A rapid in the Tagus, artificially converted into a weir, renders irrigation easy, and has thus created an oasis in the midst of the barren plateau of New Castile.
The principal right-hand tributaries, besides the Gallo and Zezere, are the Jarama, descending from the tableland of New Castile a little below Aranjuez, the Alberche and the Tietar, which collect their head waters from opposite sides of the Sierra de Gredos, and the Alagon, from the rough and broken country between the Sierras de Gredos and Gata.
The northern part of the kingdom, which was first freed from Moorish rule, is called Old Castile (Castilla la Vieja); the southern, acquired later, is called New Castile (Castilla la Nueva).
In 1833 Old Castile was divided into the provinces of Avila, Burgos, Logrono, Palencia, Santander, Segovia, Soria and Valladolid; while New Castile was similarly divided into Ciudad Real, Cuenca, Guadalajara, Madrid and Toledo.
This plateau has a natural frontier of high mountains on all sides, except on the borders of Leon and Murcia; it is also bisected by the Sierra de Guadarrama and Sierra de Gredos, which extend in a southwesterly direction across the central districts, and form the dividing line between Old and New Castile.
