TERTIARY TECTONICS 111 
the basement. The contact with the latter is partly visible and shows 
a mylonitic mass, a mixture of sandstone and slate fragments and also 
a schist-breccia. A similar thrusting dislocation may be observed 
on a tilted basement (the overthrust plane rises towards the east) 
at the northern end of the ,intermontane" section, at the northern 
head of the eastern mountain block, here called Cerro La Pampa 
(see the figure 19). The numerous fault lines, crossing the sand- 
stone zone in longitudinal directions, have caused many irregulari- 
ties in the succession and thickness of the strata. 
un 100 m 
Fig. 17. A West — East profile through the tertiary dislocation zone (the Pirca zone) along the western 
foot of the Sierra de Umango ridge. Arrow I: di ection of the overthrust of old crystalline rocks over 
red sandstones, folded by this movement. Arrow II: a later downfaulting of the late tertiary cong- 
lomerate. A=red sandstone, X = old crystalline rocks, C =late tertiary conglomerate. 
Another, but smaller zone containing folded sandstones occurs, 
as have been mentioned, at the western foot of the Sierra de Umango 
ridge and has been called the ,Pirca zone". The red sandstone 
beds at Pirca are laid into relatively gentle folds, overthrust from 
the west by a faultblock of crystalline rocks, strongly crushed in 
the contact plane (see the profile 17). The amount of overthrust 
was, bowever, quite considerable, reminding of the ,lambeaux de 
charriage“ in the Alpes on a smaller scale. Blocks of old, crystal- 
line rocks lie on the red sandstone beds (see the profile fig. 18). 
Also in some other parts of the Umango area we find signs of 
overthrusting and folding from the west. 
From the morphological circumstances which will be more closely 
exposed, may also be seen, that these movements really belong to 
