Chaco Canyon

Chaco Canyon is a remote, arid canyon cut by the Chaco Wash in north-western New Mexico, between Albuquerque and Farmington. Between about 900 and 1150 CE it was the leading centre of the Ancestral Puebloans, who built there a cluster of massive stone "great houses" that remained the largest buildings in North America until the nineteenth century. Linked by roads to dozens of outlying settlements and carefully aligned with the sun and moon, the canyon is now protected as a national historical park and World Heritage site, and it is regarded as an ancestral homeland by the Hopi and other Pueblo peoples.

Hunter-gatherers camped in the canyon for millennia; over seventy of their sites are dated to between 7000 and 1500 BCE. Their Basketmaker descendants lived in pit-house villages, and around 800, in the period archaeologists call Pueblo I, they were building crescent-shaped stone house blocks set around underground ceremonial rooms called kivas.

From around 850 the population grew rapidly. In the tenth century the first part of Pueblo Bonito, a curved row of some fifty rooms, was built, and a turquoise-working industry spread across the canyon; excavators have since recovered some 200,000 pieces of turquoise. The most ambitious construction came later, with tree-ring dating showing that most roof beams were cut between 1033 and 1092. In all, fifteen major complexes rose along a stretch of canyon floor about fourteen kilometres long. Pueblo Bonito, the largest, had at least 650 rooms and stood up to four storeys high in places. Builders used thick walls with a rubble core faced with sandstone, and hauled the timber of an estimated 200,000 conifers, on foot, from mountains up to about 110 kilometres away.

How the great houses were used is debated. Some researchers argue that only a small population lived in them all year and that larger crowds gathered for periodic ceremonies. Genetic study of burials in Pueblo Bonito indicates that one elite female line was interred there for roughly 330 years. By about 1115 at least seventy outlying pueblos in the Chacoan style stood across the San Juan Basin, many joined to the canyon by long, straight, engineered roads whose purpose remains uncertain.

Many buildings appear to be aligned with solar and lunar cycles. Pueblo Bonito and Chetro Ketl lie on an east-west line, and on Fajada Butte the so-called Sun Dagger, a spiral petroglyph lit by shafts of light between stone slabs, has been interpreted by its discoverer, the artist Anna Sofaer, and others as a calendar marking solstices, equinoxes and the 18.6-year lunar cycle.

The Chacoan system began to come apart around 1140, possibly triggered by a fifty-year drought beginning in 1130, along with erosion and heavy deforestation. Outlying communities emptied and the central buildings were eventually sealed and left. Some scholars point to violence as a factor, though the great houses show little sign of defence. The people moved south, east and west towards the Little Colorado, the Rio Puerco and the Rio Grande, and influence passed to places such as Mesa Verde and Aztec. Ute peoples were present on the Colorado Plateau from the twelfth century, and Athabaskan-speaking Navajo and Apache had moved into the region by the fifteenth century. The Navajo Nation lies west of the canyon today.

The first recorded visit was an 1823 expedition led by José Antonio Vizcarra, governor of New Mexico, at a time when the area was part of Mexico. After the United States took the region in the Mexican-American War, an army detachment surveyed the ruins in 1849. The Hyde Exploring Expedition dug at Pueblo Bonito from 1896 and shipped more than 60,000 artefacts to New York, and Richard Wetherill's homestead claim over the main ruins helped prompt the Antiquities Act of 1906. Theodore Roosevelt proclaimed Chaco Canyon National Monument in 1907, and Neil Judd excavated Pueblo Bonito for the National Geographic Society in the 1920s.

The monument was enlarged into Chaco Culture National Historical Park in 1980 and inscribed by UNESCO in 1987. Since 1981 the park has avoided large excavations, out of respect for Hopi and Pueblo beliefs, and a committee founded in 1991 gives Navajo, Hopi and Pueblo representatives a voice in its management. Fajada Butte has been closed to the public because of erosion, and energy development in the surrounding basin has prompted efforts to protect the Chacoan roads and a buffer zone around the park.