CORRASION BY GRAVITY STREAMS. 301 



From Mounts Dana and Lyell, and the Merced head- 

 waters, this younger ice mass started. Its upper limits 

 may be seen to be above the 12,000 feet level on the highest 

 peaks. Flowing down the Tuolumne Meadows its bulk 

 passed to the north by way of the Tuolumne Canon to 

 Hetch Hetchy, but quite a considerable overflow passed 

 along the Tenaya Basin with a spilling over also into the 

 Merced drainage by way of Cathedral Peak. The heavier 

 ice stream appears to have descended the Tenaya. At 

 Cathedral and Echo Peaks the flood level had dropped to 

 10,500 feet, while just above Lake Tenaya it stood at just 

 under 10,000 feet. Still falling rapidly, its mass passed by 

 Clouds' Rest, leaving that peak as a nunatak. (At an 

 earlier stage this peak was apparently buried by the ice, 

 but that is a story less certainly decipherable, and in any 

 case is not necessary to our purpose). 



Half Dome appears to have been an island in this younger 

 flood at the confluence of the Merced and Tenaya glaciers. 

 An ice fall took place in the surface of the glacier here, 

 and Glacier Point was not nearly reached by the ice 

 mass. Down stream a fairly level surface was maintained , 

 the thickness of the ice in the main valley being about 

 2,000 feet. Below this point the increase of the valley 

 cross-section induced a marked weakening of corrasive 

 power, while at the same time the steep channel bed 

 urged the glacier rapidly into much warmer regions and 

 brought about its rapid dissipation. 



As high up as the 6,000, and even the 6,500 feet contour, 

 the cross-section of the main valley appears to be less than 

 the sum of the cross-sections of its three confluents. The 

 channel slopes of the feeders are also very steep. These 

 facts imply both great relative velocity for the glacier and 

 corrasive work increased in geometrical ratio to such 

 acceleration. A basin associated with undercutting of the 



