There is a marked difference in the degree of dissection of the moraines. The lowermost and oldest is so thoroughly dissected as to exhibit but little of its original surface. The second has been greatly modified, but still possesses a ridge-like quality and marks the beginning of a noteworthy flattening of the valley gradient. The third is as sharp-crested as a roof, and yet was built so long ago that the flat valley floor behind it has been modified by the meandering stream. From this point the glacier retreated up-valley several miles (estimated) without leaving more than the thinnest veneer on the valley floor. The retreat must, therefore, have been rapid and without even temporary halts until the glacier reached a position near that occupied today. Both the present ice tongues and snowfields and those of a past age are emphasized by the presence of a patch of scrub and woodland that extends on the north side of the valley from near the snowline down over the glacial forms to the lower valley levels.
The retreatal stages sketched above would call for no special comment if they were encountered in mountains in northern latitudes. They would be recognized at once as evidence of successive periodic retreats of the ice, due to successive changes in temperature. To understand their importance when encountered in very low latitudes it is necessary to turn aside for a moment and consider two rival hypotheses of glacial retreat. First we have the hypothesis of periodic retreat, so generally applied to terminal moraines and associated outwash in glaciated mountain valleys. This implies also an advance of the ice from a higher position, the whole taking place as a result of a climatic change from warmer to colder and back again to warmer.
Fig. 137—Looking up a spurless flat-floored glacial trough near the Chucuito pass in the Cordillera Vilcapampa from 14,200 feet (4,330 m.). Note the looped terminal and lateral moraines on the steep valley wall on the left. A stone fence from wall to wall serves to inclose the flock of the mountain shepherd.
Fig. 138—Terminal moraine in the glaciated Choquetira Valley below Choquetira. The people who live here have an abundance of stones for building corrals and stone houses. The upper edge of the timber belt (cold timber line) is visible beyond the houses. Elevation 12,100 feet (3,690 m.).
But evidences of more extensive mountain glaciation in the past do not in themselves prove a change in climate over the whole earth. In an epoch of fixed climate a glacier system may so deeply and thoroughly erode a mountain mass, that the former glaciers may either diminish in size or disappear altogether. As the work of excavation proceeds, the catchment basins are sunk to, and at last below, the snowline; broad tributary spurs whose snows nourish the glaciers, may be reduced to narrow or skeleton ridges with little snow to contribute to the valleys on either hand; the glaciers retreat and at last disappear. There would be evidences of glaciation all about the ruins of the former loftier mountain, but there would be no living glaciers. And yet the climate might remain the same throughout.