They are some of the largest pre-Columbian buildings in North America, but until now it has been a mystery how the great houses of Chaco Canyon were built between 850 and 1,140AD.
The ancient structures, made from stone and wood, occupy a nearly treeless landscape, sparking decades of curiosity about the origins of more than 240,000 trees used to build them.
But now experts have proof, in the form of tree rings, that the wood must have been carried around 46 miles (75km) from surrounding mountains.
The great houses, some of which are partially standing today, are located in the high desert landscape of New Mexico, where ancestors of Puebloan people called home centuries ago.
They covered an area of 4,633 square miles (12,000 square km) around in the San Juan Basin.
The massive masonry structures required a wealth of resources to erect, including an estimated 240,000 trees incorporated as roof beams, door and window lintels, and other building elements, according to a study published in the journal PNAS.
The first excavators of the great houses in the early 20th century speculated that construction timbers were harvested locally, perhaps resulting in deforestation of the surrounding landscape.
But subsequent studies revealed the land was never rich in timber.
To work out where the wood came from, researchers from the University of Arizona used methods of tree-ring analysis.
Christopher Guiterman and his colleagues compared tree-ring growth patterns from 170 great house timbers with the tree-ring growth patterns of trees from eight potential timber-harvesting areas in the nearby mountains.
They found that the Chuska and Zuni Mountains – more than 46 miles (75km) away were the most likely sources for 70 per cent of the wood used to build the houses.
Prior to 1,020AD nearly all of the wood came from south of Chaco Canyon in the Zuni Mountains.
‘We found that a previously unrecognised timber source, the Zuni Mountains, supplied construction beams as early as the 850s in the Common Era (AD),’ the researchers wrote.
By 1,060 AD most of the beams came from the Chuska Mountains to the west.
The shift from the Zunis to the Chuskas coincided with substantial expansion of Chaco Canyon society, including enlargement of existing great houses and the addition of seven new great houses.
‘It also coincides with the proliferation of Chuskan stone tools and pottery in the archaeological recors of Chaco Canyon, further underscoring the link between land use and occupation in the Chuska area and the peak of great house construction,’ the study continued.