National Geographic : 1916 May
Photograph by O. F. Cook SOME OF 'TTI STAIRCASE FARMS OF TIIE ANCIENTS Each terrace consists of three parts-the wall and the two distinct layers of earth that fill the space behind the wall. All of the ruined terraces show the same inside structure, wherever the walls are removed. The strata that are hidden behind the walls are artificial no less than the stone facing (see also illustrations, pages 508 and 509). The underlying stratum, or artificial subsoil, is composed of coarse stones and clay, and is covered by a layer of fine surface soil two or three feet thick. The thickness of the subsoil layer depends, of course, upon the height of the terrace. Where clay or other light colored material is used for the subsoil, the difference between the two layers appears most striking; but the finer texture of the upper layer also renders it very distinct. The lower terraces of this bank are still under cultivation. In the background a part of the megalithic terraces can be seen. A ruined Inca house stands near the base of the precipice at the left. tivated lands were widened by building new terraces along the beds of the streams. In many cases the work was evidently planned so that large immovable boulders or outcrops of rock could be utilized in the building of the walls instead of being allowed to diminish the area of cultivated land. We may believe that powder or dynamite, to shatter refractory rocks, would have been very highly appreciated among the ancient Peruvians. STRAIGHTENING OF RIVI R BANKS ANDI STREAM BEDS It would be a mistake to suppose that reclamation work in the bottoms of the valleys was wholly or even principally of the nature of improving irregular land by terracing and leveling behind the walls. A large part of the surface of the valley bottoms must have been altogether bare of soil, as the unimproved portions still are-mere wastes of loose stones brought down by the torrential floods. The natural behavior of swift moun tain streams is to cut irregular channels back and forth between the walls of their valleys, but in the terraced valleys of Peru it is the regular condition to find the rivers and smaller streams confined to channels of definite width, and some times kept in straight courses for several miles at a stretch, as in the case of the Urubamba River near Pisac, and again below Ollanttatambo. In the latter in stance the river runs for nearly five miles in a straight course, and, although the ancient walls that were built to confine the river have remained intact in only a few places, the artificial nature of the channel is obvious.