130 ; SIERRA DE UMANGO 
Tambillos, separated from the broader Guandacol valley by the - 
sandstone ridge of the Cerros de San Antonio (the southern pro- — 
longation of the „intermontane sandstone zone “), has a very bro- 
ken bottom, due to a young erosion of old valley floors. This 
rejuvenation originated from the throughcutting by the Rio La 
Troya of the named sandstone ridge. Standing on the slope of the 
mountain, which rises to the east of the Tambillos depression, and 
looking westward, one obtains an excellent view of the now in- 
tricately dissected old valley floor, once flattened over the variega- 
ted series of filling sandstones. The erosion is a subsequent one, 
following the complicated fault tectonics of the strata. The nume- 
rous hills carved out in these sandstones show still on their flat- 
topped summits the remnants of the mentioned ancient valley floor. 
Upward the slopes the old surface passes into less dissected sheet 
flood cones. | 
The Rio La Troya (which is dry during the greater part of the 
year) „upstream“ irom the Tambillos depression leeds to the Guan- 
dacolinos depression, which separates the Cerro Cacho complex 
from the near situated Cordilleran border. ‘Here, as in the upper 
part of the) Vinchina valley, are no signs of young terrace cuts. — 
The valley floor is a flat sandy „medano“. 
4) Differences of the detailed sculpture in the 
crystalline rock-ground and in the terranes of 
| the continental fsediments. 
Results of the last erosion phase are visible also in the minute 
surface forms of the mountain slopes, which dissect the ground in 
a manner possible only in desert-regions. In the crystalline rock 
ground such an asperous surface has been formed as to make the 
forthcoming almost impossible. The only usable trails follow the 
gravel filled bottoms of the larger quebradas. The numberless sharp 
ridges have been called in Spanish ,,cuchillas« (= choppers). Above 
