36 FAIRBANKS AND RAMPART QUADRANGLES. 
of gold may now be different from what it was formerly, neverthe- 
less it is reasonable to suppose— and the occurrence of gold near the 
headwaters renders such a supposition entirely justifiable — that the 
deposition of auriferous material is there in progress. At present 
the streams come in closest relation in the vertical section to the bed 
rock near the heads of the valleys, and there, if anywhere within 
the valleys, downward cutting of the bed rock is in progress. The 
lower parts of the valleys have been areas of abundant deposition. 
Near the heads deposition closely folloAvs cutting and there the 
deeply buried, more or less permanently frozen pay streaks of the 
lower valleys merge into the deposits within the zone of the present 
streams' activity. 
The gravels in the valleys of the Fairbanks region are composed 
of materials derived from the bed rock in which the valleys have 
been cut and were deposited through stream action, uninfluenced by 
any general glaciation, yet under conditions somewhat different from 
those of the present. The position of the pay streak in the valley 
marks the position of an earlier drainage way as well as that part 
of the cross section of the valley which was at the time of its deposi- 
tion probably the deepest. 
The successive stages of development may have been somewhat as 
follows: (1) Elevation of the region, the surface being laden with 
much unassorted weathered material and older stream deposits; (2) a 
period of active erosion by the streams during which there was little 
opportunity for the formation of permanent deposits; (3) a period 
of deposition when the streams were nearly down to grade and when 
the pay streaks were, for the most part, laid down with their clay 
content, which may have been derived in part from the abundant 
clay of the weathered material and in part directly from the bed 
rock; and (4) a period of stream shifting, valley widening, and fur- 
ther deposition, with the gradual development of the unsymmetrical 
type of valley of the present day. This unsymmetrical shape — one 
side steep and the other a more or less gradual slope — is a character- 
istic feature of many valleys in Alaska and results probably from 
several causes, among which are local elevation or depression, litho- 
logic character and structure, and insolation, It suffices here to 
emphasize only one of these, often observed by miners — that the sunny 
side of valleys is subject to more rapid wear than the shady side, 
which remains locked in frost for a much greater part of the open 
season. Slides are of frequent occurrence on the sunny sides of the 
valleys, even on very gentle slopes, and in the course of time these 
produce an accumulation of waste which forces the stream toward 
the opposite slope of the valley. 
The greater mobility of the material w T as due probably in part 
to the greater activity of the streams, which were at that time just 
