18 BUREAU OF AMEEICAN ETHNOLOGY [bull. 52 



Pampean formation or terrane. They are no doubt thickest where 

 the old rock floor lies deepest — in the downwarps of the Rio de la 

 Plata and at Bahia Blanca. 



These two facts — that there is a continental surface which was 

 eroded on the ancient crystalline rocks, and that the surface, being 

 warped, became generally covered with the Pampean formation — 

 are the fundamental facts of the later geologic history of the pampas. 

 To these we may add the note that the region is now elevated and 

 subject to erosion. 



PAMPEAN TERRANE 



That portion of the geologic history which concerns the discussion 

 of the antiquity of man relates to the Pampean terrane. Wlience 

 were the materials derived ? How were they deposited ? What dis- 

 tinct episodes of the long process may be recognized? To what 

 epochs of geologic time do these episodes correspond ? In what con- 

 nection do they stand with man? 



It has been said that the Pampean was deposited most abundantly 

 in the deep downwarps which are now the embayments of the Rio 

 de la Plata and the Bahia Blanca. In each of these there developed 

 a system of rivers, whose modern representatives are the Parana and 

 Uruguay in the one embayment, in the other the Rio Colorado and 

 and its long northern tributary, the Gran Salado (Rio Curaco of 

 some maps) . These rivers, or their predecessors, brought, distributed, 

 and laid down the muds which were gathered in the process of denuda- 

 tion of upper watersheds and which consisted of the characteristic 

 soils of the several headwater regions. In the one case that w^as 

 central and western South America. The soils were heavy clays, 

 containing much iron and of various shades of brown to deep 

 brownish-red. Much of the Pampean terrane that lies north of 

 the Sierra de la Ventana, was derived from that region and has that 

 general character. The Colorado River system, or its ancestors, on 

 the otlier hand, flowed from the Andes and brought down sands 

 in large volume, as well as clays, producing light-colored, sandy 

 varieties of the Pampean terrane, which are sometimes so unlike 

 the brown clays that they are not called Pampean, but are de- 

 scribed as Tertiary sandstones. Within the area of the Province 

 of Buenos Aires there rose, moreover, the heights of the Sierra Tandil 

 and de la Ventana, which were eroded by rains and by winds, and 

 which contributed more or less sand, together with clays, to the 

 deposits laid down near the hills or in the valleys of streams which 

 may have flowed from them. 



Thus the sources of the Pampean earths were very unlike. The 

 accumulation of so great a mass required a long time, during which 

 conditions changed. Nevertheless, the Pampean terrane is on the 

 whole a remarkably uniform and monotonous deposit, several agen- 



