ALLUVIAL VALLEYS 171 



flow between high cUffs, the height diminishing as one 

 goes downward. Presently these barrancas become low, 

 approach each other, and at last merely mark the banks 

 of a larger bed which the floods fill. There is no trace 

 of valleys. Bailey Willis, surprised at this weakness 

 of watercourses that have, nevertheless, an appreciable 

 fall, attributes it to the fact that the cycle of erosion 

 opened by the last upheaval of the Pampa has not yet 

 had time to penetrate into the interior. In reality, 

 it means that here we are at the limit of the zone of 

 erosion by running water, and that in this climate the 

 essential factor in shaping the landscape is the wind. 



The region of the right bank of the Parana (east of 

 the Salado), which alone has a complete hydrographic 

 network, must be considered apart. From the latitude 

 of Rosario to that of Buenos Aires it is cut by flat- 

 bottomed valleys which are sometimes a hundred feet 

 deep. The excavation of these valleys is due to an 

 upheaval which raised this part of the Pampa above 

 the base-level. The rapids of the lower Carcarafia 

 also bear witness to this resumption of excavation. 

 Farther on an inverse movement has put the bottom 

 of the valleys below this level, and led to their being 

 filled up (lagoon deposits of the Lujanense of Ameghino). 

 South of Buenos Aires the upheaval has been less import- 

 ant, and the valleys are not so deep. Some of them 

 (middle Salado and its tributaries on the left bank) are 

 now occupied by long lagoons with steep banks, branch- 

 ing along the side-valleys, and these owe their origin 

 to the same negative movement, subsequent to the ex- 

 cavation of the valleys. The upheaval did not extend 

 to the eastern part of the province of Buenos Aires 

 south of the Salado, a low-lying flat area, badly drained, 

 exposed to floods, the contour of which has been minutely 

 studied in connection with the construction of a great 

 network of drainage-canals. North of Rosario, on 

 the only slightly permeable clay, the water circulates, 

 after rain, not by means of valleys in the proper sense. 



