KOYUKUK AND MOUNTAIN PROVINCES 235 



eral consists of a rolling country composed essentially of Mesozoic rocks 

 whose low, rounded hills and ridges vary from 1,000 to 3,000 feet in ele- 

 vation. It is sujjposed to represent the Koyukuk portion of the Yukon 

 plateau, but which here is not distinctly marked. The drainage, which 

 is separated from that of the Arctic slope by the Endicott mountains, 

 is southwestward. The master stream is the Koyukuk, which flows into 

 the Yukon, while the large tributaries are South Fork, John, Alatna, 

 and Alashuk rivers. The lower part of all these tributaries, as well as 

 the Koyukuk, meanders in wide valley flats, bordered by the rolling 

 country which has been noted. The Koyukuk river is navigable by 

 steamboats to Bettles, near the 67th parallel, a distance of about 530 

 miles above its confluence with the Yukon. 



MO VNTA IN PRO VINCE 



The middle or mountain province is the most striking. It consists of 

 a rugged range of mountains couij)osed of Paleozoic rocks extending 

 east and west across the field between latitudes 67° 10' and 68° 25'. 

 These mountains, for which the name Endicott is locally proposed, here 

 have a minimum width of 80 miles and an average elevation of 6,000 

 feet. Orographically the range is regarded as the northwestward con- 

 tinuation of the Rocky Mountain system of the United States 'and British 

 Columbia, which here trends nearly east and west entirely across nprtbern 

 Alaska, forming the great Transalaskan watershed between the Yukon 

 on the south and the drainages of the Arctic ocean on the north. 



In their northward and finally westward course they form a promi- 

 nent feature of the concentric geography of Alaska, and embrace in the 

 concavity which they present on the south the great basin of the Yukon 

 and its well known but not always distinct feature, the Yukon plateau. 

 West of the 153d meridian, in the region of the head of tbe Colville and 

 the Noatak rivers, the mountains decrease in elevation and seem to 

 divide or fork, forming two ranges. The northern range, continuing 

 westward, terminates in the low mountains and abrupt sea cliffs of cape 

 Lisburne at Bering sea, while the southern forms the divide between the 

 Noatak and the Kobuk rivers. 



On the southern side the rise from the rolling Koyukuk country is by 

 means of foothills, but rapid. On the north the mountains br^ak off 

 somewhat abruptly, much as they do along the edge of the Great plains 

 in the western United States, as shown in plate 41. Pronounced fault- 

 ing and uplift are evidenced by marked deformation of the strata and 

 the presence of fault scarps, sometimes recognizable for miles. Where 

 they were crossed the mountains locally present a crescentic or concave 

 front to the northward, which is followed by low concentric ridges in the 

 plateau beyond, though these grow weaker and die out farther northward. 



