20 FORTYMILE, BIRCH CREEK, AND FAIRBANKS PLACERS, [bull. 251 
shallow and swift, with an average width of perhaps 30 feet. Amer 
ican Creek is its most important tributary. 
Seventymile system. — The Seventymile, which is nearly the size oJ 
Fortymjle, has a length of about 75 miles, and flows in a general east 
erly course, through a beautiful valley, to the Yukon, about 25 miles 
below Eagle. Its upper valley is of the canyon type, but at Barnej 
Creek, about 30 miles west of Eagle, it becomes broader, and h 
bounded on the north by a high, steep ridge, and on the south by mas- 
sive spurs that descend rather abruptly from the plateau region to tin 
foot of a flat which is about one-half mile in width and from 10 t( 
20 feet above the level of the present stream. In the canyon portior 
the spurs from the hills to the south are even more clearly benched thai 
are those on the Fortymile. The surface of the most prominent bench 
500 feet above the stream, is shown in the accompanying photograpl 
(PL III, B). Farther south the hills rise to a height of aboul 
2.500 feet or more above the river, and exhibit beautifully preserved 
rock-cut benches to their summits, which are flat topped, several acres 
in extent, and often correspond in level to the truncated tops of neigh 
boring hills. The benches, except where obscured by gulches, can lx 
traced around the hills; they are especially prominent on the spur* 
and occur at corresponding levels. The vertical distance bet wee 
the step-like benches varies from a few feet to about 50 feet, tl 
rise from one to the other often showing an outcrop of the bed roc 
The rocks comprising this area are highly contorted metamorpli 
schists and granular intrusives. Whatever the bed rock or its at 
tude, the same forms have been developed and are most striking fe 
tures of the landscape. Similar forms were observed in other loca 
ties on the northern side of the Yukon-Tanana country, but nowherf 
exhibited such development as in the valley of the Seventymile 
The tributaries from the north are short and enter the river thr 
deep, narrow canyons. Some of those from the south attain a lengtli 
of from 12 to 15 miles. Bryant, Mogul, Granite, and Flume are til 
most important. These head in beautiful open valleys, which becomii 
narrower as the main river is approached, until, in the lower portions 
the streams flow through narrow canyons cut by them in the broad 
flat bench, which, at the Seventymile, is about 500 feet above tbl 
stream. 
The river can be easily forded on foot at low water, and with thi 
exception of a short portage at the falls, is generally navigable fo 
small boats nearly to Barney Creek. 
The same type of country prevails from the Seventymile to t 
Birch Creek region. Ridges like Glacier Mountain, however, a 
more prominently developed. One extending from the Seventy 
about opposite the mouth of Flume Creek for about 15 miles i 
westerly direction is the most conspicuous example. The altitud 
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