rocks fairly homogeneous, such as the crystallines. In 

 stratified rocks of differential strength the cirques will 

 tend to possess enormously steep heads by such action, 

 because of the exposure of huge vertical walls by such 

 sapping action, for example, those of the Canadian Rockies 

 and Selkirks. The reason for this amphitheatre head 

 (cirque) is that under ordinary atmospheric conditions the 

 fragments of the cirque wall are acted on only by the 

 constant gravitational force and take the lines of least 

 resistance or quickest descent, producing a symmetrical 

 geometrical figure in so doing. 



Origin of the Yosemite and associated valleys.— We are 

 now in a position to understand the later steps in the 

 history of the Yosemite Valley. From the descriptions 

 and figures supplied in the earlier portion of this paper it 

 is evident that the thalweg of the Yosemite in recent time 

 was steep ; that it descended in a fairly even manner both 

 from Lake Tenaya and the Little Yosemite along the 

 Tenaya and Merced channels respectively, to a point some- 

 what higher than the lip of the Bridal Veil Falls. A pro- 

 nounced const fiction occurred also in the valley between 

 the Cathedral Rocks and El Capitan Dome. During the 

 recent Ice Flood the Yosemite glacier gained enormously 

 kinetic energy during its descent of the steep thalweg and 

 its passage of the constriction at El Capitan. Just in front 

 of this constriction a "step" with "basined" tread was 

 formed. 1 The "step" and "tread" once formed were 

 rapidly enlarged by the process of recession. The heavy 

 quarrying action of the glacier caused the rock structures 

 of the declivity forming the channel slope near Kl Capitan 

 to be easily removed because of the notable steepness of 



~ ' From Bridal Veil Fallot., El IWtal a distance ,.f ah an I t miles Ly 

 coach, the thalweg of the Merced falls rapidly. Relatively little glacial 



the rapid disappearance of the glacier by melting. — (Andrews, Corrasion 

 'y Gravity Streams, p, 303.) 



