120 A. C. SPENCER — PACIFIC MOUNTAIN SYSTEM 



axis assumes a westerly trend, which is continued as far as mount 

 McKinley, the highest mountain of North America, where it turns 

 again, and the range assumes a southwesterly course, gradually dimin- 

 ishing in height toward the Tordrillo mountains, lying northwest of 

 Cook inlet. Between the Tordrillo and the Kenai mountains and over- 

 lapping them lies the Alaskan peninsula, the axis of which is parallel 

 to the course of the mountains on either side. Projected across the 

 Pacific ocean, this axis would follow the trend of the Aleutian islands. 

 The region lying between the Alaskan peninsula and the lower valley 

 of the Yukon river contains several more or less distinct ranges of moun- 

 tains, which must be assigned to the Pacific system, but the relations of 

 these to the adjacent Alaskan range are at present only vaguely known. 



Copper River District 



The Chugach and Wrangell mountains are drained principally by the 

 tributaries of the Copper river. The former as a range is made up of 

 mountains which have been carved by the numerous streams of the 

 region from a portion of the earth's crust which formerly existed as a 

 high plateau. The Wrangell mountains have originated essentially 

 through the upbuilding of volcanic materials on a surface which is the 

 extension of the Chugach plateau. Volcanic activity has been continu- 

 ous in this group from the time of its inception, probably in the Miocene, 

 down to the most recent time. 



The Chugach plateau is considered to have originated in the uplift of 

 a baseleveled land surface, and from the fact that this feature of erosion 

 has been found to bevel the edges of folded and upturned lower Creta- 

 ceous strata, its age is considered to be late Mesozoic or Tertiary. It is, 

 however, impossible to fix the date more closely than between these 

 limits. At the close of this long-continued erosion period the whole 

 region had been so completely reduced that all topographic evidences of 

 any dynamic revolutions of Mesozoic date were completely effaced. 

 There could have been no elevations worthy of the name of mountains ; 

 no greater eminences than low hills existed to break the monotony of 

 the extensive rolling plains. The upraising of the region was accom- 

 plished in several stages, but the intervening pauses were of ver}'' short 

 duration when compared with the earlier period of baseleveling. 



Two paragraphs adapted from the report upon the Copper River dis- 

 trict by Schrader and Spencer will indicate the character of the physiog- 

 raphy of the Chugach mountains.-^ 



* Loc. eit., pp. ()3 and Go. 



