Sierra Club Bulletin 



by obvious peculiarities of physical structure of their rocks. 

 The same is true of all valleys in this region. We give one ex- 

 ample, the upper Tuolumne Valley, which is about eight miles 

 long, and from 2000 to 3000 feet deep, and trends in a generally 

 northerly direction. If we go to its head on the base of Mount 

 Lyell, and follow it down, we find that after trending steadily 

 about two miles it makes a bend of a few degrees to the left (A, 

 Fig.6) . Looking for the cause, we perceive a depression on the 



Fig. 6. — Illustrating Bend of Upper Tuolumne Valley 



Opposite or right wall ; ascending to it, we find the depression to 

 be the mouth of a tributary valley which leads to a crater- 

 shaped ice- fountain (B) which gave rise to the tributary gla- 

 cier that, in thrusting itself into the valley trunk, caused the 

 bend we are studying. After maintaining the new trend thus 

 acquired for a distance of about a mile and a half, the huge val- 

 ley swerves lithely to the right at C. Looking for the cause, we 

 find another tributary ice-grooved valley coming in on the left, 

 which like the first conducts back to an ice- womb (D) which 

 gave birth to a glacier that in uniting with the tr ank pushed it 

 aside as far as its force, modified by the direction, smoothness, 

 and declivity of its channel, enabled it to do. Below this, the 

 noble valley is again pushed round in a curve to the left by a 

 series of small tributaries which, of course, enter on the right, 

 and with each change in trend there is always a corresponding 

 change in width or depth, or in both. No valley changes its 

 direction without becoming larger. On nearing the Big Mead- 

 ows it is swept entirely round to the west by huge glaciers, rep- 



