Ill l.C. RUSSELL — SURPAC1 GEOLOGY OF ALASKA. 



- closely woven thai when the river borders are washed away it hangs from 

 the top of the bank like a curtain, as if intended to hide the ruin the waters 

 had made. 



The gn atesl expanse of the Yukon lowlands, as already mentioned, occurs 

 just above the Lower Ramparts, and extends some 250 miles to the eastward ; 

 ii- breadth may be roughly estimated at from 75 to ltti) miles. At the 

 Lower Ramparts the river is greatly contracted, and is now deepening its 

 channel. The explanation of the presence of the lowlands above the Lower 

 Ramparts seems to be that orographic movement is taking place, and a 

 mountain range is being raised athwart the river. Above the obstruction 

 the river has Bpread out a broad Hood-plain, through which it meanders. 

 This is only a suggested explanation of the origin of the lowlands. No 

 opportunity was afforded for studying the matter in detail. It is possible 

 that a broad lake has existed above the Lower Ramparts, but no beach lines 

 were observed on the hills which would have formed the border of such a 

 lake, and besides, the material exposed in the river hanks does not suggest 

 the presence of lacustral conditions duringits deposition. The lack of evi- 

 dence of the former presence of a lake, as well as the positive evidence of 



tl 1-plain conditions, leads me to suppose that obstruction of the drainage 



by orographic movement would account for all the conditions noted. 



Whither a similar relation of lowlands to river narrows occurs in the 

 case of the swampy areas below the Lower Ramparts or not is uncertain. 

 The broad, moss-covered region of the delta belongs to another category and 

 mid m»t be considered in this connection. 



Highlands of the Upper Yukon. — Above the lowlands through which the 

 Yukon and Porcupine rivers flow near their junction, the banks of the 

 Yukon are bold, and usually rise abruptly from the river. Many of them 

 rise like Bea-cliffs directly from the water's edge to a height of four or five 

 hundred feet, and can not be passed even by a person on foot. About their 

 bases the river sweeps with such force that the ascent of the stream in a 

 -mall boat i- exceedingly difficult. 



A- one continue- to ascend, the terraces on the borders of the stream be- 

 come more and more prominent, until near the mouth of the IVllv river, 



and thence to the lakes on the Lewes they form an important element in the 

 landscape. 



At the mouth of the Telly, and for several miles below, there is a bold 

 palisade on the righl bank, formed by a basaltic escarpmenl Bome three or 

 four hundred feet high. This is the edge of a table land, formed by a lava 

 Mow which filled the valley and extended Beveral miles up the Telly. The 

 Yukon in excavating its channel occupied the Ii f junction between the 



lava coulee and the bold left bank of its former valley. The Tcllv also 



followed the border "I the coulee along it- eastern edge. 

 At the international boundary the Yukon flows through an exceedingly 



