Fig. 203—A composite sketch to represent general conditions in the Peruvian Andes. In order to have the actual facts represented the profiles of this figure were taken from the accompanying topographic sheets. The main depression on the right and the corresponding depression of the tributary profiles bear out most strikingly the conclusions concerning the erosive power of the ice. At A and B the spurs have been cut off to exhibit the profiles of tributary valleys. At 2 and 3 were tributary glaciers of such size that they entered the main valley at grade. Lesser tributaries had floors elevated above those they joined and now have a hanging character, as just above 2. D is a matterhorn; C is deeply recessed by cirques; E represents a peak just below the limit of glaciation. At F are the undissected post-mature slopes of an earlier cycle of erosion. G lies on the steep lower slopes formed during the canyon cycle of erosion. The down-cutting of the stream in the canyon cycle was generally checked by glaciation and was superseded by aggradation.
If we now turn to the valley profiles of the glaciated portions of the Peruvian Andes, we shall see the excess of ice over water erosion expressed in a manner equally convincing. To a thoughtful person it is one of the most remarkable features of any glaciated region that the flattest profiles, the marshiest valley flats, and the most strongly meandering stretches of the streams should occur near the heads of the valleys. The mountain shepherds recognize this condition and drive their flocks up from the warmer valley into the mountain recesses, confident that both distance and elevation will be offset by the extensive pastures of the finest ichu grass. Indeed, to be near the grazing grounds of sheep and llamas which are their principal means of subsistence, the Indians have built their huts at the extraordinarily lofty elevations of 16,000 to 17,000 feet.
An examination of a large number of these valleys and the plotting of their gradients discloses the striking fact that the heads of the valleys were deeply sunk into the mountains. It is thus possible by restoring the preglacial profiles to measure with considerable certainty the excess of ice over water erosion.
The results are graphically expressed in [202] . It will be seen that until glacial conditions intervened the stream was flowing on a rock floor. During the whole of glacial time it was aggrading its rock floor below b′ and forming a deep valley fill. A return to warmer and drier conditions led to the dissection of the fill and this is now in progress. The stream has not yet reached its preglacial profile, but it has almost reached it. We may, therefore, say that the preglacial valley profile below b′ fixes the position of the present profile just as surely as if the stream had been magically halted in its work at the beginning of the period of glaciation. There, b′-d-c-b represents the amount of ice erosion. To be sure the line b-c is inference, but it is reasonable inference and, whatever position is assigned to it, it cannot be coincident with b′-d, nor can it be anywhere near it. The break in the valley profile at b′ is always marked by a terminal moraine, regardless of the character of the rock. This is not an accidental but a causal association. It proves the power of the ice to erode. In glacial times it eroded the quantity b-c-d-b′. This is not an excess of ice over water erosion, but an absolute measure of ice erosion, since a′-b′ has remained intact. The only possible error arises from the position assigned b-c, and even if we lower it to b-c′ (for which we have no warrant but extreme conservatism) we shall still have left b′-c′-d-b as a striking value for rock erosion (plucking and abrasion) by a valley glacier.
A larger diagram, [203] , represents in fuller detail the topographic history of the Andes of southern Peru and the relative importance of glaciation. The broad spurs with grass-covered tops that end in steep scarps are in wonderful contrast to the serrate profiles and truncated spurs that lie within the zone of past glaciation. In the one case we have minute irregularities on a canyon wall of great dimensions; in the other, more even walls that define a glacial trough with a flat floor. Before glaciation on a larger scale had set in the right-hand section of the diagram had a greater relief. It was a residual portion of the mountain and therefore had greater height also. Glaciers formed upon it in the Ice Age and glaciation intensified the contrast between it and the left-hand section; not so much by intensifying the relief as by diversifying the topographic forms.
