202 STKUCTUKAL GEOLOGY. 



THE YAMPA PLATEAU. 

 DISPLACEMENT. 



The structural sections of Plate I embrace not only the Uinta Mountain 

 region, but also extend over the region which I have designated as the Yampa 

 Plateau. The facts relating to the present position of the formations are 

 there fully set forth. On Plate V we have separated the stereogram of the 

 Yampa region from the general stereogram, Plate III. Referring to Plate 

 V, attention is called to the Island Park sag d, d, to the Echo Park sag e, 

 to the Split Mountain cusp c, to the monoclinal flexure of Cliff Creek m, m, 

 w, to the Fox Creek flexure f, and to the Yampa fault #, g, which branches 

 midway in its course. 



A comparison of 'this stereogram with the geological map will be 

 instructive. 



The Yampa Plateau is on the south side of the Uinta Mountains, and 

 all the lines of displacement in the stereogram, except the Y ampa fault, show 

 the upheaval to be northward corresponding to the general upheaval of the 

 Uinta Mountains, but the exception mentioned exhibits a northward throw. 

 This great displacement is a flexure at its eastern end ; but it finally 

 changes into a dragged fault, where the thrown beds are flexed upward at 

 the end, and then into a clean fault ; midway in its course it branches ; the 

 northern branch is a fault as far as it has been traced ; the southern branch 

 changes from a fault to a monoclinal flexure, and finally these branches of 

 the great displacement fade out, but in a manner not clearly understood, as 

 the region is marked by an accumulation of late subaerial gravels, by soil 

 and by vegetation, and on the stereogram this uncertain area has been rep- 

 resented by broken lines. By referring to the map it will be seen that the 

 main fault with its branches is represented in the topography of the country 

 by bold cliffs. These lines of cliffs are broken by many deep gorges of 

 corrasion descending from the plateau to the lowlands where the stratigraphy 

 is plainly revealed, and the lower beds of Triassic sandstone are seen on 

 the same geographic level as the upper beds of Uinta Sandstone, so that the 

 total displacement is about 5,000 feet. These cliffs are due immediately to 



