GEOMORPHOLOGY 123 
å number of independent centripetal drainage systems with heavy 
accumulations at the base. In the Umango area the different base- 
‚levels communicate by small streams cutting across the dividing 
ridges, but their scarcity of water prohibites any sudden levelling 
between the different bases. 
According to a paper recently issued by Keidel! the mountains 
in the northwestern parts of the Argentine Republic got their prin- 
cipal relief already in preglacial times. In the formerly glaciated 
areas, it may be observed fhat the mantle of glacial deposits covers 
all the main forms. The piedmont-stream-deposits are consequently 
chiefly of preglacial age. The preglacial climate was dry as the 
present. 
In the Umango area glacial deposits are completely lacking and 
‚cannot furnish any information in this respect. It is, however, quite 
certain, that the principal drainage lines here, viz the main erosion 
lines are of old age. This is evident on regarding the giant gravel 
cones radiating from the mouth of every such valley. The cones 
are generally uplifted and dissected by younger erosion (cp. fig. 23). 
1) A first erosion phase. 
The late tertiary tectonical relief was in its initial stage probably 
built up as follows: all the great blocks were composed of the 
basement rocks and flanked by the continental mantle of sediments, 
which generally dips outward from the block cores. At an earlier time 
this mantle probably covered also the very summit of the blocks. 
The young tertiary conglomerate capped probably all the summits. 
The present strong relief was brought out chiefly by the carving 
out of the hard crystalline blocks as result of the destruction of the 
continental strata in the depressions. 
1 Hans Keidel: Ueber den Anteil der quartären Klimaschwankungen an der 
Gestaltung der Gebirgsoberfläche in dem Trockengebiete der mittleren und nörd- 
lichen argentinischen Anden. Compte Rendu. XII. Session. Congr. Géol. Intern. 
Canada 1913. Ottawa 1914. Pages 757—769. ‘ 
