310 BROOKS AND KINDLE PALEOZOIC ROCKS OF UPPER YUKON 



noteworthy are some high gravel deposits which lie on a well defined rock ' 

 bench some 200 feet above the river at Calico bluff. The section of this 

 deposit showed about 10 feet of coarse river gravel resting on bedrock, 

 succeeded by 20 feet of fine silt. The base is about 160 feet above the 

 river. The rock bench on which this deposit rests, traceable from Daw- 

 son down the Yukon to Calico bluff and beyond, appears to mark an old 

 valley floor. The same epoch of erosion has been noted in some of the 

 tributary streams, such as Fortymile, where old valley floors are found 

 above the present waterlevel and probably mark the same cycle of erosion I 

 as that of the high gravels (White channel) of the Klondike. 



At the mouth of several of the northern tributaries of the Yukon heavy 

 deposits of silts are found resting on a rock bench about 200 feet above 

 tlie river. The fact that these silts occur only along the streams whose 

 sources lay in a glaciated region suggests that they are probably of glacial 

 origin. In other words, they represent the overwash deposits of a re- 

 treating ice-sheet which, however, did not reach the Yukon. At the 

 mouth of Sheep creek these deposits are found to have a thickness of 500 

 to 1,000 feet. They are essentially buff-colored fine silt, with some clay 

 beds, showing very little evidence of stratification, but usually rest on a 

 stratum of gravel or sand. Spurr^^* has described these elevated alluvial 

 deposits at some length under the name ''^Yukon silts," and has suggested 

 a glacial origin for at least those of the upper river. When these deposits, 

 ■which are found for 2,000 miles along the Yukon and Lewes rivers, are 

 mapped in detail, it will undoubtedly be found that they include material 

 of quite diverse origin. The occurrence in them at many localities of 

 vertebrate remains adds interest to the problem of the genesis of these 

 deposits. Maddren^*^ has recentty summarized the available data bearing 

 on their distribution and origin. 



STRUCTURE 



In the foregoing pages an account of the structure of each individual 

 stratigraphic unit has been presented, so far as the facts are available. It 

 is proposed to here summarize this data and to point out the larger struc- 

 tures of the province. 



The general trend of the dominant structure lines has already been 

 referred to and emphasis has been laid on their crescentic swing through 

 the province here under discussion. Thus, in the Eampart region and 



I 



154 Geology of the Yukon gold district, pp. 200-221. 1 



1^ A. G. Maddren : Smithsonian exploration in Alaska in search of mammoth and other 

 fossil remains. Smithsonian Miscellaneous Collections, vol. xllx, 1905, pp. 1-117. 



