The only difficulty which the caravans encountered on the roads over the plain was the crossing of the rivers. They were forded. Fords with a muddy bottom on the lower course of the rivers, such as that on the Saladillo near the confluence of the Rio Tercero, were more difficult for wagons than the fords with sandy bottoms in the upper course, near the fringe of the mountains, such as those of the Rio Tercero on the Cordoba road, or of the Rio Cuarto on the road to Chile. After rain, certain parts of the plain are flooded and impassable. That is the case in the district to the south of the lower Salado, at the very spot where Pere Cardiel notices the lack of water in the dry season (1747). The direct road from Buenos Aires to the sierras was at that point exposed, alternately, to drought and flood. The line of the Southern railway, which crosses this low district, is still cut periodically on both sides of Las Flores by floods. The lack of an organized network of streams, the irregularity of the rains, the difficulty of ascertaining the inclination, and the flow of the waters over a plain which seems to the eye to be perfectly level, have led to more than one miscalculation on the part of the railways, which were constructed hurriedly, and before the general survey of the Pampa was finished. Some lines, on the Pampa or on the Chaco, have had to be partially reconstructed, and raised higher, after a series of rainy years.[106] [106] Certain duplications in the actual scheme of the railways are due to this need to correct a line that had been planned hastily and was useless. The line from Justo Daract to La Paz (1912), on the Pacific railway, avoids the steep inclinations of the first line, which followed the course of the wagon-road _via_ San Luis. The interpretation of the relief is particularly difficult in a country which has not been shaped by normal erosion. Blunders detected by later topographical inquiries were similarly committed in constructing the Patagonian railways. The colonization of that part of the plain which actually constitutes the province of Buenos Aires was late. It belongs to the era of the railways. There is only one historic road crossing this area, which remained until the last third of the nineteenth century in the hands of the Indian tribes. This is the salt road. We do not know exactly when it began to be used. In the eighteenth century, in spite of the competition of salt from Cadiz and Patagonia, imported by sea, the Pampa salt was the main part of the supply of Buenos Aires. The salt road was not abandoned until after 1810. We still have the diary of several journeys from Buenos Aires to the salt-pits. They were military expeditions. Hundreds of wagons, with a strong escort, collected at Lujan and Chivilcoy, and they reached Atreuco, west of the Guamini and Carbue lakes, after a fifteen to twenty-five days' march.