TUCSON, ARIZONA—Christopher Guiterman of the University of Arizona used the collections housed at the Laboratory of Tree-Ring Research and a technique called dendroprovenance to determine the origins of the wooden beams that were used to build the monumental great houses in Chaco Canyon, New Mexico. Over a period of four years, he compared the tree-ring patterns on 170 different beams with archived tree-ring patterns from nearby mountain ranges. “We pulled stuff out of the archive that hasn’t been looked at in 30 or 40 years. It was pretty cool to open those boxes,” Guiterman said in a press release. He found that before A.D. 1020, most of the wood used in construction came from the Zuni Mountains to the south. The Chacoans then switched around the year 1060 and harvested trees from the Chuska Mountains to the west. These results agree with the chemical and archaeological evidence. “There’s a change in the masonry style—the architectural signature of the construction. There’s a massive increase in the amount of construction—about half of ‘downtown Chaco’ houses were built at the time the wood started coming from the Chuska Mountains,” Guiterman said. To read in-depth about the ancient Southwest, go to "On the Trail of the Mimbres."