22 



Sierra Club Bulletin 



reached a climax, like an oncoming tide, and then slowly- 

 waned, the glaciers making many repeated but progressively 

 feebler re-advances, like the waves of an outgoing tide. Today 

 we know that the Glacial Epoch, so-called, really consisted of 

 several prolonged ice-tides separated by equally prolonged in- 

 tervals, during each of which the continental ice-sheet and the 

 lesser ice-bodies on our western mountain ranges shrank back 

 to their sources and perhaps vanished altogether. 



In the Sierra Nevada indications of at least two great ice- 

 floods have been clearly recognized by several observers — two 

 ice-floods that occurred manifestly at widely different times, 

 the later culminating probably only twenty thousand years ago, 

 the earlier, perhaps as much as several hundred thousand years 

 ago. The evidence is the more readily established as the later 

 ice-flood was the smaller and less extensive of the two and left 

 undisturbed the moraines — that is, the ridges of ice-carried 

 rock debris — that mark the limits of the earlier ice-flood. In 

 no part of the Sierra Nevada have these facts been ascertained 

 with more precision than in the Yosemite region and the High 

 Sierra immediately above it. Thus it is now definitely known 

 that the later ice-flood invaded the Yosemite Valley only as far 

 as the Bridal Veil Meadows, whereas the earlier ice-flood ad- 

 vanced eleven miles farther down the Merced Canon, coming 

 to a halt a short distance beyond El Portal. 



It will be clear from this that there must be from the Bridal 

 Veil Meadows upward throughout the Yosemite region and ad- 

 joining the High Sierra not one but two "ice-lines," each mark- 

 ing the culmination of an ice-flood. The pursuit of these two 

 ice-lines up towards the crest of the range was, indeed, for the 

 better part of two seasons the writer's most engrossing occupa- 

 tion. He traced them in detail and mapped them along the 

 length of the Yosemite, up through the Little Yosemite and the 

 upper Merced Basin and all its tributary canons, and also up 

 through Tenaya Canon and the great Tuolumne Basin and its 

 tributary canons. The result, it may be said, was to him, as 

 glacialist, a genuine surprise. The two ice-lines, which in the 

 lower Yosemite lie several thousand feet apart in altitude, were 

 found to approach each other as they ascend the range and ul- 

 timately to coalesce at its crest. One might reasonably have 



