This block of chains and plateaus forms the central watershed of Spain, which throws westward the sources of the Douro, Tagus, Guadiana, and Guadalquivir, and eastward those of the Xucar and Segura. The basins of these streams and their tributaries form three-fourths of the Iberian Peninsula. The rest consists mainly of the great valley of the Ebro: this hardly falls into the system, and is somewhat exceptional. It has been described as serving as a sort of wet-ditch to the main fortification of the peninsula. Starting in the western extension of the Pyrenees, quite close to the Bay of Biscay, it runs diagonally across Spain, more or less parallel to the Pyrenees, and falls into the Mediterranean between Catalonia and Valencia. It is more low-lying than the rest of the main valleys of Spain, is broader, and is not so much cramped and cut up by mountains running down to it at right angles to its course. Behind the Ebro lie, chain after chain, the parallel sierras which mark off the divisions of the great central plateau of Spain. Arteche compares them to the waves of a great petrified sea, running some higher and some lower, but all washing up into jagged crests, with deep troughs between them. The first and most northerly of these waves is that which we may call the range of Old Castile, which separates the basin of the Ebro from that of the Douro. At one end it links itself to the Pyrenean chain in the neighbourhood of Santander: at the other it curves round to join the more central sierras in the direction of Soria and Calatayud. It is the lowest of the chains which bound the central plateau of Spain, and is pierced by three practicable roads, of which the most important is that from Vittoria to Burgos.