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GEOMORPHOLOGY 129 
in many parts well developed river terraces of quite recent age. 
These terraces are not of an accumulative, but of a destructive kind, 
carved out in the sandstone terranes, the substratum of the lowlands. 
The most perfect terraces are met with along the lower course of 
| Rio Bermejo, between the Villa Union and the Paso de Lamar, that 
q 
is, in the western part of the Pagancillo lowland. Excellent terraces 
occur also in the named Paso. It is obvious, that all these terraces 
have been formed by a successive throughcutting of the sandstone 
ridge of Paso de Lamar, due to a gradual tectonical sinking of the 
Guandacol valley floor. It may also be assumed, that the breaking 
off of the barrier of the Paso de Lamar has adjusted the whole 
drainage of the Pagancillo lowland and the connecting Vinchina 
valley during the last erosion phase. The Lomas of the Villa Union 
have thence been carved out by this adjustment, as later on the 
very river terraces along the edge of the flood plain of the Rio 
Bermejo. 
Upward from the Villa Union no more terraces are visible except 
in its near vicinity to the northeast. The upper part of the Vin- 
china valley is, as far as I have seen, only an even sandy floor (so 
called ,medano“), and the sheet flood cones on both sides are 
not dissected. This part of the valley lies yet aside from the in- 
fluence of the Paso-de-Lamar-adjustment. 
In the Bolson of Jaguel the results of the latest erosion are de- 
veloped only to a very small degree. Almost the whole basin is 
an flat accumulation plain. The Jaguel river, crossing the plain 
from NW, has no marked valley, except where it approaches the 
sandstone ridge to the east, the outlet-canyon down to the 
Vinchina valley. Here a backward erosion carves out the alluvial 
accumulations of the plain, and some terraces have been formed. 
The erosion tends, however, to extend its work over the whole plain 
until it would be paralyzed by alluvial depositions from the sur- 
rounding slopes. 
The large tectonical depression of Zapallar — Nacimientos — 
