792 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



Q 5-6. YTTKON VALLEY. 



Spun ™ distinguished an occurrence of Tertiary strata which he described as 

 the Twelvemile beds and provisionally referred to the Miocene or Tliocene. These 

 are now regarded as of Kenai (Eocene) age, according to Brooks (personal com- 

 munication) . Spurr says : 



Twelvemile Creek is one of the branches of Mission Creek. Near the point of junction of 

 these two streams are exposed in numerous outcrops slightly consolidated and coarse-bedded 

 grayels and sands which carry seams of light-brown sandy lignite. Fragments of wood, con- 

 siderably altered, having a diameter of as much as 2^ inches, are found, but no fossil leaves or 

 shells. Occasional pieces of black, shiny lignite; such as is found in the Kenai series and less 

 abundantly in the Mission Creek series, occur, but in such a manner as to lead to the suspicion 

 that they are fragments derived from these older rocks. The beds are slightly folded, having 

 a dip of about 20°. Near the mouth of Twelvemile Creek the dip is southward, though about 

 a mile above Twelvemile Creek, on Mission Creek, the dip is to the north, though the strike 

 remains nearly the same; thus a shallow syncline is indicated. 



The rocks of the Mission Creek series, which underlie these beds, are sharply folded, and 

 although no actual contact was seen, it is probable that an unconformity exists. Below Mis- 

 sion Creek the rocks of the Kenai series are well indurated, forming compact conglomerates and 

 sandstones, and they are apparently conformable with the rocks of the underlying Mission 

 Creek series, with which they are infolded. It appears, therefore, that the Twelvemile beds 

 can not be correlated with the Kenai series, and yet they are older than the horizontal Pleis- 

 tocene silts and gravels which are exposed all along the course of the Yukon. They may be 

 provisionally referred to the Miocene or Pliocene. From their nature they seem to be fresh- 

 water lake deposits. 



Q 7. UPPER YTTKON. 



A belt of coal-bearing, probably fluviatile deposits crosses the international 

 boundary in the Yukon Valley. The occurrences on the Alaskan side of the line 

 are described by Brooks and Kindle : ^"^^ 



The Kenai formation has usually been described as occurring in local basias, either of 

 lacustrine or fluviatUe origin. That described here appears, in part at least, to be of fluviatile 

 origin. The distribution * * * iadicates that a belt of the Kenai rocks runs parallel to 

 the Yukon from Eagle to Birch Creek. While this zone has not been traced continuously, it is 

 sufficiently well known to justify the opinion that it was originally deposited as one continuous 

 belt. * * * The conglomerate of the Kenai on Woodchopper Creek and Seventymile has 

 been found to be auriferous, which lends weight to the assumption that the deposits are of 

 fluviatile origin. 



R 5-6. ARCTIC COASTAL PLAIN. 



In traversing the Colville region Schrader ^°** distinguished the Eocene and 

 Pliocene strata as the Colville series, which he describes as follows : 



The Colville series is named from the large river along which it occurs and is excellently 

 exposed. It is a series of Tertiary terranes which underhes the flat tundra country, or Arctic 

 Coastal Plaia, that succeeds the more southerly roUing plain formed by the Upper Cretaceous 

 or Nanushuk series, which it apparently unconformably overlies, beiag itself in turn uncon- 

 formably overlaiu by the Gubik sands. * * * 



The series consists principally of heavy-bedded silts, soft sandstone, hmestone, shale, and 

 lignite. It is best exposed along the Colville in the region of the mouth of the Anaktuvuk, where 

 it forms steep-faced bluffs about 200 feet in height, extending for a number of nules both up 

 and down the river. 



