The hills which rise like islands out of the alluvial plain are not all incapable of being crossed, and the roads do not always skirt them. The road from Buenos Aires to Peru runs north of 30 deg. 40' S. lat. on the very axis of the granite peneplain which forms the northern part of the Sierra de Cordoba. The Dean Funes ridge, which begins with an altitude of 2,500 feet between the Sierra Chica and these tablelands, has always been used for communication between Cordoba and the north-western provinces. There the railway has taken the place of the primitive track. Another important track crosses the Sierra de Cordoba in the north of the Pampa de Achala, and used to join Cordoba with Villa Dolores and the north of the San Luis province. The southern part of the Sierra de Cordoba and the Sierra de San Luis are, on the other hand, an insurmountable obstacle, which diverts southward the high road to Chile _via_ Achiras, San Jose, del Morro, and San Luis. The sierras of the Buenos Aires province are not so high and extensive. They are, moreover, broken into isolated hills with the plain passing between them. As early as 1822 Colonel Garcia pointed out the importance, in connection with the migrations of the Indian tribes, of the passage between the Sierra Amarilla and the Sierra de Curaco, that is to say, the Olavarria ridge. It is there that the first railway between Buenos Aires and Bahia Blanca crosses the line of sierras. It then skirts the Sierra de la Ventana, to the north, by the Pigue ridge, between the mass of Curumalan and the Puan hills. The dunes of the western Pampa also are an impediment to traffic, not so much because of their height as because of the looseness of the ground. The strip between General Acha and Toay was very trying for the stage-coaches. Travellers had to cross the dunes on foot during the winter season, when the horses were in a bad condition.[105]