320 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



wedges. Along such a sheared zone the streams would 

 rapidly deepen their beds. Even where the rocks are not 

 sheared, but merely intersected with vertical joints, it is 

 easy to see how, as erosion progressed, the slabs would 

 crumble or tumble off along the joint planes, leaving verti- 

 cal faces. If now a tongue of ice should pass through the 

 valley and clear out the talus and other detritus and round 

 off the projecting shoulders and spurs, and as it retreats 

 leave terminal moraines as barriers to form the valley floor, 

 we seem to have sufficient means for the accomplishment 

 of all we now see in Yosemite Valley. 



