In his Political Essays on New Spain, Leon Alexander von Humboldt called attention to the importance of irrigation in the wheat production of colonial Mexico. Extensive systems of irrigation existed not only in the western Bajio described by Humboldt, but along the River Laja and certain smaller streams toward the east. The Bajio has no clearly defined physical boundaries. The flat, interconnected valleys of the Bajio, lying between 1,700 and 2,000 meters above sea level, are old lakebeds formed by the disruption of drainage patterns through volcanic activity beginning in the Cretaceous period. Located within the southern range of the subtropical calms, the Bajio has a moderately arid climate with a markedly seasonal pattern of rainfall. By the end of the colonial era, the frontier society of the Bajio had evolved into a relatively populous region with a balanced economy based on agriculture, stock raising, mining, industry, and trade.