STRUCTURE 299 



This series has a thickness of about 3,500 feet and carries an Upper 

 Carboniferous and Permian fauna. It is in this series that the uncon- 

 formity by erosion, if it exists, is likely to occur. It is suggested by the 

 apparent overlap of the beds ; moreover, a widespread unconformity is 

 known to exist at this horizon in Colorado and elsewhere. 

 ■ Above these in typical development are the light red, thin- bedded 

 sandstones, generally classed as Triassic, which in turn are succeeded by 

 shales and limestones carrying characteristic Jurassic fossils. 



The accompanying map (figure 2), of which the topographic base is 

 a copy of a portion of the Hayden Peak quadrangle of the Forest Reserve 

 Survey by the U. S. Geological Survey, shows the distribution of the 

 above described beds as far as they occur on the area mapped. Unfor- 

 tunately the map extends only a few miles south of the Iron Creek fault, 

 so that a large part of the Paleozoic exposures do not appear on it. 



Structure 



In the area represented the beds do not strike due east and west, but 

 to the north of east on the east side of the valley and to the north of west 

 on the west side, which means that the valley runs approximately in the 

 axis of a secondary anticlinal fold, with axis at right angle to that of the 

 main anticline. In dip the beds steepen to the southward from the aver- 

 age angle of 5 degrees in the Uinta quartzite to 10—17 degrees in the area 

 mapped; then to 25 degrees just south of it, and finally to -45 or even 

 60 degrees along a varying line that follows the southern flanks of the 

 ranges. A typical cross-section of the range is given in figure 2, which 

 is reduced and somewhat generalized from an actual section drawn to 

 natural scale on a line following the top of the spur east of the Duchesne. 

 J list north of the main crest of the range there is generally found to be 

 a sharp break line in the Uinta quartzites, on the south of which they in- 

 cline from 2 to 5 degrees southward, and on the north dip steeply north- 

 ward up to angles of 45 degrees or more. There has evidently been fault- 

 ing in the region where this remarkable change of dip takes place, but, 

 owing to the similarity in lithologieal constitution of the rocks on either 

 side, neither the throw nor even the location of the fault can be deter- 

 mined. It is not, however, continuous along a given east and west line, 

 as seems to have been assumed by Powell, who probably reasoned from 

 the most strongly marked of these faults, which runs along the northern 

 edge of the Red Creek Archean body, in the Browns Park region; this 

 fault, however, has a strike of from 16 to 20 degrees to the north of 

 east and crosses Green river to the south of Horseshoe canyon; hence 



