| GEOMORPHOLOGY FE KN DA! 
Id) THE POSTPLIOCENE EROSION AND ACCUMULATION. 
General conditions of erosion. 
With the great late tertiary uplift-movements a new geogıaphical 
cycle began in the andean regions. From the present morphologi- 
cal forms in the Umango area may be concluded, that this cycle 
_ must still be in a little advanced stage. The elevations are to their 
extension still determined by the fault lines. This statement is app- 
licable even to the most intensely faulted grounds, where every hill 
is a little horst. The youthful stage of the erosion is also manifes- 
ted by the deep and narrow profiles of the valleys, the rapidly de- 
scending erosion-curves on the slopes and also by the still pre- 
served relics of upland plains. 
The general topographical situation of the Sierra de Umango 
area in the interior part of the Republic may be characterized as 
follows: The area lies aside from the pampean plain and in a high 
position with respect to the local baselevel. The mountairs form, 
as said, really only a part of the Cordilleran bulk, except some 
more isolated parts (Cerro Villa Union, a. 0.). 
Examining indeed the erosion-forms of the Umango area, be- 
longing no doubt to the physiographical province of the pam- 
pean sierras with their very rough and dissected surface, one reveals 
the dependency of the erosion in the last named sierras as well as 
in the Umango mountains upon the possibility to attack ihe more 
or less isolated blocks from all sides, and upon the existing great 
level differences. Westward again in the Cordillera the vast high- 
land-area misses deep dissected valleys, and thence the linear ero- 
sion has but slightly progressed. The Cordilleran upland surface 
is still preserved over wide areas. To these belongs the part lying 
immediately to the west off the Umango area and contrasting with 
the later in a very conspicuous manner. 
The principal outlet-course for the whole drainage in the Umango 
area, leading down to the pampean plain, is represented by the 
