RAINFALL

The annual rainfall of Morococha is as follows:

The mean for the above six years amounts to 40 inches (1,024 mm.). This is a value considerably higher than that for Caylloma or Santa Lucia. The greater rainfall of Morococha is probably due in part to its more northerly situation. An abnormal feature of the rainfall of 1908, the rainiest year, is the large amount that fell in June. Ordinarily June and July, the coldest months, are nearly or quite rainless. The normal concurrence of highest temperatures and greatest precipitation is of course highly favorable to the plant life of these great altitudes. Full advantage can be taken of the low summer temperatures if the growing temperatures are concentrated and are accompanied by abundant rains. Since low temperatures mean physiologic dryness, whether or not rains are abundant, the dryness of the winter months has little effect in restricting the range of Alpine species.

The seasonal distribution of rain helps the plateau people as well as the plateau plants. The transportation methods are primitive and the trails mere tracks that follow the natural lines of topography and drainage. Coca is widely distributed, likewise corn and barley which grow at higher elevations, and wool must be carried down to the markets from high-level pastures. In the season of rains the trails are excessively wet and slippery, the streams are often in flood and the rains frequent and prolonged. On the other hand the insignificant showers of the dry or non-growing season permit the various products to be exchanged over dry trails.

In Spanish times the ritualistic festivals, incorporated with fairs, followed the seasonal movement. Today progress in transportation has caused the decadence of many of the fairs but others still survive. Thus two of the most famous fairs of the last century, those of Vilque (province of Puno) and Yunguyo (province of Chucuito), were held at the end of May and the middle of August respectively. Copacavana, the famous shrine on the shores of Titicaca, still has a well-attended August fair and Huari, in the heart of the Bolivian plateau, has an Easter fair celebrated throughout the Andes.

Cochabamba

Cochabamba, Bolivia, lies 8,000 feet above sea level in a broad basin in the Eastern Andes. The Cerro de Tunari, on the northwest, has a snow and ice cover for part of the year. The tropical forests lie only a single long day’s journey to the northeast. Yet the basin is dry on account of an eastern front range that keeps out the rain-bearing trade winds. The Rio Grande has here cut a deep valley by a roundabout course from the mountains to the plains so that access to the region is over bordering elevations. The basin is chiefly of structural origin.

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.

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.

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.

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 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.

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.