172 THE PLAIN OF THE PAMPAS 



but along broad and almost imperceptible depressions 

 (canadas) where the current is slow, and the water 

 dries up in the dry season. Their general relations are 

 not yet known. 



The loose deposits of the Pampean offer little resis- 

 tance to erosion. The cycles are run through rapidly, 

 and the traces of earlier cycles are faint, and are soon 

 effaced. I 



An ancient erosion-surface, dissected by the existing 

 valleys, has survived in the south-west of the Pampean 

 plain, thanks to the presence on the surface of a sheet 

 of hard limestone, the tosca. The tosca is the result 

 of the concentration of calcareous elements contained 

 in clay at the surface in a dry climate. The formation 

 of it implies a prolonged stability of the surface on 

 which it has accumulated. Like the deep decom- 

 position-soils in moister regions, it indicates a peneplain 

 on which erosion has ceased. The bed of tosca covers 

 the whole district between the Sierra de Tandil and 

 the Sierra de la Ventana, the south-western slope of 

 the Ventana, and most of the area of the central Pampa. 

 In the north it does not go beyond the line from Buenos 

 Aires to San Rafael. Its eastern limit goes almost by 

 Ingeniero Malmen, Monte Nievas, and Atreuco, where 

 it joins the southern bank of the lagoons of Carhue 

 and Guamini in the east.* In some places the tosca is 

 about forty feet thick. 



To-day the region of the tosca forms a plateau cut by 

 narrow valleys, sometimes 200 feet deep, west of the 

 Sierra de la Ventana and in the central Pampa. These 



' Certain features of the hydrographic network clearly have the 

 character of having been superimposed : that is to say, the path of 

 the watercourses has been bequeathed to the actual plain by former 

 erosion-surfaces, which have now disappeared, on which the valleys 

 were originally imposed. That is why in the district of the confluence 

 of the Colorado and the Chadi-Leuvu the valleys pass from Pampean 

 deposits to the crystalline sierras, which were at one time entirely 

 covered \\ith water. 



» In the vicinity of San Luis and Cordoba the hard strata which 

 are called tosca are beds of eruptive ashes. 



