For decades after the cliff dwellings of Mesa Verde became famous, nobody could say how old they were. Guesses ran from a few centuries to thousands of years. The answer came from an unexpected direction: an astronomer in Arizona who had gone looking for the rhythm of sunspots in the growth of trees.
Reading the rings
Andrew Ellicott Douglass, director of the Steward Observatory at the University of Arizona, began studying ponderosa pines around Flagstaff early in the twentieth century. Each year a tree adds a ring, wide in wet years and narrow in dry ones, and across a region the trees record the same sequence of good and bad years. Douglass learned to match these patterns from tree to tree, and then from living trees back into old timbers whose inner rings overlapped the outer rings of younger wood. Tree-ring dating is matching, not counting.
Encouraged by archaeologists, he collected beams from the ruins of the Southwest. By the late 1920s he had two long sequences, one running back from the present and one floating, undated, from the ancient pueblos, but nothing to join them. In June 1929 a charred beam excavated at a ruin near Show Low, Arizona, catalogued as HH-39, helped close the gap, and the floating sequence suddenly had dates. The story of that night has grown into a legend; the real work took years of patient matching on either side of it.
Dating Mesa Verde
Douglass published the results in National Geographic in December 1929, including the first tree-ring dates for Mesa Verde, from beams taken out of the park's ruins. The cliff dwellings proved to be of the 1200s, built and rebuilt within a few generations; Cliff Palace, Balcony House and Spruce Tree House were all under construction in the same decades. Archaeologists have since gathered more than 4,000 tree-ring dates from well over a hundred sites in the park, a calendar of building activity down to the year in which a tree was cut.
The rings also showed something else. From 1276 to 1299, Douglass found, the region suffered a great drought, with some of the driest years in his entire record. Construction in the cliff dwellings stopped in the early 1280s, as the drought set in.
The drought and the leaving
Douglass suggested that the drought drove the people out, and for decades the great drought was the explanation. It no longer seems enough. The people of the Mesa Verde region had lived through many droughts in the centuries before; their descendants among the Pueblos of New Mexico and Arizona and the Hopi describe the movement south as a migration with its own reasons, and archaeologists now point to a combination of drought, crowding, violence and social change pulling people toward kin already living along the Rio Grande and in the Hopi country.
Nor was the place abandoned in the sense the word suggests. Pueblo people did stop building and repairing the cliff dwellings in the late 1200s, but Mesa Verde has remained part of their history and their homeland ever since, a landscape their descendants still visit and name as their own. A visitor at Mesa Verde National Park can see the beams themselves, set in the walls where their builders put them, holding the dates of the years they were cut.