120 SIERRA DE UMANGO 
The sinking down of this bolson evolved before the high seated — 
conglomerate in the Sierra de Umango ridge was deposited, because 
this conglomerate lies unconformably over the tilted strata of the © 
eastern border of the bolson (or exactly, over their southern conti- 
nuation). The strata were naturally tilted up together with the for- 
mation of the bolson. | 
The above named conglomerate, situated on the ridge of the 
Sierra de Umango, is, as has been stated, preserved in a smaller 
downfaulted area, which seems to be of a later origin than the 
valley-iorming dislocations. | 
The nearly smooth ground, which forms a kind of highland plain 
between the Cerro Cacho and the Sierra de Umango ridge, may 
be considered as a relatively sunken part of ihe older tertiary le- 
velled surface. No remnants of the sediment cover are preserved 
in this area, and that depends naturally on the relatively high po- 
sition of the plain, which slopes rapidly down to the north and 
south.‘ 
The crystalline horst-blocks of the Umango area have in many 
respects the same character as the pampean sierras, formed as they 
are by similar dislocations. But the conspicuous phenomenon of 
tilted blocks, as generally developed in ihe last named sierras,! is 
hardly seen in the Umango area. The Famatina mountain blocks 
seem, however, to follow this rule, according to which the western 
borders of the mountains have been more uplifted. Also the Cerro 
Villa Union has a tendency to a similar behaviour, although worse 
developed, while an upland plain is but little preserved. The western 
slope is, however, higher and steeper than the eastern. Tilting does 
not occur in the principal part of the Umango mountains. They 
form a part of the Cordilleran late tertiary uplift zone, and in this 
respect the area lies aside from the pampean sierras-region, while 
the Nevado de Famatina may already belong to the last named. 
1 Juan Rassmuss: Rasgos geolögicos generales de las sierras pampeanas. — 
(1916). 
