Sierra Madre (syāˈrä mäˈħrā) [key], chief mountain system of Mexico, consisting of the Sierra Madre Oriental, the Sierra Madre Occidental, and the Sierra Madre del Sur and forming the dissected edges of the vast central Mexican plateau; a volcanic belt along the plateau's southern edge links the three sierras. Extending from northwest to southeast through Mexico from the U.S. border, the rugged Sierra Madres, 6,000–12,000 ft (1,829–3,658 m) high, with deep, steep-sided canyons ( barrancas ), have long been a barrier to east-west travel. The terrain ranges from permanently snow-covered peaks to hot, tropical valleys; and from the humid, thickly vegetated seaward slopes to the dry, largely barren interior-facing slopes. Agricultural products vary according to the climate. Lumbering is done in the N Sierra Madre Occidental. The Sierra Madres have a great wealth of minerals including iron ore, lead, silver, and gold. The mountains are sparsely populated, with settlement limited to mining towns and agricultural communities. The Sierra Madres hold good potential for hydroelectric-power development, and several stations have been built in the northern ranges. The Sierra Madre Oriental ōryĕntälˈ, beginning in barren hills S of the Rio Grande, runs for c.700 mi (1,130 km) roughly parallel to the coast of the Gulf of Mexico, ranging from 10 to 200 mi (16–320 km) inland. It reaches an elevation of 18,700 ft (5,700 m) in Citlaltépetl, which belongs also to the volcanic belt, Cordillera de Anáhuac. This belt, which divides Mexico in half at about lat. 19° N and includes the peaks Popocatépetl and Iztaccíhuatl, on the other end joins the Sierra Madre Occidental ōkˌsēdĕntälˈ. This range, paralleling the Pacific coast for c.1,000 mi (1,610 km), extends SE from Arizona. Its main escarpment is more abrupt than that of the eastern cordillera. From c.5,000 ft (1,520 m) in the north, elevations reach over 10,000 ft (3,048 m) in the south. The Sierra Madre del Sur dĕl sŏrˈ is a tumbled, broken mass of uptilted mountains that touch the Pacific coast but form into no clearly defined range. It spreads over S Mexico between the volcanic belt and the Isthmus of Tehuantepec and forms the natural harbor of Acapulco.