68 CHESTER R. LONGWELL 



Therefore the original topographic expression of the fault has been 

 reversed by differential erosion. This anomalous relation indicates 

 that the structure is more complex than is apparent at the borders of 

 the mountain (Fig. 4). 



Overthrust relations. — The key to the older structure of Call- 

 ville Mountain is furnished by Muddy Peaks Basin. Pro- 

 nounced doming has affected the western part of the mountain, 

 the strata on the sides of the dome dipping at a maximum of 

 45°. Erosion has stripped the thick Paleozoic beds from the 



■^ •L^s-a-aatsci 



Fig. 4. — Portion of the north front of Callville Mountain. In the foreground, 

 Triassic sediments are masked by recent waste. Outcrops of Jurassic sandstone 

 partly covered by talus near base of cliff. 



summit of the dome, exposing cross-bedded Jurassic sandstone 

 beneath. The sandstone forms the floor of an erosional basin, 

 which has been divided into two lobes by a faulted tongue of 

 Tertiary sediments. Limestone walls rise abruptly around the 

 margins of the basin, and are retreating rapidly due to sapping 

 of the comparatively weak sandstone beneath. Remnants of the 

 limestone, lying almost horizontally, cap sandstone hills near 

 the center of the basin. The contact of limestone and sandstone, 

 well exposed at many places, is plainly one of overthrust, and is 

 essentially parallel to the bedding both below and above. Immedi- 

 ately above the thrust surface a breccia has been developed, the 

 thickness ranging from a few feet to at least 75 feet. Most of the 

 fragments are of Hmestone, but near the base some pieces of sand- 



