This describes how villages were rebuilt on top of the previous one with increasingly
sophisticated construction techniques. In this case it was over a period of about 150
years.
It also talks about how cold it got in the winter, which led me to theorize that
perhaps all these ruins are, ironically, slums. The rich people enjoyed life in the
pleasant valley near the pleasant river to the north while these poor folk eked out a
living high in the cliffs, the cheapest real estate around, even though they roasted in
summer and froze in winter. Those on top of the mesa might not have been the poorest of
the poor because there probably would have been grazing opportunities, that is if they
herded animals. Recall that this was long before the introduction of the horse by the
Spanish, and that there is little or no evidence of animal husbandry.