In contrast, a more mountainous zone surrounds the Meseta and serves as the region's natural border. In the north of the Province of Guadalajara, bordering Madrid and Segovia, is a mountain range forming part of the Sistema Central, among which can be distinguished the mountain ranges Pela, Ayllón, Somosierra, Barahona and Ministra, with the headwaters of the rivers Jarama, Cañamares and Henares. The Sistema Central also penetrates the northwest of the Province of Toledo: a southwest to northeast sub-range known as the Sierra de San Vicente, bordered on the north by the Tiétar and on the south by the Alberche and the Tagus, rising up to its maximum heights at the summits of Cruces (1373 m), Pelados (1331 m) and San Vicente (1321 m).