In each of the two areas mentioned above—Chaco Canyon and Mesa Verde—archeological work has revealed a continuous occupation of the sites and the immediate vicinity. At the great Aztec ruin, however, there is still some doubt as to what really happened. In two different time periods there seem to be strong architectural relations to both the Chaco and Mesa Verde centers, as well as close ties in ceramics and other items of material culture. Part of the intriguing mystery at Aztec is whether these similarities represent actual migrations from those centers on a fairly large scale, or an exchange of ideas, or small groups of migrants who strongly influenced the local population. In the midst of the populous Animas Valley, along the edge of an old river terrace, early in the 1100’s a large multistoried stone pueblo was built in an architectural style reminiscent of that in Chaco Canyon. Did a large migrant group from the Chaco area—or some other area where Chaco-like people were living—move into the Animas Valley and erect this structure? Or did some of the local citizenry decide to join in a community effort and copy the building techniques of their neighbors to the south? If so, what was the impetus which launched the local people upon this ambitious project? Perhaps a small group of highly skilled technicians, under the leadership of a few “priests” or medicine men, came from the Chaco area into the Animas Valley. Once established there, by persuasion, teachings, or by religious magic and psychological control, they may have prevailed upon some of the local population to join them and to build their homes and kivas of sandstone blocks, in the traditional Chaco style. We may never know the exact answer to these questions, but wherever the people came from, whoever they may have been, whatever the guiding impetus, Aztec pueblo, like Rome, was not built in a day. Dates from tree rings—as described later—indicate that the pueblo was built between A.D. 1110-1124, with the major construction periods in 1111 and 1115. Probably a small group, or just a clan, moved into the site about 1110, and finding it suitable for habitation erected the first small part of the pueblo. The next year a much larger group, perhaps several clans or more, joined the earlier settlers and more than 50 percent of the pueblo was finished. Then, in 1114 or 1115, a third wave of migrants arrived and essentially completed the pueblo, except for the one-story row of rooms which closed off the south side. It is possible that some of the indigenous Animas population joined these newcomers and moved in with them. From 1115 until about 1124 or 1125, occasional rooms were added as new quarters were necessary for newly married couples and as old rooms were used as refuse dumps.