Fig. 91—Looking down the canyon of the Majes River to the edge of the cloud bank formed against the Coast Range back of Camaná.
Fig. 92—Topographic and climatic cross-section to show the varying positions of the cloud bank on the coast of Peru, the dry terrace region, and the types of stream profiles in the various belts.
If the fog and cloud bank extend westward indefinitely, the entire day may be overcast or the sun appear for a few moments only through occasional rifts. Generally, also, it will make an appearance just before sunset, its red disk completely filling the narrow space between the under surface of the clouds and the water. I have repeatedly seen the ship’s passengers and even the crew leave the dinner table and collect in wondering groups about the port-holes and doorways the better to see the marvelous play of colors between sky and sea. It is impossible not to be profoundly moved by so majestic a scene. A long resplendent path of light upon the water is reflected in the clouds. Each cloud margin is tinged with red and, as the sun sinks, the long parallel bands of light are shortened westward, changing in color as they go, until at last the full glory of the sunset is concentrated in a blazing arc of reds, yellows, and purples, that to most people quite atones for the dull gray day and its humid air.
At times the clouds are broken up by the winds and scattered helter-skelter through the west. A few of them may stray into the path of the sun temporarily to hide it and to reflect its primary colors when the sun reappears. From the main cloud masses there reach out slender wind-blown streamers, each one delicately lighted as the sun’s rays filter through its minute water particles. Many streamers are visible for only a short distance, but when the sun catches them their filmy invisible fingers become delicate bands of light, some of which rapidly grow out almost to the dome of the sky. Slowly they retreat and again disappear as the rays of the sun are gradually shut off by the upturning curve of the earth.
The unequal distribution of precipitation in the climatic zones of western Peru has important hydrographic consequences. These will now be considered. In the preceding figure four types of stream profiles are displayed and each has its particular relation to the cloud bank. Stream 1 is formed wholly upon the coastal terraces beneath the cloud bank. It came into existence only after the uplift of the earth’s crust that brought the wave-cut platforms above sea level. It is extremely youthful and on account first of the small seepage at its headquarters—it is elsewhere wholly without a tributary water supply—and, second, of the resistant granite that occurs along this part of the coast, it has very steep and irregular walls and an ungraded floor. Many of these “quebradas” are difficult to cross. A few of them have fences built across their floors to prevent the escape of cattle and burros that wander down from the grassy hills into the desert zone. Others are partitioned off into corrals by stone fences, the steep walls of the gorge preventing the escape of the cattle. To these are driven the market cattle, or mules and burros that are required for relays along the shore trail.
Stream 2 heads in the belt of rains. Furthermore it is a much older stream than 1, since it dates back to the time when the Coast Range was first formed. It has ample tributary slopes and a large number of small valleys. A trickle of water flows down to become lost in the alluvium of the lower part of the valley or to reappear in scattered springs. Where springs and seepage occur together, an olive grove or a garden marks the spot, a corral or two and a mud or stone or reed hut is near by, and there is a tiny oasis. Some of these dots of verdure become so dry during a prolonged drought that the people, long-established, move away. To others the people return periodically. Still others support permanent settlements.
Stream 3 has still greater age. Its only competitors are the feeble, almost negligible, streams that at long intervals flow east toward the dry zone. Hence it has cut back until it now heads in the desert. Its widely branched tributaries gather moisture from large tracts. There is running water in the valley floor even down in the terrace zone. At least there are many dependable springs and the permanent homes that they always encourage. A valley of this type is always marked by a well-defined trail that leads from settlement to settlement and eastward over the “pass” to the desert and the Andean towns.
Stream 4 is a so-called “antecedent” stream. It existed before the Coast Range was uplifted and cut its channel downward as the mountains rose in its path. The stretch where it crosses the mountains may be a canyon with a narrow, rocky, and uncultivable floor, so that the valley trails rise to a pass like that at the head of stream 3, and descend again to the settlements at the mouth of 4. There is in this last type an abundance of water, for the sources of the stream are in the zone of permanent snows and frequent winter rains of the lofty Cordillera of the Andes. The settlements along this stream are continuous, except where shut-ins occur—narrow, rocky defiles caused by more resistant rock masses in the path of the stream. Here and there are villages. The streams have fish. When the water rises the river may be unfordable and people on opposite sides must resort to boats or rafts.[24]
EASTERN BORDER CLIMATES
On windward mountain slopes there is always a belt of maximum precipitation whose elevation and width vary with the strength of the wind, with the temperature, and with the topography. A strong and constant wind will produce a much more marked concentration of the rainfall. The belt is at a low elevation in high latitudes and at a high elevation in low latitudes, with many irregularities of position dependent upon the local and especially the minimum winter temperature. The topographic controls are important, since the rain-compelling elevation may scatter widely the localities of maximum precipitation or concentrate them within extremely narrow limits. The human effects of these climatic conditions are manifold. Wherever the heaviest rains are, there, too, as a rule, are the densest forests and often the most valuable kinds of trees. If the general climate be favorable and the region lie near dense and advanced populations, exploitation of the forest and progress of the people will go hand in hand. If the region be remote and some or all of the people in a primitive state, the forest may hinder communication and retard development, especially if it lie in a hot zone where the natural growth of population is slow.... These are some of the considerations we shall keep in mind while investigating the climate of the eastern border of the Peruvian Andes.