The mountains to the south-west of the Peak of Peñalara (7,870 feet), which is the culminating point of the sierra, sink down rapidly, and at the Alto de la Cierva (6,027 feet) the chain divides into two branches, of which the northern forms the water-shed between the Duero and the Tajo, whilst the more elevated southern chain joins the Sierra de Guadarrama to the Sierra de Gredos, but is cut in two by the defile excavated by the river Alberche, which rises to the north of it.
The Sierra de Gredos is, next to the Pyrenees and the Sierra Nevada of Granada, the most elevated mountain chain of Spain, for in the Plaza del Moro Almanzor it attains a height of 8,680 feet, and thus reaches far beyond the zone of trees. Its naked summits of crystalline rocks remain covered with snow during more than half the year. The country extending along the southern slope {381} of these mountains is one of the most delightful districts of all Spain. It abounds in streams of sparkling water; groups of trees are dotted over the hill-slopes and shield the villages; and Charles V., when he selected the monastery of St. Yuste as the spot where he proposed to pass the remainder of his days, exhibited no mean taste. In former times the foot of the sierra was much more frequented, for the Roman road known as Via Lata (now called Camino de la Plata) crossed immediately to the west of it, by the Puerto de Baños, and thus joined the valley of the Duero to that of the Tajo.
The Sierra de Gata, which lies beyond this old road, has a course parallel with that of the Sierra de Gredos, and this parallelism is observable likewise with respect to the minor chains and the principal river beds of that portion of Spain. The Sierra de Gata rises to a height of 5,690 feet in the Peña de Francia, thus named after a chapel built by a Frankish knight. Within its recesses are the secluded valleys of Las Batuecas and Las Hurdes.
In the eastern portion of New Castile the country is for the most part undulating rather than mountainous, and, if the deep gorges excavated by the rivers were to be filled up, would present almost the appearance of plains. The most elevated point of this portion of the country is the Muela de San Juan (5,900 feet), in the Montes Universales, thus called, perhaps, because the Tajo, the Júcar, the Guadalaviar, and other rivers flowing in opposite directions take their rise there.
The Sierra del Tremendal, in the district of Albarracin, farther north, is said to be frequently shaken by earthquakes, and sulphurous gases escape there where oolitic rocks are in contact with black porphyry and basalt. Several triassic hills in the vicinity of Cuenca are remarkable on account of their rock-salt, the principal mines of which are those of Minglanilla.
Farther south the height of land which separates the rivers flowing to the Mediterranean from those tributary to the Tajo and Guadiana is undulating, but not mountainous. We only again meet with real mountains on reaching the head-waters of the Guadiana, Segura, and Guadalimar, where the Sierra Morena, forming for 250 miles the natural boundary between La Mancha and Andalusia, takes its rise. Seen from the plateau, this sierra has the appearance of hills of moderate height, but travellers facing it from the south see before them a veritable mountain range of bold profile, and abounding in valleys and wild gorges. Geographically this sierra belongs to Andalusia rather than to the plateau of the Castiles.
In the west, judging from the courses of the Tajo and the Guadiana, the country would appear to subside by degrees into the plains of Portugal; but such is not the case. The greater portion of Estremadura is occupied by a mountain mass consisting of granite and other crystalline rocks. The sedimentary strata of the region bounded in the north by the Sierras of Gredos and Gata, and in the south by the Sierra de Aroche, are but of small thickness. In former times these granitic mountains of Estremadura retained pent-up waters of the lakes which then covered the interior plateaux, until the incessant action of water forced a passage through them. Their highest summits form a range between the rivers Guadiana and Tajo known as the Sierra of Toledo, and attain a height of 5,115 feet in {382} the Sierra de Guadalupe, famous in other days on account of the image of a miracle-working Virgin Mary, an object of veneration to Estremeños and Christianized American Indians.
Fig. 141.—DEFILE OF THE TAJO IN THE PROVINCE OF GUADALAJARA.
Geologically the series of volcanic hills known as Campo de Calatrava (2,270 feet) constitute a distinct group. They occupy both banks of the Guadiana, and the ancient inland lake now converted into the plain of La Mancha washed their foot. From their craters were ejected trachytic and basaltic lavas, as well as ashes, or negrizales, but acidulous thermal springs are at present the only evidence of subterranean activity.