which extend right up to the summits (Fig. 134). In contrast are the uncultivated soil-less slopes of the mountains and the bare valley walls of the deeply intrenched streams. The distribution of the fields thus brings out strongly the principal topographic relations. Where the softer slopes are at too high a level, the climatic conditions are extreme and man is confined to the valley floors and lower slopes where a laborious system of terracing is the first requirement of agriculture.

The appearance of the country after the mature slopes had been formed is brought out in Fig. 122. The camera is placed on the floor of a still undissected, mature valley which shows in the foreground of the photograph. In the middle distance is a valley whose great depth and steepness are purposely hidden; beyond the valley are the smoothly graded, catenary curves, and interlocking spurs of the mature upland. In imagination one sees the valleys filled and the valley slopes confluent on the former (now imaginary) valley floor which extends without important change of expression to the border of the Cordillera. No extensive cliffs occur on the restored surface, and none now occur on large tracts of the still undissected upland. Since the mature slopes represent a long period of weathering and erosion, their surfaces were covered with a deep layer of soil. Where glaciation at the higher levels and vigorous erosion along the canyons have taken place, the former soil cover has been removed; elsewhere it is an important feature. Its presence lends a marked softness and beauty to these lofty though subdued landscapes.

The graded mountain slopes were not all developed (1) at the same elevation, nor (2) upon rock of the same resistance to denudation, nor (3) at the same distance from the major streams, nor (4) upon rock of the same structure. It follows that they will not all display precisely the same form. Upon the softer rocks at the lowest levels near the largest streams the surface was worn down to extremely moderate slopes with a local relief of not more than several hundred feet. Conversely, there are quite unreduced portions whose irregularities have mountainous proportions, and between these extremes are almost all possible variations. Though the term _mature_ in a broad way expresses the stage of development which the land had reached, _post mature_ should be applied to those portions which suffered the maximum reduction and now exhibit the softest profiles. At no place along the 73rd meridian was denudation carried to the point of even local peneplanation. All of the major and some of the minor divides bear residual elevations and even approximately plane surfaces do not exist.

Among the most important features of the mature slopes are (1) their great areal extent--they are exhibited throughout the whole Central Andes, (2) their persistent development upon rocks of whatever structure or degree of hardness, and (3) their present great elevation in spite of moderate grades indicative of their development at a much lower altitude. Mature slopes of equivalent form are developed in widely separated localities in the Central Andes: in every valley about Cochabamba, Bolivia, at 10,000 feet (3,050 m.); at Crucero Alto in southern Peru at 14,600 feet (4,450 m.); several hundred miles farther north at Anta near Cuzco, 11,000 feet to 12,000 feet (3,600 to 3,940 m.), and Fig. 129 shows typical conditions in the Vilcabamba Valley along the route of the Yale Peruvian Expedition of 1911. The characteristic slopes so clearly represented in these four photographs are the most persistent topographic elements in the physiography of the Central Andes.

The rock masses upon which the mature slopes were formed range from soft to hard, from stratified shales, slates, sandstones, conglomerates, and limestones to volcanics and intrusive granites. While these variations impose corresponding differences of form, the graded quality of the slopes is rarely absent. In some places the highly inclined strata are shown thinly veiled with surface débris, yet so even as to appear artificially graded. The rock in one place is hard granite, in another a moderately hard series of lava flows, and again rather weak shales and sandstones.

Proof of the rapid and great uplift of certain now lofty mountain ranges in late geologic time is one of the largest contributions of physiography to geologic history. Its validity now rests upon a large body of diversified evidence. In 1907 I crossed the Cordillera Sillilica of Bolivia and northern Chile and came upon clear evidences of recent and great uplift. The conclusions presented at that time were tested in the region studied in 1911, 500 miles farther north, with the result that it is now possible to state more precisely the dates of origin of certain prominent topographic forms, and to reconstruct the conditions which existed before the last great uplift in which the Central Andes were born. The relation to this general problem of the forms under discussion will now be considered.

The gradients of the mature slopes, as we have already seen, are distinctly moderate. In the Anta region, over an area several hundred square miles in extent, they run from several degrees to 20° or 30°. Ten-degree slopes are perhaps most common. If the now dissected slopes be reconstructed on the basis of many clinometer readings, photographs, and topographic maps, the result is a series of profiles as in Fig. 127. If, further, the restored slopes be coördinated over an extensive area the gradients of the resulting valley floors will run from 3° to 10°. Finally, if these valley floors be extended westward to the Pacific and eastward to the Amazon basin, they will be found about 5,000 feet above sea level and 4,000 feet above the eastern plains. (For explanation of method and data employed, see the accompanying figures 127-128). It is, therefore, a justifiable conclusion that since the formation of the slopes the Andes have been uplifted at least a mile, or, to put it in another way, the Andes at the time of formation of the mature slopes were at least a mile lower than they are at present.

Further proof of recent and great uplift is afforded by the deeply intrenched streams. After descending the long graded slopes one comes upon the cliffed canyons with a feeling of consternation. The effect of powerful erosion, incident upon uplift, is heightened by the ungraded character of the river bed. Falls and rapids abound, the river profiles suggest tumultuous descents, and much time will elapse before the river beds have the regular and moderate gradients of the streams draining the mature surface before uplift as shown in the profiles by the dotted lines representing the restored valley floors of the older cycle. Since the smooth-contoured landscape was formed great changes have taken place. The streams have changed from completely graded to almost completely ungraded profiles; in place of a subdued landscape we now have upland slopes intersected by mile-deep canyons; the high-level slopes could not have been formed under existing conditions, for they are being dissected by the present streams.

Since the slopes of the land in general undergo progressive changes in the direction of flatter gradients during a given geographical cycle, it follows that with the termination of one cycle and the beginning of another, two sets of slopes will exist and that the gradients of the two will be unlike. The result is a break in the descent of the slopes from high to low levels to which the name “topographic unconformity” is now applied. It will be a prominent feature of the landscape if the higher, older, and flatter gradients have but little declivity, and the gradients of the lower younger slopes are very steep. In those places where the relief of the first cycle was still great at the time of uplift, the erosion forms of the second cycle may not be differentiated from those of the first, since both are marked by steep gradients. In the Central Andes the change in gradient between the higher and lower slopes is generally well marked. It occurs at variable heights above the valley floors, though rarely more than 3,000 feet above them. In the more central tracts, far from the main streams and their associated canyons, dissection in the present erosion cycle has not yet been initiated, the mature slopes are still intact, and a topographic unconformity has not yet been developed. The higher slopes are faced with rock and topped with slowly moving waste. Ascent of the spur end is by steep zigzag trails; once the top is gained the trail runs along the gentler slopes without special difficulties.

It is worth noting at this point that the surface of erosion still older than the mature slopes herewith described appears not to have been developed along the seventy-third meridian of Peru, or if developed at one time, fragments of it no longer remain. The last well-developed remnant is southwest of Cuzco, Fig. 130. I have elsewhere described the character and geographic distribution of this oldest recognizable surface of the Central Andes.[41] Southern Peru and Bolivia and northern Chile display its features in what seems an unmistakable manner. The best locality yet found is in the Desaguadero Valley between Ancoaqui and Concordia. There one may see thousands of feet of strongly inclined sediments of varying resistance beveled by a well-developed surface of erosion whose preserval is owing to a moderate rainfall and to location in an interior basin.[42]

The highest surface of a region, if formed during a prolonged period of erosion, becomes a surface of reference in the determination of the character and amount of later crustal deformations, having somewhat the same functions as a key bed in stratigraphic geology. Indeed, concrete physiographic facts may be the _only_ basis for arguments as to both epeirogenic and orogenic movements. The following considerations may show in condensed form the relative value of physiographic evidence:

1. If movements in the earth’s crust are predominantly _downward_, sedimentation may be carried on continuously, and a clear geologic record may be made.

2. Even if crustal movements are alternately downward and upward, satisfactory conclusions may be drawn from both (a) the nature of the buried surfaces of erosion, and (b) the alternating character of the sediments.

3. If, however, the deformative processes effect steady or intermittent uplifts, there may be no sediments, at least within the limits of the positive crustal units, and a geologic record must be derived not from sedimentary deposits but from topographic forms. We speak of the _lost intervals_ represented by stratigraphic breaks or unconformities and commonly emphasize our ignorance concerning them. The longest, and, from the human standpoint, the most important, break in the sedimentary record is that of the present wherever degradation is the predominant physiographic process. Unlike the others the _lost interval_ of the present is not lost, if we may so put it, but is in our possession, and may be definitely described as a concrete thing. It is the physiography of today.

Even where long-buried surfaces of erosion are exposed to view, as in northern Wisconsin, where the Pre-Cambrian paleo-plain projects from beneath the Paleozoic sediments, or, as in New Jersey and southeastern Pennsylvania, where the surface developed on the crystalline rocks became by depression the floor of the Triassic and by more recent uplift and erosion has been exposed to view,--even in such cases the exposures are of small extent and give us at best but meager records. In short, many of the breaks in the geologic record are of such long duration as to make imperative the use of physiographic principles and methods. The great Appalachian System of eastern North America has been a land area practically since the end of the Paleozoic. In the Central Andes the “lost interval,” from the standpoint of the sedimentary, record, dates from the close of the Cretaceous, except in a few local intermont basins partially filled with Tertiary or Pleistocene deposits. Physiographic interpretations, therefore, serve the double purpose of supplying a part of the geologic record while at the same time forming a basis for the scientific study of the surface distribution of living forms.

The geologic dates of origin of the principal topographic forms of the Central Andes may be determined with a fair degree of accuracy. Geologic studies in Peru and Bolivia have emphasized the wide distribution of the Cretaceous formations. They consist principally of thick limestones above and sandstones and conglomerates below, and thus represent extensive marine submergence of the earth’s crust in the Cretaceous where now there are very lofty mountains. The Cretaceous deposits are everywhere strongly deformed or uplifted to a great height, and all have been deeply eroded. They were involved, together with other and much older sediments, in the erosion cycle which resulted in the development of the widely extended series of mature slopes already described. From low scattered island elevations projecting above sea level, as in the Cretaceous period, the Andes were transformed by compression and uplift to a rugged mountain belt subjected to deep and powerful erosion. The products of erosion were in part swept into the adjacent seas, in part accumulated on the floors of intermont basins, as in the great interior basins of Titicaca and Poopó.

Since the early Tertiary strata are themselves deformed from once simple and approximately horizontal structures and subjected to moderate tilting and faulting, it follows that mountain-making movements again affected the region during later Tertiary. They did not, however, produce extreme effects. They did stimulate erosion and bring about a reorganization of all the slopes with respect to the new levels.

This agrees closely with a second line of evidence which rests upon an independent basis. The alluvial fill which lies upon all the canyon and valley floors is of glacial origin, as shown by its interlocking relations with morainal deposits at the valley heads. It is now in process of dissection and since its deposition in the Pleistocene had been eroded on the average about 200 feet. Clearly, to form a 3,000-foot canyon in hard rock requires much more time than to deposit and again partially to excavate an alluvial fill several hundred feet deep. Moreover, the glacial material is coarse throughout, and was built up rapidly and dissected rapidly. In most cases, furthermore, coarse material at the bottom of the glacial series rests directly upon the rock of a narrow and ungraded valley floor. From these and allied facts it is concluded that there is no long time interval represented by the transitions from degrading to aggrading processes and back again. The early Pleistocene, therefore, seems quite too short a period in which to produce the bold forms and effect the deep erosion which marks the period between the close of the mature cycle and the beginnings of deposition in the Pleistocene.

The alternative conclusion is that the greater part of the canyon cutting was effected in the late Tertiary, and that it continued into the early Pleistocene until further erosion was halted by changed climatic conditions and the augmented delivery of land waste to all the streams. The final development of the well-graded high-level slopes is, therefore, closely confined to a small portion of the Tertiary. The closest estimate which the facts support appears to be Miocene or early Pliocene. It is clear, however, that only the culmination of the period can be definitely assigned. Erosion was in full progress at the close of the Cretaceous and by middle Tertiary had effected vast changes in the landscape. The Tertiary strata are marked by coarse basal deposit and by thin and very fine top deposits. Though their deformed condition indicates a period of crustal disturbance, the Tertiary beds give no indication of wholesale transformations. They indicate chiefly tilting and moderate and normal faulting. The previously developed effects of erosion were, therefore, not radically modified. The surface was thus in large measure prepared by erosion in the early Tertiary for its final condition of maturity reached during the early Pliocene.

It seems appropriate, in concluding this chapter, to summarize in its main outlines the physiography of southern Peru, partly to condense the extended discussion of the preceding paragraphs, and partly to supply a background for the three chapters that follow. The outstanding features are broad plateau areas separated by well-defined “Cordilleras.” The plateau divisions are not everywhere of the same origin. Those southwest of Cuzco (Fig. 130), and in the Anta Basin (Fig. 124), northwest of Cuzco, are due to prolonged erosion and may be defined as peneplane surfaces uplifted to a great height. They are now bordered on the one hand by deep valleys and troughs and basins of erosion and deformation; and, on the other hand, by residual elevations that owe their present topography to glacial erosion superimposed upon the normal erosion of the peneplane cycle. The residuals form true mountain chains like the Cordillera Vilcanota and Cordillera Vilcapampa; the depressions due to erosion or deformation or both are either basins like those of Anta and Cuzco or valleys of the canyon type like the Urubamba canyon; the plateaus are broad rolling surfaces, the punas of the Peruvian Andes.

There are two other types of plateaus. The one represents a mature stage in the erosion cycle instead of an ultimate stage; the other is volcanic in origin. The former is best developed about Antabamba (Figs. 122 and 123), where again deep canyons and residual ranges form the borders of the plateau remnants. The latter is well developed above Cotahuasi and in its simplest form is represented in Fig. 133. Its surface is the top of a vast accumulation of lavas in places over a mile thick. While rough in detail it is astonishingly smooth in a broad view (Fig. 29). Above it rise two types of elevations: first, isolated volcanic cones of great extent surrounded by huge lava flows of considerable relief; and second, discontinuous lines of peaks where volcanic cones of less extent are crowded closely together. The former type is displayed on the Coropuna Quadrangle, the latter on the Cotahuasi and La Cumbre Quadrangles.

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]