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fit later knowledge of the stratigraphic and lithologic conditions and remain in common usage, these being now designated, respectively, Birch Creek schist, Rampart group, and Tahkandit limestone.
The first topographic mapping in this region was done in 1898, when E. C. Barnard made a reconnaissance topographic map of the Fortymile quadrangle. The northern part of this quadrangle includes the Yukon River and contiguous territory from the international boundary downstream to the Tatonduk River, as well as the Seventymile River and American Creek Basins, and this map remains still a suitable base for geologic mapping in the southeast corner of the area under consideration.
Some further geologic observations were made in 1899 by Brooks,5 in the course of a traverse from Lynn Canal to Eagle.
The accumulation of detailed stratigraphic data was begun in 1902 by Collier, who traversed the Yukon from Dawson, Yukon Territory, to the Yukon Delta, giving particular attention and study to the Mesozoic and Tertiary coal-bearing terranes. During 1903 Arthur Hollick visited the same general region, in order to make more extensive collections of fossil plants from the coal-bearing beds. In 1903 Prindle began a systematic study of the geology and mineral resources of the Yukon-Tanana region, which continued intermittently until 1911. Coincidentally with this geologic work, a reconnaissance topographic map of the Circle quadrangle south of the Yukon was prepared in 1903, 1904, 1905, and 1908, by T. G. Gerdine, D. C. Witherspoon, R. B. Oliver, J. W. Bagley, and G. T. Ford. Prindle, in these years, outlined the fundamental geologic features of this region and laid a lasting geologic foundation for subsequent work. Prindle's traverse of 1903 from Eagle to Fairbanks by way of Circle and the traverse in 1911 by Prindle and the writer both cover a part of the area here described and form a part of the groundwork for the following publications:
Prindle, L. M., The gold placers of the Fortymile, Birch Creek, and Fairbanks regions, Alaska: U. S. Geol. Survey Bull. 251, 1905.
Prindle, L. M., The Yukon-Tanana region, Alaska-Description of Circle quadrangle: U. S. Geol. Survey Bull. 295, 1906.
Prindle, L. M., The Fortymile quadrangle, Yukon-Tanana region, Alaska: U. S. Geol. Survey Bull. 375, 1909.
Prindle, L. M., and Mertie, J. B., jr., Gold placers between Woodchopper and Fourth of July Creeks, upper Yukon River: U. S. Geol. Survey Bull. 520, pp. 201-210, 1912.
Prindle, L. M., A geologic reconnaissance of the Circle quadrangle, Alaska: U. S. Geol. Survey Bull. 538, 1913.
Brooks, A. H., A reconnaissance from Pyramid Harbor to Eagle City, Alaska, including a description of the copper deposits of the upper White and Tanana Rivers: U. S. Geol. Survey Twenty-first Ann. Rept.. pt. 2, pp. 331-391, 1900.
Collier, A. J., Coal resources of the Yukon Basin: U. S. Geol. Survey Bull. 218, 1903.
An important contribution to the geology of this area was made in 1906 by Brooks and Kindle,' who made a boat traverse from the international boundary to Circle, spending about two months in a study of the section exposed along the river. Their paper not only contained additional stratigraphic data of great value but also attempted for the first time a general correlation of the geologic terranes of interior Alaska.
In 1911 and 1912 the International Boundary Survey prepared a topographic map embracing a strip 2 to 3 miles wide on each side of the boundary from the Yukon River to the Arctic Ocean. A geologic survey along the boundary was also made, and for this purpose the strip was divided into a northern section extending from the Porcupine River to the Arctic and a southern section extending from the Yukon River to the Porcupine. The southern part of the southern section is within the area here described. The geology of the southern section was studied by Cairnes, representing the Geological Survey of Canada. One of the interesting results of Cairnes's work was the discovery in this area of a well-developed sequence of lower Paleozoic rocks, including rocks of Cambrian age.
The triangle bounded on the east by the international boundary, on the northwest by the Porcupine River, and on the southwest by the Yukon River contains unquestionably the most complete Paleozoic section in Alaska, but it is as yet almost unexplored. Wishing, if possible, to obtain additional data on this Paleozoic section, A. H. Brooks, chief Alaskan geologist of the United States Geological Survey, in 1915 dispatched Eliot Blackwelder upon a boat trip from the international boundary to Circle. Blackwelder spent perhaps a month in this work, and from the data so collected he prepared a geologic report on this river section. His report was never published but was available to the writer, both in the field and in the office, and was of great assistance in the preparation of the present report.
Further paleontologic field work was done in 1918 by G. H. Girty, of the Geological Survey, who made extensive fossil collections from the type Mississippian and Permian localities along the Yukon at Calico Bluff and at the mouth of the Nation River, respectively.
SCOPE OF THE PRESENT REPORT
During the summer of 1925 the writer, assisted by M. M. Knechtel, made another boat traverse from the international boundary to Circle, for the purpose of correlating the work of earlier investigators.
'Brooks, A. H., and Kindle, E. M., Paleozoic and associated rocks of the upper Yukon, Alaska: Geol. Soc. America Bull., vol. 19, pp. 255-314, 1908.
Cairnes, D. D., The Yukon-Alaska international boundary between Porcupine and Yukon Rivers: Canada Geol. Survey Mem. 67, 1914.
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.
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