164 GEOLOGY OF THE HIGH PLATEAUS. 



of Tertiary time, and probably during the Cretaceous epoch. It may 

 belong to a class of flexures produced near the close of the Cretaceous, of 

 which several instances are found in the district, chiefly in its southeastern 

 portions. They all involve the Cretaceous beds in the displacements when- 

 ever they are present, but not the Tertiaries, which, when found in contact, 

 ovei'lie them unconformably. After the upturning of this flexure it may 

 have stood as a long narrow ridge near the western shore line of the great 

 Cretaceous-Eocene lake and been subject to a considerable amount of 

 degradation, which removed the Cretaceous beds and finally planed down 

 the whole mass until it stood but little above the common level. In the 

 oscillations of the shore line during the Green River epoch it would seem 

 to have been overflowed by the waters of the lake during the last stages 

 of its existence, receiving a thin deposit of the beds of that period, which 

 have since been nearly all removed, though just enough traces of them are 

 left to render it certain that they once extended over it in a sheet which is 

 locally very thin. At some epoch subsequent to that of the latest deposi- 

 tion a fault occurred, cutting along these Jurassic beds, throwing up the 

 western side into a great "hog-back." By the subsequent denudation of 

 the overlying Tertiaries the highly-inclined Jurassic beds are left project- 

 ing- above them and also above the continuation of these Tertiaries on the 

 eastern or thrown side of the fault. Thus they form a narrow belt between 

 the interrupted Tertiary formations. The fault is directly in the prolonga- 

 tion of the Sevier fault, but the throw is reversed relatively to it. It is 

 designated on the stereogram as the East Gunnison fault, and its northern 

 continuation is found on the west side of San Pete Valley, extending nearly 

 and perhaps quite to the base of Mount Nebo, though its details have not 

 been examined in that vicinity. The sections across this Jurassic Wedge, 

 as I have termed it, will be found in Mr. Howell's delineations (Plate 3), 

 sections 1 to 13. 



On the west side of the Sevier Valley runs another fault parallel to 

 the foregoing and presenting similar and even homologous features, but 

 with the throw on the opposite side. Both in linear and vertical extent the 

 dimensions of this displacement (termed the West Gunnison fault) are less 



