Mesa Verde National Park's "Hemenway House view" cliff dwellings in a canyon wall and the rim above, scorched by a fast-moving wildfire in southwestern Colorado's Montezuma County. Tree ring dating from logs used in the park's dwellings indicates that construction and refurbishing of the cliffside communities was continuous from about 1190 through 1260 A.D. The Anasazi, or "ancient ones" -- ancestors of later Puebloan Indians -- constructed the cliff dwellings and abandoned them by 1300, for reasons that are debated to this day. Causal theories range from the ravages of drought and flight from enemies to (in the extreme) the demands of a religously fanatic leader. Photo by Carol Highsmith, 2016.