GEOLOGIC HISTORY 761 



West of Aguaray the crest of the Sierra de Aguarague is only about a 

 thousand feet above the bordering lowlands. Its eastern front rises boldly 

 in a steep fault-scarp which overlooks the plain. At this place the fault- 

 plane has a hade of about 25 degrees and pitches toward the west. The 

 strata on its upthrow side dip about 10 degrees toward the west and 

 strike north 25 degrees east, approximately parallel to the trend of the 

 mountain range and the strike of the fault-plane. From this point south- 

 ward the structure of the Sierra de Aguarague is better described as a 

 tilted fault-block than as a faulted anticline, so inconspicuous is the fold- 

 ing in comparison with the faulting. Ten miles south of Tartagal the 

 displacement along the Aguarague fault decreases rapidly and a mile or 

 two farther southward it becomes so slight that none of the resistant beds 

 of the Tacuru formation are lifted by it above the level of the plain. 



Geologic History 

 devonian period 



The first recorded event in the geologic history of this region dates 

 back to the Devonian period. At that time the entire area seems to have 

 been submerged beneath the waters of an epicontinental sea, the outlines 

 of which may have been much as indicated on the recently published 

 paleogeographic map of Dr. Bonarelli (1921, plate 7). In the region 

 now occupied by the Sierras de Santa Cruz the calcareous shales and 

 bituminous limestones of the Espejos formation were accumulated in this 

 sea. Farther to the southward the clastic sediments were somewhat 

 coarser and formed the carbonaceous shales and intervening sandy strata 

 of the Los Monos formation. The locality of the Sierra de Mandiyuti 

 seems to have been nearer the shores of this ancient sea than were the 

 areas now occupied by the Sierra de Aguarague ; in any event, the equiva- 

 lent beds in the former place contain a much greater percentage of sand. 



The total thickness of these Devonian sediments is not known, but ap- 

 parently sedimentation was long continued. That epoch seems to have 

 been terminated late in Devonian time by an uplift which changed this 

 portion of the Andean geosyncline to an area of erosion. At the north, 

 between Cochabamba and Santa Cruz, this late Devonian diastrophism 

 crumpled and flexed the newly formed strata into a great series of anti- 

 clines and synclines. Toward the south, in the area now occupied by the 

 Front Ranges described herewith, the movement seems to have been a 

 simple uplift. 



L — Bull. Geol. Soc. Am., Vol. 33, 1921 



