178 THE ICE AGE IN NORTH AMERICA. 



bia River, and southward to the Mexican boundary, there is 

 neither any bowlder-clay nor scorings indicative of a general 

 southward-moving ice-mass. On the contrary, the great areas 

 of Quaternary material are evidently subaerial, not subglacial. 

 The rocks outside the limit of local mountain glaciers show no 

 traces either of the rounding, scoring, or polishing which are 

 so conspicuously preserved in the regions overridden by the 

 northern glacier. Everything confirms the generalization of 

 Whitney as to the absence of general glaciation. 



Wherever in the fortieth parallel area a considerable mount- 

 ain-mass reached a high altitude, especially when placed where 

 the Pacific moisture-laden wind could bathe its heights, there 

 are ample evidences of former glacial action, but the type is 

 that of the true mountain glacier, which can always be traced 

 to its local source. In extreme instances, in the Sierra Nevada 

 and Uintah Ranges, glaciers reached forty miles in length, and, 

 in the case of the Sierra Nevada, descended to an altitude of 

 2,000 or 2,500 feet above sea-level. Over the drier interior 

 parts of the Cordilleras the ancient glaciers usually extended 

 down to between 7,000 and 8,000 feet above the sea. In the 

 case of the Cottonwood Glacier of the Wahsatch, a decided ex- 

 ception, the ice came down to an altitude of 5,000 feet. . . . 



Not more than a thirtieth part of the entire surface of the 

 fortieth parallel area was ever covered by glacial ice. It is 

 characteristic of the canons of these extinct glaciers that they 

 give evidence of a gradual recession of the ice from its greatest 

 extension until it is entirely melted. This retiring from its 

 greatest bulk was not a continuous retrogression, but was 

 marked by pauses at certain places long enough to permit the 

 accumulation of considerable terminal moraines. In ascend- 

 ing one of the larger canons, as of the southern Uintah, there 

 is observed a series of successive terminal moraines, and in 

 passing to the upper heights of the ranges it is found that, in 

 the great snow amphitheatres, glacial markings, rock-polish- 

 ing, and the arrangement of morainal matter are evidently 

 fresher than in the lower levels or points of greatest exten- 

 sion. 



Whatever the greater causes may have been, the Cordilleran 

 surface south of the State of Washington was free from an ice- 



