760 K. F. MATHER FRONT RANGES OF THE ANDES 



of the range throughout this portion of its extent. A few miles farther 

 southward, in the vicinity of Villamontes, the topography indicates that 

 displacement along the fault-plane is less and less, until near Rio Pilco- 

 mayo it dwindles practically to nothing. 



Detailed examinations of the structure of this range were also made 

 in the Quebrada de Los Monos. At this locality the Sierra de Aguarague 

 appears to be a simple anticline, little modified or entirely unmodified by 

 faulting. On the east flanks of this anticline the strata dip eastward at 

 angles between 40 degrees and 56 degrees. Near the crest of the fold, on 

 its west limb, the shales of the Los Monos formation dip toward the west 

 at angles between 50 degrees and 80 degrees. No certain indication of 

 faulting was observed at this locality, although it is entirely possible that 

 some breaking and slipping of the strata have taken place here. The 

 ridge on the west side of the Los Monos Valley, where it trends north- 

 ward along the crest of the anticline, is higher than that on the east side 

 of the same valley, and there may be a fault parallel to the longer axis 

 of the anticline between these two valley walls, in a similar position to 

 that in the gorge of Rio Vitiacua, 30 miles to the north. If so, the west 

 side is the upthrow side and the displacement could be only about two or 

 three hundred feet. 



Southward from Villamontes for many miles the trail swings far out 

 among the forest-clad knolls and low hills of the plain, so that it is im- 

 possible to get a good view of the mountains from it. Almost nothing is 

 known, therefore, concerning the appearance of the Sierra de Aguarague 

 west and northwest of Soto. West of Yahuacua the range displays an 

 uneven, but roughly horizontal crest, formed by westward-dipping sand- 

 stones which overlook a magnificent fault-scarp 1,500 feet in height. 

 Along the fault trace is a discontinuous depression parallel to the major 

 axis of the range. This is bordered on the east by hogbacks with strata 

 inclined steeply toward the east. The structural conditions in this part 

 of the sierra are therefore roughly similar to those in the vicinity of Rio 

 Vitiacua. 



Farther southward, between the latitude of Caiza and that of Yacuiva, 

 the Sierra de Aguarague is appreciably lower. Its slopes are heavily 

 wooded and there are no bold cliffs formed either by erosion along a 

 fault-plane or by the slopes of transverse canyons. Apparently in this 

 portion of its length the Aguarague fault is again becoming the dominant 

 structural feature. Its position is nearer the eastern front of the range 

 and the east-sloping cuestas dwindle to disappearance. South of Yacuiva, 

 in the Argentine, the eastern front of the range is almost everywhere a 

 fault-scarp varying between 1,500 and 1,000 feet in height. 



