Geol.— Vol. I.] TURNEK— ORIGIN OF YOSEMITE VALIEY. 301 



V-shaped canyon has been cut, and nowhere in the V-can- 

 yon are there, so far as known, any traces of glacial action, 

 either in the form of the canyon or in the existence 

 of morainal material or of polish on the rocks. The same 

 is true of that portion of the Merced Canyon near Big 

 Meadow, west of Yosemite Valley (See Plate XXXVI-^). 



In both cases, however, the existence of morainal material 

 on the ridges on either side of these V-shaped canyons 

 shows that the ice at one time covered the area now occu- 

 pied by these river-cut canyons, and if the present canyon 

 system was then in existence the glacier must have pro- 

 truded as a tongue still farther west in the lower rugged 

 canyon. Of this there is no evidence. However, the 

 matter needs much more investigation, for at this altitude 

 (2,500-4,000 feet) glacial markings would rapidly disap- 

 pear, and on account of the steep slopes of the canyons 

 morainal material might easily be removed. 



Perhaps the strongest evidence that the inner rugged 

 canyon at this point is of more recent date than the earlier 

 glaciation is its V-shape, characteristic of river erosion. 

 Even after all traces of glacial markings were removed by 

 weathering and all morainal material removed by erosion, 

 we should expect that the U-shape character of the canyon, 

 if it had been occupied by ice, would still be evident.^ 



Another way to account for the V-shape character of the 

 canyons at the western limit of the glaciated area is to sup- 

 pose that erosion there has been very rapid. The water 



1 In comparing sections of the deep Sierra canyons drawn to scale from the topo- 

 graphic maps of the United States Geological Survey, often no great difference in form is 

 apparent; but when seen on the ground certain differences in details are very plain. 

 Thus, there is usually less talus in a glaciated canyon, it having been removed by the 

 ice as morainal material; the projecting shoulders of the side and hummocks in the floor 

 show rounding; and there is usually more of a floor, or flat space along the bottom. 

 While the deep river canyons exhibit in cross-section a rather flat V-shape, the much 

 talked-of U-shape of glacial canyons is often not so apparent. Such canyons, however, 

 do exist, as for example, that of Bubb's Creek, a tributary of King's River. The U-shape 

 of this canyon is fiuelj" seen in a photograph taken by ISIr. J. N. Le Conte from near the 

 Grand Sentinel on King's River. Notwithstanding that the difierence between cross- 

 sections of the two types of canyons is far from being so marked as the difference be- 

 tween the letters V and U, nevertheless the large floor space of the glaciated canyon 

 gives it a tendency to U-form. Moreover, if exact contour maps of the two types of can- 

 yons should be made on the scale of 1000 feet to the inch (about 1/12,000, the present 

 scale of the published maps of the Geological Survey being 1/125,000 or nearly two miles 

 to the inch), the points of difference would probably be clearly apparent in the cross- 

 sections. 



