a GEOMORPHOLOGY 119 
4 The narrowing is due to an intervening ridge of sandstones, the 
Cerro San Antonio, running north (see the map!). 
£ 4 
: 
| 
i 
The tectonical valleys of Tambillos-Guandacolinos. 
These valleys may be regarded as a tectonical continuation of 
i the Guandacol valley to the NW, separated from it by the named 
| * sandstone tidge of Cerro San Antonio. The Precordilleran border still 
- forms the western wall, here a more abrupt one. To the east rise 
the high slopes of the Umango mountains. On this side there are 
_ observed vertical gliding planes with tectonical striae. The valley 
of Tambillos is filled with sandstones of the continental series well 
exposed by the erosion. They are mostly dipping westward. The 
Guandacolinos valley more to the northwest lies higher than the 
Tambillos valley and thence the bottom sandstones here seem to 
have been worn away by erosion. Crystalline hills are therefore 
not unusual here. This last named valley cuts obliquely across the 
Umango-schist-structure. It ends where the Umango complex gains 
the Cordilleran border. 
The Jaguel bolson. 
It has already been stated, that the Umango structure is separa- 
ted to the north from the similar structure in the Catamarca moun- 
tains by the depression of the Jaguel bolson, where the old rocks 
are covered with a mighty formation of sandstones and recent allu- 
vions. The Jaguel bolson is a typical kettle-formed „graben“, sur- 
rounded on all sides by mountain walls, except to the east, where 
a low sandstone ridge rises, which forms the northern continuation 
of „the intermontane sandstone zone“. To the west and east the 
slopes of the walls consist of the continental sandstones, dipping 
inward. To the south there are no sandstones visible as flanc- 
cover, but the old Umango structure is here cut across, dissappea- 
ring rapidly beneath the young accumulations. The same peculiari- 
ties seem to be dominant also on the northern border of the 
bolson. 
