The Toluca Valley is a valley in central Mexico, just west of the Valley of Mexico (Mexico City). The valley runs north south for about 35 km surrounded by mountains, the most imposing of which is the Nevado de Toluca Volcano. It is one of the highest valleys in Mexico and for this reason has a relatively cold climate. Since the 1940s, there has been significant environmental degradation in the valley, with the loss of forests, soil erosion, falling water tables and water pollution due to growth in industry and population. In the pre Hispanic period, it was a buffer region between the Aztec and Tarascan Empires.