The retreat of the steps, the abrasion of the rock, and the sapping of the cirques at the valley heads excavated the upper valleys so deeply that they are nearly all, as W. D. Johnson has put it, “down at the heel.” Thus, above Arma, one plunges suddenly from the smooth, grassy glades of the strongly glaciated valley head down over the outer slopes of the lowermost terminal moraine to the steep lower valley. Above the moraine are fine pastures, in the steep valley below are thickets and rocky defiles. There are long quiet reaches in the streams of the glaciated valley heads besides pretty lakes and marshes. Below, the stream is swift, almost torrential. Arma itself is built upon alluvial deposits of glacial origin. A mile farther down the valley is constricted and steep-walled—really a canyon.
Though the glaciers have retreated to the summit region, they are by no means nearing extinction. The clear blue ice of the glacier descending from Mt. Soiroccocha in the Arma Valley seems almost to hang over the precipitous valley border. In curious contrast to its suggestion of cold and storm is the patch of dark green woodland which extends right up to its border. An earthquake might easily cause the glacier to invade the woodland. Some of the glaciers between Choquetira and Arma rest on terminal moraines whose distal faces are from 200 to 300 feet high. The ice descending southeasterly from Panta Mt. is a good illustration. Earlier positions of the ice front are marked by equally large moraines. The one nearest that engaged by the living glacier confines a large lake that discharges through a gap in the moraine and over a waterfall to the marshy floor of the valley.
Retreat has gone so far, however, that there are only a few large glacier systems. Most of the tributaries have withdrawn toward their snowfields. In place of the twenty distinct glaciers now lying between the pass and the terminal moraine below Choquetira, there was in glacial times one great glacier with twenty minor tributaries. The cirques now partly filled with damp snow must then have been overflowing with dry snow above and ice below. Some of the glaciers were over a thousand feet thick; a few were nearly two thousand feet thick, and the cirques that fed them held snow and ice at least a half mile deep. Such a remarkably complete set of glacial features only 700 miles from the equator is striking evidence of the moist climate on the windward eastern part of the great Andean Cordillera, of the universal change in climate in the glacial period, and of the powerful dominating effects of ice erosion in this region of unsurpassed Alpine relief.
THE VILCAPAMPA BATHOLITH AND ITS TOPOGRAPHIC EFFECTS
Fig. 141—Composite geologic section on the northeastern border of the Cordillera Vilcapampa, in the vicinity of Pampaconas, to show the deformative effects of the granite intrusion. There is a limited amount of limestone near the border of the Cordillera. Both limestone and sandstone are Carboniferous. See [Appendix B]. See also Figs. [142] and [146]. The section is about 15 miles long.
The main axis of the Cordillera Vilcapampa consists of granite in the form of a batholith between crystalline schists on the one hand (southwest), and Carboniferous limestones and sandstones and Silurian shales and slates on the other (northeast). It is not a domal uplift in the region in which it was observed in 1911, but an axial intrusion, in places restricted to a narrow belt not more than a score of miles across. As we should expect from the variable nature of the invaded material, the granite belt is not uniform in width nor in the character of its marginal features. In places the intrusion has produced strikingly little alteration of the country rock; in other localities the granite has been injected into the original material in so intimate a manner as almost completely to alter it, and to give rise to a very broad zone of highly metamorphosed rock. Furthermore, branches were developed so that here and there tributary belts of granite extend from the main mass to a distance of many miles. Outlying batholiths occur whose common petrographic character and similar manner of occurrence leave little doubt that they are related abyssally to a common plutonic mass.