Throughout their 2000-year history on the Colorado Plateau, Anasazi populations tended to cycle between periods of clustering and dispersal, occupation and abandonment. Yet the period of 700-1130 A.D. was singular for all the various peoples of the Southwest. Technological and social innovations such as pottery for boiling foods, food-sharing and storage, together with a more settled way of life dependent on agriculture, coincided with a period of adequate rainfall. The result was a rapid increase in population.
Skeletal studies show that increases in fertility, not a decrease in mortality, were responsible for the sudden rise in populations. Over several decades, numbers increased dramatically and the Anasazi spread across the landscape. By approximately 1075-1100 A.D., people were more densely packed and widely distributed in the Southwest than ever before, a peak of population not reached again until historic times.
On Black Mesa as in many northern locations, with the exception of a brief interval from about 1030 to 1050 A.D., the density of people increased 10-fold over the course of a few generations. Indigenous population growth alone cannot account for it; significant numbers of people must have moved into the area from the surrounding countryside. Mesa tops were not alone in attracting population growth; in Grand Canyon, during the last half of the eleventh century, communities were established in scores of locations deep within the canyon.
The outposts of the Anasazi realm expanded to the edges of the Colorado Plateau and beyond. Anasazi settlements ranged west to central Utah and the Virgin river, south to the Mogollon Rim in Arizona, and east to the Rio Grande Valley in New Mexico. This demographic shift anticipated the urbanization of the United States during the twentieth century, but on a much smaller scale.
Success, however, may well have been the seed of eventual decline. A severe drought began in 1130 and lasting for 50 years. At the same time another much longer cycle unrelated to precipitation was causing water tables to drop and initiating a period of arroyo-cutting. Anasazi settlements that were on the edges of the realm, or those closer in but inhabititing poor farmland, were the first to be abandoned and left in ruins. By late in the twelfth century many localities on the more arid western extent of the Pueblo homeland were deserted.
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The Anasazi "collapse"
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