124 SIERRA DE UMANGO 
During the first phase of erosion, a mighty cover of coarse cong- 
lomerate (the late tertiary) was really destroyed to an extensive 
degree which is indicated here and there by visible remnants of 
quaternary shingle, composed of the same boulders as the young 
tertiary conglomerate, thus representing a re-deposition of tertiary- — 
material. These quaternary shingle deposits were afterward destroyed 
by subrecent erosion to a considerable degree. They occur at loca- 
lities, where there are signs of dislocation of later date. 
Such a remnant of quaternary shingle, afterward dislocated, is 
visible at the southern end of the Cerro Potrero Viejo, at the eastern 
border of the ,intermontane sandstone zone“. A recent river gorge 
2) 
> 
x G A 
Fig. 21. An E—W profile from the vicinity of Altillo, E border of the „intermontane zone", 
showing tertiary and diluvial dislocations. CD=diluvial conglomerate. X = micaschist. G= 
Gondwana sandstone. A=red sandstone. C=tertiary conglomerate. A= red sandstone 
(cretaceous ?) 
is carved out through the gravel and also through the older strata 
to the west, against which the gravel has been dislocated (see 
the profile, fig. 21). The gravel being distinctly bedded with 
much sandy material has been laid into a flexure. Upward the 
profile is limited by a gentle eastward dipping surface of a young 
sheetflood. The quaternary dislocation had here a direction parallel 
to the tertiary of the intermontane zone, as the profile figure shows. 
Another remnant of diluvial gravels, to some higher degree har- 
dened to conglomerate, was found in the valley bottom of the Rio 
La Troja, at the western foot of the.Cerro Villa Union. The gravels 
form here a mighty vertical wall facing the eastern margin of the 
river, but slope gently to the east, toward the recent mountain 
fans. In the river cuts the gravel shows almost horizontal layers. 
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