294 GEOLOGY OF THE HIGH PLATEAUS. 



rocks present a great deal of variation in their aspect. A booty of lavas so 

 enormous as that which caps the Aquarius cannot be discussed with profit 

 until it has been studied long and patiently, and inasmuch as my own 

 observation has been extremely superficial, I do not feel justified in attempt- 

 ing to give any further account of them. 



The structure of the plateau is best studied upon the southern slopes. 

 Here the most striking feature is a large monoclinal, already alluded to as 

 a companion to the "Water Pocket fold. It comes up from the southeast, 

 crossing the lower end of Potato Valley, and trends along the slopes north- 

 westwardly, disappearing beneath the lava-cap. The throw of the mono- 

 cline is to the westward. Upon its flanks the Cretaceous system is turned 

 up and dips westward beneath the southwestward extension of the general 

 plateau mass. The edges of its strata are truncated by erosion, and over 

 them lies unconformably the Tertiary. (See Atlas Sheet No. 7, Section 

 No. 7.) The upthrow of the monocline heaves up the Jurassic white sand- 

 stone, which is seen rolling up in a huge wave 1,200 to 1,800 feet high 

 across the lower end of Potato Valley. The position of this flexure rela- 

 tively to the plateau mass is peculiar and very striking ; indeed, at first 

 sight it appears altogether anomalous. We are accustomed in the western 

 regions to see the strata rolled up on the flanks of a mountain range like a 

 great wave urged onward towards a coast and bi^eaking against its rocky 

 barriers. But the Escalante flexure is like a wave sweeping along parallel 

 to the coast, the crest-line of the wave being perpendicular to the trend of 

 the shore. Its line of strike runs up the slope and disappears beneath the 

 Tertiary near the summit of the plateau. A fine steam of water (Winslow 

 Creek) runs upon this monocline parallel to its strike, precisely as Water 

 Pocket Creek runs upon and parallel to the course of that flexure. 



The age of the Escalante monocline is evidently Pre-Tertiary. It has 

 been exhumed b)' the general erosion after having been buried beneath 

 Eocene strata, and after these strata had been overflowed in great part 

 at least by many hundreds of feet of lavas. The stream had its course 

 laid out prior to this erosion, and held its position after it had cut through 

 lavas and Eocene beds into the underlying Jurassic sandstones. 



The area included between the Escalante fold on the west and the 



