WILLIS] GENERAL GEOLOGIC NOTES 31 



of the Parana and the ahuvial islands on the farther side, and it is 

 apparent that the great river is corrading — literally cutting away — the 

 bank faster than talus or gullies can develop. But though the river is 

 a powerful agent of erosion it has not accomplished a great deal in 

 widening its flood plain at this favorable point. The work is only 

 begun. 



In describing the warped surface of the old continent it was stated 

 that the Parana and the Rio de la Plata occupied a downwarp. 

 Southwest of their depression the Pampean plain rises very gently in 

 an upwarp to an altitude of between 30 and 40 meters above sea. 

 The highest part of the plam lies in a Ime which trends from northwest 

 to southeast, some 50 kilometers southwest of Buenos Aires. About 

 50 kilometers still farther southwest there is another depression whose 

 axis is approximately parallel to that of the Rio de la Plata and which 

 is occupied by one of the several salt rivers (Rio Salado) of the coun- 

 try. This depression constitutes the eastern portion of the Province 

 of Buenos Aires, in latitude 36°, and is an area in which extensive 

 drainage canals have been thought necessary. The general elevation 

 of the plain rises from near sea level to 25 meters in the valley of the 

 Salado, but there is no perceptible slope and the extraordinary flat- 

 ness of the surface is such that during the rainy seasons of wet years 

 rain water has stood over many square kilometers where during drier 

 years there spread the grassy plain. The channel of the Salado, 

 meandering through this flat, is shown in the photograph (pi. 25). 

 It will be noticed that the river is in a peculiar state of equilibrium. 

 It is not deeply corrading, neither is it aggrading the channel. It 

 has sufficient fall to carry away the silt which it brings and therefore 

 does not build up its banks above the neighboring plain, as is the habit 

 of rivers in their deltas, nor does it appear to be obstructed. Yet its 

 force is not sufficient to excavate its channel to a greater depth below 

 the surface than is required to carry its waters. The river may be 

 said to flow practically at base level. The writer observed these 

 relations in the vicinity of the stations called Villanueva and General 

 Belgrano, and noted them as an illustration of a surface which, 

 although elevated considerably above sea level, and traversed by 

 streams, does not exhibit any of the features sculptured by running 

 water. This condition may be attributed to the fact that confluent 

 streams have not yet developed on the gentle slopes that descend 

 from the northeast and from the southwest toward the Rio Salado. 

 Due allowance must be made for the effects of wind erosion, which 

 has produced very broad and shallow hollows in the plain, in which 

 the rain waters gather and evaporate instead of running off. Never- 

 theless, the period of time is short during which such a surface, when 



