136 1. <. RUSSELL — SURFACE GEOLOGY OF ALASKA. 



wild landscape. Frequently a large de'bris stream will bifurcate above and 

 be joined by secondary branches, forming a dendritic Bystem of the same 



ueral character a< that presented by high-grade mountain streams. In 

 fact, the depressions occupied by the debris streams are also lines of water 

 drainage, but the grade being exceedingly sleep, they discharge their waters 

 quickly, and are therefore usually dry. Their slopes arc usually upwards of 

 thirty degrees, and not infrequently appear to approach the perpendicular. 

 I have observed similar streams of d€bris on the steep mountains of the Arid 

 Region, hut they arc there less conspicuous — perhaps on account of the ab- 

 sence of a general covering of moss and lichens on the undisturbed rock 

 surfaci - 



T'lhis Slopes "i- Screes. — All of the mountains in the nonglaciated portion 

 of Alaska arc flanked with great accumulations of angular d£bris derived 

 from the steep .-dupes above them. This material forms a pediment about 

 the mountains and accumulates especially in the mouths of steep gorges. 

 Many of the talus slopes are fed by the debris streams just described. 



The limestone ranges along the Yukon near the international boundary 

 are particularly noticeable for the magnitude of the talus slopes about them. 

 While enjoying Mr. McGrath's hospitality, I climbed the mountains a few 

 miles north of his station, near Belle Isle, and had a tar-reaching view over the 

 surrounding country from an elevation of about .'>,000 feet above the river. 

 The crest of the range visited is composed of compact earthy limestone in nearly 

 vertical strata, striking nearly east ami west, conformably with the trend of the 

 mountains. This range retains its prominence for fully fifty miles eastward 

 of the national boundary and was in full view while suhsequentlyjourneying 

 up the Yulcm. Its crest is composed of blade like crags of rock forming an 

 exceedingly sharp crest line, flanked by vast slopes of loose angular stones 

 on either side. The rock is fresh and undecomposed, but everywhere 

 shattered and fissured. The upper portions of the talus slopes, like the crags 

 rising above them, are bare of vegetation. At a lower level they are covered 

 with moss, increasing in thickness as one descends, and finally, at an eleva- 

 tion of about 2,000 feet above the river, merging with the nearly universal 

 forest covering of the count ry. 



The e litions just described prevail throughout the nonglaciated portions 



of Alaska and the North West Territory, hut not in the recently glaciated 

 area of tin- upper Yukon region. 



Absenct ofDibrisin tin Glaciated Region. — In the glaciated region drained 

 by the Lewes, and also throughout southern Alaska, there is a remarkable 

 absence of de'bris on the mountains, It is evident that the ice movement in 

 this region -wept the surface clear of previously accumulated fragmental 

 material. < Mi i he south Bide of the Coast mountains the d£bris carried away 

 by the ice was deposited in lie- ocean ; on the north Bide the ice movement was 



