ocr: astride the narrow waist ofa valley that is the best route between the Valley of Mexico and the Valley of Puebla. The Valley of Puebla, in turn, is the gateway to the lowlands along the Gulf of Mexico. The lower part of Teotihuacan's valley isa rich alluvial plain, watered by permanent springs and thus independent of the un- certainties of highland rainfall. The inhabit- ants of the valley seem early to have dug channels to create an irrigation system and to provide their growing city with water. Even today a formerly swampy section at the edge of the ancient city is carved by channels into ch ...
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