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This trip differed from previous boat trips down the Yukon, in that an attempt was made to penetrate as far back from the river as possible, in order to obtain some idea of the areal distribution and regional trend of the geologic formations. Side trips were made up the Eagle, Seventymile, and Tatonduk Rivers and Fourth of July, Coal, Woodchopper, and other creeks, and much of the country several miles back from the river on both sides was visited. The present report, however, is based also in considerable measure upon the reports of earlier workers in this region, chiefly Collier, Prindle, Brooks, Kindle, Blackwelder, and Girty, and is an attempt to interpret all available data in the light of the writer's experience in this and other parts of the interior of Alaska.

GEOGRAPHIC FEATURES

RELIEF AND DRAINAGE

The Eagle-Circle district is a part of the great central plateau province of interior Alaska, which is bounded on the south by the Alaska Range and on the north by the Brooks Range. The Alaska Range is a continuation of the Coast Range province of the Cordilleras; but the Brooks Range, to the north, may represent either the Rocky Mountain Range of the Cordillera, bending northwestward into Alaska, or a separate mountain range of Arctic rather than of Pacific affinity. The term plateau, as applied to the great interior province, is somewhat misleading, as this province itself includes high areas that almost qualify as mountain ranges. The Ogilvie Mountains, which cross the international boundary just north of the Yukon, are apparently an integral part of the Rocky Mountains proper, but they extend only a short distance northwest into Alaska. Similarly, other small mountain groups that enter Alaska from Yukon Territory-for example, the Keele Mountains, just south of the Porcupine River, and other small unnamed mountain groups between the Yukon and Tanana Rivers-may be considered parts of the Cordilleran system, but their exact physiographic and structural placement in that system can not at present be given. If the Brooks Range represents the Rocky Mountains as developed along the east side of our western Cordillera, the interior province corresponds areally, though not necessarily physiographically or structurally, with the Great Basin province of the Western States. If, however, the Brooks Range represents an Arctic system of mountains, the interior province must represent the residuum of the western Cordillera minus the Coast Range. Under either interpretation it will probably be found that the conventional threefold division of the Cordillera into Coast Range, Great Basin, and Rocky Mountain provinces will require modification in Alaska.

The central plateau province in this longitude has a width from north to south of 350 to 400 miles, through the center of which flows the Yukon River in a northwesterly direction. The Porcupine and Tanana Rivers are the two largest tributaries of the Yukon, the former flanking the Brooks Range on the south and the latter flanking the Alaska Range on the north. The Yukon River proper, however, is the master stream of the area here described, the Porcupine and Tanana Rivers lying beyond its limits. The larger tributaries of the Yukon between Eagle and Circle, on the north side, named in order downstream, are Eagle Creek, the Tatonduk River (Sheep Creek), Hard Luck Creek, the Nation River (Tahkandit River), and the Kandik River (Charley Creek); those on the south side named in the same order, are Mission Creek, the Seventymile River, Trout Creek, Michigan Creek, Fourth of July Creek, Logan (Jewett) Creek, Washington Creek, the Charley River, Sam Creek, Coal Creek, Woodchopper Creek, Webber Creek, and Thanksgiving Creek. The Yukon River at the international boundary is about 800 feet above sea level. A number of ridges in the Circle quadrangle have altitudes of 4,000 to 5,000 feet, and one unnamed mountain at the headwaters of the Charley River rises to an altitude of 6,340 feet. The maximum relief south of the Yukon in this district is therefore 5,500 feet. North of the Yukon, where the Ogilvie Mountains cross the international boundary, several mountains are known to exceed 5,000 feet in altitude, and it is very probable that a topographic survey will show within this district, west of the boundary, mountains 6,000 or even 7,000 feet high. The relief north of the Yukon is therefore at least as great as that south of the Yukon and possibly greater. The general aspect of the Yukon Valley as seen downstream from the international boundary is shown in Plate 1, A.

SPECIAL PHYSIOGRAPHIC FEATURES

The ridge tops in the Yukon-Tanana region are rather flat, and many of the spurs leading laterally from the ridges are also flattopped, as well as extraordinarily long; this form, together with a noticeable tendency toward uniformity of ridge altitude in certain districts, has led earlier workers to regard this entire area between the Yukon and Tanana Rivers as a great dissected peneplain. This physiographic classification is not altogether proper. To be sure, the base-level of erosion in this region has been lowered since the Pleistocene epoch, as shown by the river-cut benches along the Yukon and its tributary streams. But the belief does not seem warranted that erosion in the Yukon-Tanana region had progressed prior to the regional lowering of base-level, to such a stage that a peneplain, in the ordinarily accepted use of that term, had been developed. It

is now well known that much of the flatness of the tops of ridges and spurs and the abnormal elongation of the spurs are due to erosional processes peculiar to a sub-Arctic climate, such as solifluxion, nivation, altiplanation, and the like. The ground in much of interior Alaska is perpetually frozen, and much of the present movement of débris is accomplished by the processes named, which depend primarily upon the fracturing and heaving caused by alternate freezing and thawing. Moreover, the rugged mountains of this region are by no means isolated monadnocks surrounded by lower country of nearly uniform relief. If a peneplain is a land surface that has been worn down almost to some base level, then the Yukon-Tanana region is not a peneplain. It should rather be regarded as an area in which a mature but not old topography had been carved prior to the late lowering of the base level.

The stream-cut benches constitute another topographic feature that has attracted the attention of every geologist who has visited this region. These are particularly well developed on a number of the streams tributary to the Yukon from the south. On the Fortymile, Seventymile, and Charley Rivers, for example, a well-developed bench about 500 feet above the level of the present streams forms a very prominent topographic feature. This bench, though very prominent, is only one of several such old valley levels. Along the Seventymile River just above the falls a low bench about 12 feet high is seen on the north side of the valley; then after another rise of 4 or 5 feet a great flat stretches northward to the hills. On the south side of the Seventymile River remnants of similar low benches are seen, succeeded to the south by a prominent bench about 125 feet high, which is in turn succeeded by flat gravel-covered spurs 1,500 feet long and 500 feet above the present valley level. The 125-foot bench is very persistent. It follows down the south side of the present Seventymile Valley to the point where the river turns northward toward the Yukon, then veers off to the east. This turn indicates that the lower part of the old valley of the Seventymile River had a somewhat different course from the lower part of the present valley and suggests an old confluence with the Yukon somewhere above Calico Bluff. The valley of the Seventymile River is but one of many in this district that show well-developed benches, and such topographic forms are evidence of an interesting physiographic history that will eventually be deciphered when large-scale topographic maps are available and detailed studies can be made.

The Yukon River also has river-cut benches, but these seem to be developed in greater number and perfection farther upstream, in Yukon Territory above Dawson. Near the mouth of Woodchopper Creek, however, where the Yukon flows for some distance in a relatively narrow channel, there is a well-developed rock-cut terrace

about 700 feet above the present river level. A part of this sharply defined terrace along the south side of the river, above and below the mouth of Woodchopper Creek, is shown in Plate 1, C. A similar bench at the mouth of Washington Creek is shown in Plate 1, B.

In general, the plateau province of interior Alaska has not been glaciated, but locally in some of the higher mountain groups alpine glaciation on a small scale has occurred. In the headwaters of the Charley River, for example, within the limits of the area shown on the accompanying map, Prindle and the writer in 1911 mapped two small areas of morainal deposits. Other areas in the valleys of the Charley and Salcha Rivers were also mapped, and detailed work would doubtless reveal many others. These areas of glaciation are confined principally to the granitic rocks, which constitute the country rock on and near the Yukon-Tanana watershed. One of the best examples of this local alpine glaciation may be seen near the head of the Charley River, in the valley of a creek called by Prindle and the writer Moraine Creek. This creek heads against a mountain 6,284 feet high that lies between two forks of the Charley River and somewhat north of the main Yukon-Tanana divide. The valley has the typical U-shaped glacial form, with a floor about 500 feet wide. At its lower end is a terminal moraine about 400 feet thick, which extends downstream 114 miles into the main valley of the Charley River, where it thins gradually to 100 feet. Here the morainal material extends downstream to an altitude of somewhat less than 3,000 feet. The glaciated valley of Moraine Creek, the confluence of this creek with the main fork of the Charley River, and the morainal material from Moraine Creek that has nearly filled the main valley are shown in Plate 2, A.

SETTLEMENTS AND POPULATION

The two principal settlements within this district are Eagle and Circle, both on the southwest bank of the Yukon River, the former a few miles west of the international boundary and the latter about 120 miles in an air line downstream. Eagle is a picturesque little settlement built upon a terrace that is well above the high-water level of the Yukon even at times of severe flooding after the spring break-up. It undoubtedly occupies the best town site of the upper Yukon in that it is safe from floods and yet is located alongside a deep channel of the river, which favors the landing and departure of river craft. Eagle is the supply point for the Fortymile, Seventymile, and American Creek mining districts and is also the port of entry in coming downstream from Yukon Territory. The permanent

Prindle, L. M., A geologic reconnaissance of the Circle quadrangle, Alaska: U. S. Geol. Survey Bull. 538, pp. 34-35, 1913.

summer white population is perhaps 40 people, but the winter population is larger, owing to the fact that some of the miners from the near-by mining districts spend the winter in Eagle. On the Seventymile River and on American Creek, adjacent to Eagle, there are 20 to 30 people engaged in mining, and just upstream from Eagle is a settlement of natives.

Circle is at the upper end of the Yukon Flats, upon a great river flood plain. It is the supply point for the Birch Creek mining district, to the south. It has at present a summer population of less than a score of white people, but, like Eagle, it has an augmented winter population, which is derived from the near-by Birch Creek mining district. There are also a considerable number of natives living permanently at and near Circle.

Between Eagle and Circle are two smaller settlements, Nation and Woodchopper, the former on the southwest bank of the Yukon about 2 miles below the mouth of the Nation River and the latter on the same side of the river just above the mouth of Woodchopper Creek. Only two men live permanently at Nation, but 8 or 10 others are engaged in mining on the near-by Fourth of July Creek. Similarly at Woodchopper the population consists mainly of the 15 or 20 men engaged in mining and prospecting on Woodchopper, Coal, and Sam Creeks.

A few trappers and prospectors also live along the river between Eagle and Circle, but the total white population of this district immediately contiguous to the Yukon, not including the Fortymile and Birch Creek mining districts, is probably less than 100.

TRAILS AND TRANSPORTATION

The Yukon River is the arterial highway of this region, being traversed by river craft in summer and by dog sleds in winter. Few summer roads have yet been made in this part of Alaska. A wagon road connects Eagle with American Creek and extends on southward as a pack trail to the Fortymile district, another extends out from Circle to the Birch Creek mining district, and during the summer of 1925 a short road was being constructed from Nation up Fourth of July Creek. Much of the freighting is done in winter by horse and dog sleds, but these winter trails are of little use for summer transportation.

Supplies for this region, including the Fortymile and Birch Creek districts, are received mainly by way of Skagway and Whitehorse and thence down the Yukon through Canadian territory by river boats. The Alaska Railroad does not serve the upper Yukon region, and the costs of passenger and freight transportation are high. A new summer road has recently been built to connect Fairbanks with

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