Yosemite National Park 201 



washed away, yet remnants may be identified by their con- 

 stituent materials exposed in the sections at the banks of the 

 river. 



The materials of which these moraines are built are of special 

 interest. Most conspicuous are large smoothly rounded boul- 

 ders, but mixed with these are cobbles, pebbles, angular frag- 

 ments of rock of all sizes, and much sand and mud. Many 

 rounded boulders and cobbles are highly polished and in places 

 scratched and scored, like many rock floors in the High Sierra. 

 This shows that they have been brought down by a glacier, 

 that they have been ice-worn. The polish could only have been 

 imparted by long-continued abrasion with the impalpably fine 

 rock floor which is produced by the grinding action of glaciers. 

 Among the boulders and cobbles there are some that represent 

 rock types not known to occur in the Yosemite region, though 

 they do occur in the mountains and rock floors of the high- 

 lands above the valley. The occurrence of these foreign mate- 

 rials, rounded and polished boulders and irregular fragments, 

 with sand and mud, in a confused mixture with rocks from 

 near-by formations, suggests the work of ice. 



The moraine just above the Bridalveil Meadow marks the 

 farthest limit reached by the Yosemite Glacier during the last 

 stage of glaciation, the Wisconsin what may be called the 

 latest chapter in glacial history. During the earlier stages of 

 glaciation the ice extended many miles farther down the valley 

 as far as El Portal, to be referred to later. Above the El 

 Capitan Bridge no further moraines are to be found in the 

 Yosemite Valley for a distance of about 5 miles. There, near 

 the head wall that marks the upper end of the valley, occurs 

 the largest and most conspicuous moraine of all, a hummocky 

 ridge 50 to 60 feet high and half a mile long. That this ridge 

 is a moraine of glacial deposit is shown by the fact that rounded 

 and polished boulders of rock fragments such as occur in the 

 High Sierra region far above the valley have been taken from a 

 cut in the road which crosses the ridge. 



