The Cantabrian Mountains bound Leon and Old Castile towards the north-west and north, but broad mountain ranges run out from these immediately to the east of the Peña Labra, and form the water-shed between the basin of the Duero and the head-stream of the Ebro. These ranges are known by various names. They form first the Páramos of Lora (3,542 feet), which slope gently towards the south, but sink down abruptly to the Ebro, which flows here in a gorge many hundred feet in depth. The water-shed to the east of these continues to the mountain pass of the Brujula, across which leads the road (3,215 feet) connecting Burgos with the sea. Beyond this pass the so-called Montes of Oca gradually increase in height, and join the crystalline Sierra de Demanda, culminating in the Pico de San Lorenzo (7,554 feet). Another mountain mass lies farther to the south-east. It rises in the Pico de Urbion to a height of 7,367 feet, and gives birth to the river Duero. The water-shed farther on is formed by the Sierra Cebollera (7,039 feet), which subsides by degrees, its ramifications extending into the basins of the Ebro and Duero. The Sierra de la Moncayo (7,905 feet), a crystalline mountain mass similar to the San Lorenzo, but exceeding it in height, terminates this portion of the enceinte of the central plateau. The broad ranges beyond offer no obstacles to the construction of roads, but there are several rugged ridges to the south of the Cebollera and Moncayo, which force the Duero to take a devious course through the defile of Soria. Numantia, the heroic defence of which has since been imitated by many other towns of the peninsula, stood near that gorge.

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Fig. 139.—PROFILE OF THE RAILWAY FROM BAYONNE TO CADIZ.

(Altitudes in feet.)

The average height of the mountains separating the basin of the Duero from that of the Tajo is more than that of those in the north-east of Old Castile. The mountains gradually increase in height towards the west and south-west, until they form the famous Sierra de Guadarrama, the granitic rocks of which bound the horizon of Madrid in the north. It constitutes a veritable wall between the two {380} Castiles, and the construction of the roads which lead in zigzag over its passes of Somosierra (4,680 feet), Navacerrada (5,834 feet), and Guadarrama (5,030 feet) was attended with difficulties so considerable that Ferdinand VI., proud of the achievement, placed the statue of a lion upon one of the highest summits, and thus recorded that the “King had conquered the mountains.” This sierra forms a natural rampart to the north of the plains of Madrid, and many sanguinary battles have been fought to secure a passage through them. The railway to Madrid avoids them, but the depression of Ávila, through which it passes, is nevertheless more elevated than the summit of the Mont Cenis Railway.

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Fig. 140.—SIERRAS DE GREDOS AND DE GATA.

Scale 1 : 800,000.