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Show 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 whenever they are present, but not the Tertiaries, which, when found in contact, overlie 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 deposition 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 projecting 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 prolongation 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 |
