ENDICOTT MOUNTAINS. 43 



Since the Yukon Plateau, as to whose age there is slight difference of opinion, 

 has been shown by Dawson," Spurr,* and other writers to be due to Eocene or 

 early Xeocene erosion, if the present writer is correct in his supposition concern- 

 ing the representation of this plateau feature in this northern field, the interrela- 

 tions of the several features here make it obvious that the Endicott Plateau, which 

 is certainly post-Lower Carboniferous, must be at least pre-Neocene and is probably 

 considerably older, and that the Koyukuk Plateau is at least post-Eocene and possibly 

 considerably younger. 



DRAINAGE. 



In the portion of the range crossed by the Survey party the drainage is 

 principally southward into the Koyukuk. The master stream is John River, 

 which rises near the northern edge of the range. The main drainage ways are 

 therefore transverse, extending across the strike and trend of the rocks, as well as 

 across the trend of the range. The. small tributaries, being nearly always controlled 

 by rock structure, flow in general along the strike and enter the master stream at 

 right angles, producing a rectangular drainage system. 



Though John Ri\-er Valley is intramontane and contains some canyons, it is 

 broad and in general open. A portion of it, near the middle part of the range, 

 seems to lie in a syncline in the Fickett series, trending a little east of south. The 

 valley probably averages about 1% miles in width, from base to base of the mountains. 

 Portions, however, are much wider and contain flats, through which the river freely 

 describes great bends from side to side. The present stream channel has apparently 

 been sunk into several older valley floors, as is shown by the bed-rock benches along 

 the sides of the valley. Of these benches the most pronounced occur at heights of 

 1,700 feet, 600 feet, and about 100 feet above the present stream, and seem to mark 

 stages of comparative rest in the progress of orographic uplift. It is probable that 

 the 600-foot bench may be correlated with the benching noted on the Koyukuk, in 

 the region of Red Mountain. 



At the head of John River, benches sloping northward against the present drain- 

 age seem to denote that a considerable area lying at the head of this stream formerly 

 drained northward, through the Anaktuvuk and the Colville, into the Arctic Ocean, 

 instead of southward, through the Koyukuk and the Yukon, to Bering Sea, as at 

 present. 



The bed-rock benching and the topogi-aphy of the lower side slopes of the valley 

 are frequently found to have been materially modified by ice action, which has 



"Dawson, G. M., The physiographical geology of the Rocky Mountain region in Canada: Trans. Royal Soc. Canada, 

 vol. 3, 1890, sec. 4, pp. 1-74. 



SSpurr, J. E., Geology of the Yukon gold district, Alaska: Eighteenth Ann. Rept. U. S. Geol. Survey, pt. 3, 1898, 

 pp. 257-265. 



cSchrader, F. C, Preliminary report on a reconnaissance along the Chandlar and Koyukuk rivers, Alaska, in 1S99: 

 Twenty-first Ann. Rept. TJ. S. Geol. Survey, pt. 2, 1900, p. 468. 



