Fig. 180—Urubamba Valley between Ollantaytambo and Torontoy, showing (1) more moderate upper slopes and steeper lower slopes of the two-cycle mountain spurs; (2) the extensive alluvial deposits of the valley, consisting chiefly of confluent alluvial fans heading in the glaciated mountains on the left. See [Fig. 179].
Fig. 181—Glacial features of the Central Ranges (see [Fig. 204]). Huge lateral moraines built by ice streams tributary to the main valley north of Chuquibambilla. That the tributaries persisted long after the main valley became free of ice is shown by the descent of the lateral moraines over the steep border of the main valley and down to the floor of it.
The Pleistocene deposits fall into three well-defined groups: (1) glacial accumulations at the valley heads, (2) alluvial deposits in the valleys, and (3) lacustrine deposits formed on the floors of temporary lakes in inclosed basins. Among these the most variable in form and composition are the true glacier-laid deposits at the valley heads. The most extensive are the fluvial deposits accumulated as valley fill throughout the entire Andean realm. Though important enough in some respects the lacustrine deposits are of small extent and of rather local significance. Practically none of them fall within the field of the present expedition; hence we shall describe only the first two classes.
The most important glacial deposits were accumulated in the eastern part of the Andes as a result of greater precipitation, a lower snowline, and catchment basins of larger area. In the Cordillera Vilcapampa glaciers once existed up to twelve and fifteen miles in length, and those several miles long were numerous both here and throughout the higher portions of the entire Cordillera, save in the belt of most intense volcanic action, which coincides with the driest part of the Andes, where the glaciers were either very short or wanting altogether.
Since vigorous glacial action results in general in the cleaning out of the valley heads, no deposits of consequence occur in these locations. Down valley, however, glacial deposits occur in the form of terminal moraines of recession and ground moraines. The general nature of these deposits is now so well known that detailed description seems quite unnecessary except in the case of unusual features.
It is noteworthy that the moraines decrease in size up valley since each valley had been largely cleaned out by ice action before the retreat of the glacier began. Each lowermost terminal moraine is fronted by a great mass of unsorted coarse bowldery material forming a fill in places several hundred feet thick, as below Choquetira and in the Vilcapampa Valley between Vilcabamba and Puquiura. This bowldery fill is quite distinct from the long, gently inclined, and stratified valley train below it, or the marked ridge-like moraine above it. It is in places a good half mile in length. Its origin is believed to be due to an overriding action beyond the last terminal moraine at a time when the ice was well charged with débris, an overriding not marked by morainal accumulations, chiefly because the ice did not maintain an extreme position for a long period.
In the vicinity of the terminal moraines the alluvial valley fill is often so coarse and so unorganized as to look like till in the cut banks along the streams, though its alluvial origin is always shown by the topographic form. This characteristic is of special geologic interest since the form may be concealed through deposition or destroyed by erosion, and no condition but the structure remain to indicate the manner of origin of the deposit. In such an event it would not be possible to distinguish between alluvium and till. The gravity of the distinction appears when it is known that such apparently unsorted alluvium may extend for several miles forward of a terminal moraine, in the shape of a widespreading alluvial fan apparently formed under conditions of extremely rapid aggradation. I suppose it would not be doubted in general that a section of such stony, bowldery, unsorted material two miles long would have other than a glacial origin, yet such may be the case. Indeed, if, as in the Urubamba Valley, a future section should run parallel to the valley across the heads of a great series of fans of similar composition, topographic form, and origin, it would be possible to see many miles of such material.