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 Carcarana 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 important, 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, branching along the side-valleys, and these owe their origin to the same negative movement, subsequent to the excavation 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, 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 resistance to erosion. The cycles are run through rapidly, and the traces of earlier cycles are faint, and are soon effaced.[83] [83] 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 with water.