OCR Text |
Show 162 geology; of the high plateaus. mainly to erosion, the degradation of 1,500 to 2,000 feet of beds haying proceeded unequally, although the stratification still retains its sensible horizontality. Upon the southwestern shoulder there is considerable complication of the displacement. Two or three sharp faults, running north and south, include between them a long block from 2 to 3 miles in width, which has dropped, the amount of the fall varying from 600 to 1,700 feet. The length of this block is at least 27 miles and may be greater. It is much complicated by minor fractures, and a portion of its southern extension into the Cretaceous terrace south of the Wasatch Plateau has been described and illustrated by Mr. G. K. Gilbert* as an instance of a "zone of diverse displacement." The general appearance and relations of this complicated downthrow suggest that the upper recurving branch of the great monoclinal was subject to tension during the uplift, and the beds, being unable to stretch, were rent apart, allowing the block to sink. The Cretaceous terrace, upon which we may look down while standing upon the southern terminus of the Wasatch Plateau, is no doubt, from a structural point of view, a part of that plateau: but the loss of its Tertiary beds by erosion has reduced its altitude to a level 1,500 to 2,000 feet lower. It continues the structural features southward to plateaus next in order, forming a kind of connecting-link between the northern and southern uplifts. Its chief deformation is due to the sunken block already described. The two faults between which it has fallen increase for a time their throw as they continue southward, reaching a maximum of nearly 3,000 feet, and then decreasing to zero at points about 18 and 20 miles, respectively, south, of the Wasatch Plateau. The structural depression thus produced has been called Gunnison Valley, but, this name being preoccupied, it should be used provisionally. It contains abundant evidence of its origin, for the Tertiary beds are seen to abut against the Cretaceous along the lines of faulting, and the latter beds tower far above them. The drainage of this valley is to the westward, through a deep canon called Salina Canon, which is a clearly defined, but by no means uncommon example of a general fact, which is repeated so frequently throughout the entire Plateau Country that *Amer. Jour. Science; also, Geol. Uinta Mountains, J. W. Powell. The minor fractures are too small to appear effectively upon the stereogram, and have been omitted, but the main faults are introduced. |