INTRODUCTION 13Figure A.4 Highland valley in Central Mexico. Photograph by the authors of the text.
only the largest and most important city of Mexico but is also now the second largest
city in the world (only Tokyo is larger).
The Central Plateau is marked by volcanic features and unique hydrological pat-
terns. A line of high volcanos, some of them still active, forms the southern rim; on
the eastern flank is the Sierra Madre Oriental, whereas the Sierra Madre Occidental
forms the western escarpment.
The plateau itself is pitted by both large, flat basins, many of which once con-
tained lakes, and eroded volcanic peaks. Among the largest basins are Mexico, Puebla,
Toluca, Guadalajara, and a series of linked basins that form the Bajío of Guanajuato.
Many of the lakes no longer exist, some of them because of sedimentation and des-
iccation, and others—such as the five lakes in the Valley of Mexico—because of ar-
tificial draining. A few lakes, such as Lake Pátzcuaro in Michoacán, are still viable.
Much of the plateau itself lies in Tierra Templada, although the higher basins and
surrounding mountains are Tierra Fría zones. Several of the volcanic peaks—such as
Orizaba, Popocatepetl, Iztaccihuatl—are snow-covered year-round, and ice has been
obtained from these peaks for centuries.
The Central Plateau is drained by three major river systems. With its headwaters
in the Toluca Basin just west of Mexico City, the Lerma River flows west to the Pacific,
forming the longest river system in the region. Tributaries of the Balsas River flow
from basins in the southeastern portion of the plateau and drain into the Pacific
Ocean. The Panuco River and its tributary, the Moctezuma River, form one of the
largest drainage systems of Mexico’s Atlantic watershed. Both rivers have their head-
waters in the northeastern part of the Central Plateau.
The volcanic range that forms the southern boundary of the Central Plateau,
overlooking the Balsas Depression formed by the Balsas River, defines the second