WILLI si GENERAL GEOLOGIC NOTES 39 



1. Beneath the Pampean is an ancient hind surface which was 

 formerly a plain or nearly so, it having been long subjected to ero- 

 sion. The plain subsided and became covered with the alluvium 

 of rivers which brought silt from the north and especially from the 

 west. The rate of subsidence and the rate of sedimentation were 

 related to one another somewhat as they now are in the Mississippi 

 embayment, so that the surface of the river deposits remained 

 above sea level although the mass was sinking. Marine deposits, 

 therefore, do not enter the area. 



2. The alluvial deposits constitute what is now called tlu^ Pam- 

 pean terrane. They resemble the deposits which are made by the 

 Hwang River in the delta plains of eastern China, and which consist 

 of eolian loess redistributed by river action. The loess of China 

 originates in the deserts of central Asia, and by inference the Pampean 

 earths originated under similar arid conditions. The lands to the 

 east of the Andes now have an arid climate because the high moun- 

 tains dr}^ out the western winds. If this liypothesis of the origin 

 of the Pam2)ean earths be valid there is a relation between the Pam- 

 pean and the Cordillera, since the elevation of the Cordillera was 

 recjuired to establish the genetic conditions for the ])roduction of the 

 loess that was subsequently deposited as the alluvium of the Pampean 

 terrane. By this hypothesis it seems probable that most of the 

 Pampean is of late Tertiary age, but there is nothing in the process 

 of deposition which precludes Quaternary age for the later part of 

 the formation. 



3. During the deposition of the Pampean terrane eolian processes 

 involving erosion and redeposition were certainly active. They 

 appear to have been particularly active at certain epochs which 

 alternated with others of dominant fluviatile action. This alterna- 

 tion may have been local and have occurred in different places at 

 different times, but the phenomena as they are displayed in the 

 sections on the eastern coast of Argentina suggest that there were 

 one or more epochs when wind erosion prevailed over a wide area 

 and was favored by some special climatic condition, such as aridity. 

 The deposition of secondary limestone appears also to indicate 

 climatic variation, and the two lines of evidence — wind erosion and 

 limestone deposition — appear to coincide in their indications. Hence 

 it is inferred that Pampean history has been varied by climatic 

 change, and it is suggested that the ultimate subdivision of the 

 Pampean terrane into distinct formations will be based on climatic 

 variation. The writer has not formed any opinion as to the rela- 

 tions which might exist between such a classification and any of 

 thos^B which may have been published. It would seem probable, 

 however, that no attempt to classify the Pampean terrane, a mass 

 of lenticular alluvial deposits produced by several rivers, according 



