200 DESCRIPTIVE GEOL'JGY. 



In its simplest form, the great Uinta fold may be likened to a flat 

 arch, in which, by reason of too sharp a bend at either angle, the joints 

 have been partly broken near the extremities, and the crest of the arch 

 forced bodily upward, without dislocation in the centre. Actually the 

 principal displacement has occurred on the line of the northern angle of the 

 arch, that along the southern angle having been only local and of limited 

 extent. The main geological axis of the range follows approximately the 

 line of the northern face of the higher peaks. South of this line, the strata 

 slope southward at a gentle angle, generally about 5*^, but bend sharply 

 downward at the southern extremities of the spurs, forming thus a second 

 axis. Along the flanks of the range, where the denudation of the Tertiary 

 covering has been of sufficient depth, are exposed the upturned edges of the 

 younger portion of the conformable series, which once formed the crest of 

 the arch, the Triassic, Jurassic, and Cretaceous formations. 



The Triassic formation is represented by the Red Bed group in a thick- 

 ness of about 2,500 feet, principally of sandstones. At its base is a series of 

 clayey beds, over which are the characteristic red sandstones of the forma- 

 tion in a thickness of about 1,200 to 1,500 feet, about equally divided by a 

 thin but persistent bed of limestone, the summit of the formation consisting 

 of massive white and buff, cross-bedded sandstones, which generally form 

 prominent ridges. This formation is, as far as observed, barren of fossils, 

 and its horizon has been determined by its strati graphical position between 

 well-defined fossiliferous strata^ 



The Jurassic formation, though thin, contains the only considerable 

 development of limestone in the three groups. Its thickness, which varies 

 somewhat in different portions of the range, averages about 600 to 800 feet, 

 in which the maximum observed thickness of limestones, which are highly 

 fossiliferous, was 200 to 300 feet, the remainder being made up of sand- 

 stones, shales, and clay beds, remarkable, where well exposed, for their 

 bright, variegated colors. 



The Cretaceous formation consists of over 10,000 feet of beds of sand- 

 stones and clays, carrying coal seams, which are most abundant in the uj)per 

 part of the series. In the division of this formation into four groups, as 

 carried into this region, the Dakota group consists of about 500 feet of rather 



