DATE OF THE GLACIAL EPOCH. 271 



the last Glacial epoch terminated about 80,000 years ago. Other investigators 

 have approached the problem in different ways and reached widely discor- 

 dant results. At present even an approximate measurement in years of the 

 time that has elapsed since the last great retreat of the glaciers of the north- 

 ern hemisphere seems impossible. Considering that Lake Lahontan fluc- 

 tuated synchronously with llie advance and retreat of the glaciers during 

 the Glacial epoch, we nuist conclude that its last evaporation followed the 

 last great retreat of the glaciers. Our studies in the Far West have shown 

 that there is no reason for supposing that the retreat of the Sierra Nevada 

 glaciers was a sudden event, partaking of the nature of a catastrophe, or 

 that the evaporation of the lakes which were supplied by the melting ice, 

 was a matter of a few years. On the contrary, the glaciers are believed to 

 have retreated slowly; with many pauses, and the evaporation of the lakes 

 to have extended through centuries. Even if the Glacial epoch can be 

 proven to have terminated 80,000 years ago, there is no reason for consid- 

 ering that desiccation of the lakes followed that event within many thou- 

 sand years. As stated at the beginning of this paragraph, we have no defi- 

 nite evidence to show that tlie Quaternary lakes were flooded a certain 

 number of years since; but one familiar with the shore jjhenomena displayed 

 in the valleys of the Far West cannot fail to be impressed with the perfec- 

 tion with which these structures have been preserved. In many instances 

 the embankments of gravel are as perfect in contour and as regular in 

 slope as if constructed but a few years ago. Subaerial erosion is reduced 

 to a minimum in such instances, however, for the reason that the structures 

 are porous and absorb nearly all the rain that falls upon them, allowing it 

 to percolate quietly through their interstices. Changes of temperature have 

 but little power to alter their forms, owing to the large size of the inter- 

 spaces and the readiness with which moisture is removed. The only ele- 

 ments of subaerial erosion to which gravel embankments seem open to 

 attack are the wind and the beating of rain During the lapse of centuries 

 even these slow processes must effect appreciable changes, but as yet this is 

 scarcely apparent in the embankments built in Lake Lahontan. It is evi- 

 dent that gravel embankments in arid regions, so situated that they are not 

 within the reach of stream erosion, may be considered among the most per- 



