SOUTHWESTERN AECHEOLOGY — ROBERTS 511 



had been done. It pointed out blank spots in the record, indicated 

 clearly the districts where investigations were needed, and centered 

 attention on a number of general problems previously overlooked. 



Within the last decade a new method of obtaining chronological 

 evidence, one making possible absolute rather than relative dating, 

 was developed. This contribution came, not from an archeologist, 

 but from an astronomer. 



Dr. A. E. Douglass, of the University of Arizona, in making a 

 study of sun spots and their effects on climatic conditions in the 

 Southwest, turned to the growth rings of trees in an effort to obtain 

 evidence on the occurrence of drought periods and the intervals of 

 moisture. In doing this he discovered that the rings formed definite 

 patterns by groups of years, and as a consequence he developed a 

 system whereby he could tell whether the trees from which logs were 

 cut were growing at the same time or to what degree their life cycles 

 overlapped. Beginning with trees whose cutting date was Imown, 

 he has been able to devise a type chart going back to about 700 A. D.^ 

 In obtaining evidence to substantiate his own theories he was forced 

 to resort to timbers from ruins for material antedating living trees, 

 and thus furnished the archeologists with an extremely valuable time 

 scale. Now when beams are found in a ruin it is possible to check 

 their rings with the historical chart and, provided the outer sur- 

 faces have not been damaged or removed, tell the year of their cut- 

 ting. Of course, the timber may not have been placed in the struc- 

 ture immediately after the tree was felled, and occasionally a log 

 was no doubt reused. It is possible though, to gage the results by a 

 careful consideration of the archeological aspects of the site, and a 

 date is assured which closely approximates the year or years when 

 the dwellings were erected. 



Continued work with the tree-ring dating system, or dendrochro- 

 nology, as it is now called, demonstrated that the type chart would 

 not function in all cases. Material from some sections did not cor- 

 relate properly because of local variations in characteristic ring pat- 

 terns. For this reason it has been necessary to develop supple- 

 mentary charts. This work is being carried on by a number of Dr. 

 Douglass' students, and newly dated ruins are constantly being added 

 to an already sizable list. Kivalry between workers to find earlier 

 and earlier dates has in one or two instances caused misunderstand- 

 ings. Correct use of the system requires that the announced date 

 should be the outermost ring in the timber. Occasionally the date 

 of the earliest discernible ring has been proclaimed in such an am- 

 biguous way that the implication was that that was the year when 

 the log was cut and the building erected. One such case led to 



» Douglass, 1935. 



