the Muddy Mountains, Nevada, etc. 59 



formable on the Mesozoic rocks. Sa^vtootli Ridge is an 

 overturned block of Kaibab limestone with local over- 

 thrust relations to younger formations. Apparently its 

 structure is the result of intense local compression. 



In general, faults do not seem to be continuous from the 

 Muddy Mountains to the Virgin Range, but the Rogers 

 Spring fault is a possible exception. The eastern bound- 

 ary of the schist ridge northeast of St. Thomas is proba- 

 bly a fault, and it is directly in line with the monoclinal 

 folding of the clays northeast of Bluepoint. 



It is worthy of note that both faulting and folding 

 appear to be much more intense in this region near the 

 Plateaus than in knowm parts of the Basin Ranges far- 

 ther west. Instead of a gradual dying out of the effects 

 of deformation toward the east, those effects are intensi- 

 fied in a border zone, and the change to strata which are 

 relatively little deformed appears abruptly, at a well 

 defined line. 



Physiography. 



Old surfaces of aggradation and erosion. — In all of the 

 wide intermontane valleys remnants of former aggrada- 

 tion surfaces are preserved as mesas, which usually have 

 a capping of caliche, a firm conglomerate with a rich 

 matrix of calcium carbonate or gypsum deposited on 

 evaporation of ascending ground-water. The highest 

 surface, represented by Mormon Mesa, lies 700 to 800 feet 

 above present stream grades, and successively younger 

 surfaces are 250, 100, and 40 feet above the streams. All 

 of the surfaces are underlain by coarse gravel and sand to 

 a depth ranging from a few feet to 300 feet. The two 

 lower levels form narrow belts bordering streams, and 

 may appropriately be termed terraces. The upper sur- 

 faces are much more extensive, and are not terraces in the 

 ordinary sense of the term. Remnants of the highest sur- 

 face show that it once extended through St. Thomas Gap 

 and covered all of the area north of CalMlle Mountain ex- 

 cept a few scattered peaks, and rock-cut benches border- 

 ing these higher points indicate that the base-level corre- 

 sponding to the aggradation surface remained stationary 

 for a considerable period. In Callville Mountain a dis- 

 tinct fall line marks the elevation at which the surface met 

 the mountain walls. Many stream channels on the surface 

 of the mountain have easy grades and comparatively wide 



