The Geographical Setting
The Occidente is the largest of the areas that made up ancient Mesoamerica, as well as the most diverse from an ecological perspective. It is not a single geographical or cultural unit, as can be seen by its great cultural diversity in Prehispanic times. The Occidente occupies several physiographic settings, including a good number of diverse ecological niches. According to the geographical study carried out by West (1964), the Occidente extends over the following areas of northwestern México: the Mesa Central, the Cordillera Neovolcánica, the Sierra Madre Occidental and the Coastal Lowlands that border the Pacific Ocean. The Occidente is almost impossible to define as a single unit using physical or biological criteria, because it is an area of contact and transition between at least six physiographic regions -the Northwestern Coastal Plain, the Sierra Madre Occidental, the Neovolcanic Axis, the Altiplano Central, the Sierra Madre del Sur and the Balsas Depression- and four bio-geographical provinces: Sinaloense, Sierra Madre Occidental, Volcánica Transversal and Nayarit-Guerrero. From the perspective of its physical and biological geography, the Occidente appears to be a region characterized by diversity and transition, and these are the traits that probably best define it (Jardel 1994:18).
No discussion of the Occidente's geographical setting would be complete without mentioning its rivers and lakes (Figure 3), the discussion of which is based largely on Tamayo and West (1964). The Mexican Pacific watershed receives less rainfall and covers much less area than the Atlantic one, and is characterized by surface streams with relatively small discharge. There are only two large drainage basins in this region: the Lerma-Santiago and Balsas systems. Even most permanent currents in the Pacific watershed are characterized by a markedly seasonal regime, and many of the small rivers are intermittent.
The Lerma-Santiago system is one of the most extensive hydrographical basins in Middle America. The present-day Lerma drainage system originates in the marshes and lakes in the southern end of the Toluca Valley; downstream, many tributaries join the river as it traverses portions of the present-day states of México, Querétaro, Guanajuato, Michoacán, and Jalisco. The Lerma is a slow-moving river with a slight gradient and many meanderings along its course. The Grande de Santiago River originates in Lake Chapala and flows across the southern part of the Sierra Madre Occidental to reach the Pacific Ocean in the state of Nayarit. Its most important tributaries are the following rivers: Verde, Juchipila, Bolaños, Apozolco and Guaynamota.
Lake Chapala occupies a large basin some 80 km. long (east-west). It is the only lake that remains from a series of Late Tertiary stepped-basins that received abundant discharges from the Lerma, Duero and Zula Rivers. Lake Chapala has been much affected by pollution in recent years and has lost much of its water due to hydraulic projects that supply urban and industrial areas such as Guadalajara and México City.
Geology, hydrology, topography and climate combine to give the Occidente its characteristic cover of vegetation. The most abundant botanical configuration is the deciduous tropical forest, followed by a high-altitude configuration typified by conifers and Quercus. Less widespread is the sub-deciduous tropical forest. In the driest parts of the area -particularly towards the north- we find mainly thorny underbrush (matorrales), grasslands and xerophytic plants (Rzedowski and Equihua 1987:14).
What follows is an overview of Prehispanic cultural development in the Occidente from the earliest times to the arrival of the Spanish in the sixteenth century. Period names such as Formative, Classic, Postclassic and their equivalents are used here in a purely chronological sense, with no implications for the cultural development associated with each period (see Table 1).
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