118 SIERRA DE UMANGO 
„euesta“ facing west. This dislocation was mentioned by Boden 
bender (1911). To the west the lowland is bordered by the up- — 
bent edges of the same strata, here forming the ridges of Lomas 
Coloradas (Cerro Colorado) and Cerro Bola. In the bottom of the 
lowland the strata seem to lie almost horisontally. 
Simultaneously with the sinking down of the Pagancillo lowland 
TLS ee 
al oe 
many disturbances occurred: between the Cerro Villa Union and the j 
Cerro Bola, causing the marked downfold of the strata now sepa- - 
rating the two mountains. The axis of this synclinale runs NE. — 
But while the strata on the eastern side of the Cerro Bola dip to 
the east, a great bend of the strata may be observed when regar- 
ding the mountain from the northern side. 
The Guandacol valley. 
The most extensive lowland in these regions, the valley of Guan- 
dacol or of the Rio Bermejo with its continuation far to the south, 
is the dividing line between the Precordilleran chain and the Cerro 
Villa Union — Cerro Bola. I have studied only the eastern wall 
of the valley, as the western wall was mapped by Rassmuss. It 
is interesting to see, that the great fault line, which forms the 
eastern wall, is younger than the dislocations of the Cerro Villa 
Union on its southeastern and eastern sides. In the southern edge of 
the last named mountain the SE-ward tilted strata of the Gondwana 
series are vertically cut across to the west by the above named 
great fault line. The downfold between the Cerro Villa Union 
and the Cerro Bola is consequently older than the Guandacol 
valley. | 
The young fault line along the eastern wall of the Guandacol valley 
was already described by Bodenbender (1894). It is a very | 
long one forming the western border of the Cerro Villa Union, the 
Cerro Bola, the Cerro Rajado and the Sierra del Valle Fertil more 
to the south [see the map by Bodenbender (1911)]. Toward the 
north the valley is rapidly getting narrower and the bottom rises. 
