Dr. A. E. Douglass, director of Steward Observatory, University of Arizona, established the tree-ring chronology for dating Southwestern ruins. This chronology is based upon the facts that solar changes affect our weather and weather in turn the trees of the arid Southwest, as else-where, and that such affects are recorded in the variation of tree-ring growth during wet and dry years. Thus the tree-ring record of living trees has been extended into the past by arranging beams from historic pueblos in their proper sequence so that the inner rings of one match the outer rings of its predecessor, and in turn match the rings of the living trees. After completing the series from living trees and pueblos, of known dates, the record has been continued through the cross-sections of prehistoric beams of fir and pine that were chopped with the stone axes. The continuation of this chronology is only limited by the finding of earlier beams than those used in the established chronology.
The National Geographic Society tree-ring expedition took, in all, 49 beam sections from ruins within Mesa Verde National Park. During 1932 and 1933 further tree-ring research was carried on in this area and additional dates have been secured. Presuming that the year of cutting the timber was the year of actual use in construction, the following dates have been established for the major cliff dwellings:
| Mug House, A. D. 1066 | Long House, A. D. 1204-11 | |
| Cliff Palace, A. D. 1073-1273 | Square Tower House, A. D. 1204-46 | |
| Oak Tree House, A. D. 1112-84 | Spruce Tree House, A. D. 1230-74 | |
| Spring House, A. D. 1115 | New Fire House, A. D. 1259 | |
| Hemenway House, A. D. 1171 | Ruin No. 16, A. D. 1261 | |
| Balcony House, A. D. 1190-1272 | Buzzard House, A. D. 1273 |
Since considerable tree-ring material from these ruins remains yet to be examined, the dates given above are not final. On the basis of present evidence, Cliff Palace, the largest and most complex cliff house within the park, shows an occupancy of 200 years.
It is an interesting fact that all of the dates fall just short of the beginning of the great drought, which the tree-ring chronology shows commenced in 1276 and extended to 1299, a period of 24 years.
[2] The Secret of the Southwest Solved by Talkative Tree Rings, by A. E. Douglass: National Geographic Magazine. December 1929.
DISCOVERIES OF RECENT YEARS
In 1923 Roy Henderson and A. B. Hardin discovered the largest and finest watchtower that had yet been found. The tower was circular, 25 feet in height and 11 feet in diameter. Loopholes at various levels commanded the approach from every exposed quarter.