The History of a Portion of Yampa River, Colorado, and Its Possible Bearing on that of Green River

Hancock, E.

Few regions offer more interesting geologic problems relating to drainage than the Uinta Mountains, in Utah and Colorado, and the area immediately east of them. In fact, the writer's attention was primarily attracted to this field by the diversity of opinion regarding the antecedent origin of Green River. Although the present paper deals mainly with that portion of Yampa River east of Juniper Mountain, the conclusions reached are believed to have a definite bearing on the Green River problem itself. The paper is introduced by a brief discussion of the structural features of the region east of the Uinta Mountains, for a clear understanding of the relation of the minor uplifts to the great Uinta fold will better enable the reader to appreciate the possible bearing which the writer's conclusions may have in the solution of that problem. The main range of the Uinta Mountains is a broad, flat-topped anticline, which has an easterly trend and a length of over 150 miles and which separates the Green River Basin on the north from the Uinta Basin on the south. The conspicuous portion of the Uinta fold terminates in northwestern Colorado, but along the continuation of its axis to the east lies a long, gentle anticline which reaches the foothills of the Park Range. This anticline was called by White1 "the inceptive portion of the Uinta fold." The axis of the anticline is coincident with the low, broad valley known as Axial Basin, and in a recent report Gale2 refers to it as the Axial Basin anticline. According to White, along the eastern end of the main fold the strata abruptly dip east beneath the younger Tertiary beds. Upon the Axial Basin anticline have been developed two local uplifts which he called the Junction Mountain upthrust and the Yampa Mountain upthrust. These he regarded as the result of a locally intensified application of the same force by which the main fold was elevated. The peculiar relation which exists between these two "upthrusts" and the course of Yampa River renders it necessary that the reader shall clearly understand their character.