128 | SIERRA DE UMANGO 
of erosion was thence rejuvenating by way of linear cutting in the 
sheet flood planes and by! a deepening of the corresponding valley 
profiles. Some of the cones are to such a degree dissected by the 
young erosion, as to be changed into a „bad land“ sculpture (north- 
ern end of the Cerro La Pampa). Many curious hilly forms have 
been shaped where the sheet flood gravel has endurated to a hard 
bed, mostly like mesetas and separated by steep canyons. The 
Lomas of Villa Union, as mentioned, remnants of a great cone- — 
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Fig. 23. A wiew of the eastern side of the Sierra de Umango ridge, looking north, along the „inter- 
montane zone“. The pic ure demonsrates the formation of piedmont (gravel covered slopes) cut 
across by young erosion. The arrow indicates the overthrust plane between the mountain block and 
the sediment zone. The stream bed of the Quebrada Cordobés in the foreground. 
surface extending from the Nevado toward the Pagancillo low- 
land, have all a table shape, due to the hardened gravel sheet 
very resistant to the erosion. | 
The young erosion has also worked laterally in more advanced 
stages and thereby reduced the cones into flattopped ridges radia- 
ting from the apex of the former cone (see the terraces NE from 
the Villa Union.) 
In the valley bottoms, except in the Guandacol valley, there are 
