208 THE ANDES OF SOUTHERN PERU 



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 modi- 

 fied by the meandering stream. From this point the glacier re- 

 treated 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 lati- 

 tudes. They would be recognized at once as evidence of successive 

 periodic retreats of the ice, due to successive changes in tempera- 

 ture. To understand their importance when encountered in very 

 low latitudes it is necessary to turn aside for a moment and con- 

 sider 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. 



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 



