Between about A.D. 850 and 1140, the archaeology of Chaco Canyon in northwestern New Mexico reveals the rapid construction of large communal structures where smaller settlements had existed previously and shows that the locality became the core of an extensive regional system in the Four Corners region of the northern Southwest integrated by formal trails, the circulation of nonlocal goods, and the sharing of ritual items. Researchers debate the role of increased sociopolitical complexity in this development, but less attention…