In the 1190s the Pueblo people began moving to cliff dwellings after having lived on the Mesa tops for 600 years. By 1270 they began migrating away and by 1300 they were gone.
According to Wikipedia, nomadic people inhabited the area as early as 7500 BCE. Permanent residents started building the cliff dwellings around 1200 CE, but abandoned them around 1285. This information is consistent with what the National Park Service guides told us on the tour.
Some of the structures date as early as about 600CE. The most recent were dated to about 1285CE.
They don't know and the thinking changes over the years.
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