28 BUREAU OF AMERICAN ETHNOLOGY [bull. 52 



is distinctly a drifted formation and occurs with the irregularity 

 which characterizes drifts. 



It is doubtful whether we may safely speak of an Upper Pampean 

 epoch in the sense of a definite division of geologic time. We have 

 seen that eolian loess occurs as a characteristic constituent of the 

 older Pampean terranes. The material for the formation of eolian 

 drifts has been available and winds to erode and redeposit it have 

 been active in later epochs also. From the time when the earliest 

 alluviums of the Pampean terrane were formed to the present, 

 deposits possessing the physical characters of the Upper Pampean 

 have been developed. Thus it seems impracticable to distinguish an 

 Upper Pampean formation on physical characters alone. There is 

 paleontologic evidence, but it rests primarily on the physical, for the 

 biologic lines of descent can not be established except by observation 

 of the stratigraphic sequence. Those fossils which have been found 

 in the superficial eolian loess have been assigned to the Upper Pam- 

 pean, because they occurred in a position above the older Pampean 

 and in material having Upper Pampean characteristics. In these 

 criteria, however, there is nothing by which to distinguish the oldest 

 Upper Pampean loess from the most recent, and it is not impossible 

 that the range of time represented by such loess deposits corresponds 

 with a large part or all of the Quaternary period. 



While the writer is thus in doubt as to the stratigraphic and chron- 

 ologic value to be given the term Upper Pampean, there are cer- 

 tain relations which serve to set an earliest date before which the 

 Upper Pampean did not develop in the superficial position in 

 which it is now recognized. These limiting relations are physio- 

 graphic and climatic. As will be seen by referring to the description 

 of the Arroyo de Ramallo, to that of Mar del Plata, and to other 

 occurrences of the Upper Pampean, the Upper Pampean deposits 

 occupy peculiar positions in the eroded surface of the older forma- 

 tions. In so far as this may be generally true they could not have 

 been deposited until after the surface had been eroded, and the ero- 

 sion could not have taken place until the older formations had been 

 elevated above base level. There is thus a recognizable effect of 

 deformation which intervenes between the Upper Pampean and any 

 older formation. 



The Upper Pampean did not develop, however, immediately after 

 the elevation of the region. The relations which may be seen between 

 Buenos Aires and Rosario show that shallow valleys were formed by 

 small confluent streams that grew out of the Parana and that they 

 afforded the appropriate locus for deposition of the Upper Pampean. 

 (Plate 4.) Valley erosion by these local streamlets does not seem 

 consistent with simultaneous valley-filling by winds. The two seem 



