GEOLOGIC STRUCTURE 753 



tremely broad, shallow synclinal trough comparable to the Big Horn 

 Basin. 



FLORIDA FAULT 



. As observed from the trail passing through Palissa and Florida, the 

 bold front of the Sierra de Florida appears to be a fault-scarp only 

 slightly modified by erosion since movement took place along the fault- 

 plane. The west side of the fault-plane is, of course, the upthrow side, 

 and the displacement must amount to at least six or eight thousand feet. 

 The Bermejo-Tacuru beds composing the sierra strike in a general north- 

 south direction parallel to the trend of the fault-plane and display gentle 

 dips toward the west away from the fault. Their dip, however, is not 

 uniform, and many minor undulations both in degree and in direction 

 are noted. One important anticlinal axis is roughly defined by the north- 

 west dip of the strata in the hills immediately south of Mount Carobana 

 and the southwest dip of the same beds in Mount Cuchilla and the hills 

 south of it. The attitude of the beds adjacent to the fault on the down- 

 throw side was not observed, nor is the hade of the fault-plane known. 

 By analogy with the conditions observed far to the south, it may, how- 

 ever, be safely inferred that the Florida fault pitches steeply toward the 

 west, and that the beds on its east side are dragged upward into almost 

 vertical positions. 



LIMON FAULT AND ANTICLINE 



Although the general structure of the regions occupied by the Sierra 

 de Limon, the piedmont lowland immediately to the east, and the Limon 

 Valley is comparatively simple, and therefore easily determined, the de- 

 tails of structure are in places somewhat obscure and were not determined 

 with accuracy. The steep escarpment along the east front of the Sierra 

 de Limon is a fault-scarp. The fault is in all probability a thrust along 

 a steeply inclined plane near the crest of a slightly overturned anticlinal 

 fold. Displacement is measured in thousands of feet; the west side is 

 the upthrow side. The mountains are in the main anticlinal ; the Limon 

 Valley is synclinal. 



Three or four miles northwest of Abapo the exposed beds of Tatarenda 

 shale strike north 42 degrees east and dip 80 degrees toward the south- 

 east. These are evidently on the eastern limb of the faulted anticline. 

 Three or four miles farther westward the beds in the upthrow block 

 which form the mountains west of the fault-scarp appear to dip gently 

 westward. 



In the region, 6 to 8 miles south of Rio Grande, along the trail from 



