A Baffling Missing Persons Case
The Sonoran Desert holds the clues to one of history's great missing persons cases, the collapse of distinct but connected 1,000-year-old civilizations throughout the Southwest sometime in the 1400s, just before the arrival of the first Spanish explorers. Human beings have occupied the Sonoran Desert for at least 10,000 years and for perhaps as long as 40,000 years. During the last Ice Age, they hunted mammoths, camels, ground sloths, and other big game across grasslands and oak woodlands. But when the planet's climate shifted at the end of the last Ice Age, the grasslands gave way to desert. A host of desert-adapted plants and animals moved into the new habitat, mostly from dry, hot areas farther south.
Human beings also adapted to the shift from grassland and woodlands to desert. For thousands of years, people wandered across the desert in small bands, taking advantage of the varied resources. The Sonoran Desert benefits from weather patterns that provide reliable winter rains from Pacific storms plus a six-week-long, often-spectacular monsoon rainy season in the summer as a result of storms blowing up from the Gulf of California and the Gulf of Mexico. As a result, small bands could move with the seasons and take advantage of the resources of the sky islands, where they could effectively change the season by changing their elevation.
At some point, these bands of wandering hunters and gatherers discovered agriculture. Archaeologists still do not know whether the people of the Sonoran Desert learned how to cultivate corn, beans, and squash on their own or whether they picked up the seeds and the methods from the older and more complex civilizations of Mexico and South America. In any case, the desert people first learned to dry farm (without irrigation ditches or wells) by taking advantage of the twice-a-year rainy season and lack of frequent freezes. Gradually, between 1,000 and 1,500 years ago, they started building irrigation works to water crops on the floodplains of the great desert rivers of the Sonora, like the Colorado, Gila, Verde, and Salt. These rivers each drain thousands of square miles of mountainous terrain, which means that the low deserts through which they ran could support much more diverse and numerous cultures than virtually any other desert in the world.
These people built a bewilderment of complex, innovative, exquisitely adapted civilizations throughout the Southwest, mostly based on farming and supplemented by hunting and gathering up wild foods like saguaro fruit and the bean pods of mesquite. They included the Ancestral Puebloans, or Ancient Pueblo People, often referred to as the Anasazi, in the North, the Hohokam in central Arizona, and a host of other related people, like the Sinagua, Mogollon, Salado, and others. They established trading networks that exchanged shells from the coast of California for turquoise dug up from the hills of New Mexico and parrots captured in the jungles of Mexico. But after thriving despite droughts and floods and the vagaries of the desert for 1,000 years, all these desert cultures first converted their sprawling villages and cities into fortresslike cliff dwellings and then abandoned even them. One major archaeological dig attempting to figure out what happened to these ancient civilizations took place on the shores of the Salt River, one of the most important rivers in the Sonoran Desert. Archaeologists unearthed huge villages built 800 years ago along the riverbanks on the edge of what had become a reservoir providing drinking water and flood control for Phoenix, some 100 miles to the west.
Here, the Salado people built huge platform mounds: a set of mud and stone rooms built atop an earlier, filled-in structure from which a growing elite could survey their domain. A network of these mounds was constructed by the Salado people in the cradle of the Tonto Basin, where they thrived for perhaps 500 years thanks to miles of irrigation canals that enabled them to water thousands of acres of farmland. But they disappeared along with all the other desert civilizations some 600 years ago. The dig uncovered pot shards, tumbled walls, ancient bones, and enigmatic figurines and unearthed an ancient observatory, all of which helped provide clues to the mystery.
The dig was triggered by a $347-million project to raise the height of Roosevelt Lake dam. That project was triggered by studies of ancient tree rings showing that the dam could be hit by floods four times the size of any on record. Until researchers put together thousands of tree-ring growth patterns from ancient living trees, downed logs, and the logs cut for roof beams in 1,000-year-old ruins, no one had any idea about the flood danger. Trees put on a thick growth ring in wet years and a thin one in dry years, which means the growth rings over a large area indicate rainfall patterns going back for nearly 2,000 years. The analysis demonstrated that the Sonoran Desert could suffer far greater floods than anyone suspected based on the records of the past century or two. Moreover, the tree-ring data proved that the Sonoran Desert could also go through droughts lasting for 30 years or more. Those discoveries made the 1,000-year survival of the Salado civilization all the more impressive.
The Tonto Basin dig uncovered several long, oversized rooms that were dominated by massive mud and stone columns, pillars that once held beams to support high ceilings. Yet no one lived in the enigmatic structure, as evidenced by the lack of cooking hearths and household debris. Archaeologist David Jacobs, a medic in Vietnam who had been estranged from his own time and culture ever since, managed to solve the mystery of these mysterious structures. Smallish, bearded, supple, and weathered as richly worn leather, Jacobs sometimes rode with the Hells Angels between digs. He discovered that the huge, unoccupied rooms were used as solar observatories, since the doorways and pillars were oriented so that light would stream through the door and reach an alcove on the opposite wall only at dawn on summer solstice, the longest day of the year. This enabled the Salado to calibrate both their ceremonies and harvest.
The platform mounds turned out to be the key to reconstructing the Salado civilization. The pillar site was the oldest mound: a ceremonial center built about a.d. 1000 oriented toward the sun and forming the focal point of a loose alliance of mostly independent farmers and gatherers. But nearby platform mounds built some 200 years later presented a very different picture. The largest, dubbed schoolhouse mound, constituted a bustling, L-shaped assemblage of about 115 rooms that served as an economic center dominating huge tracts of surrounding farmland. Some 200 people lived on the mound, guarding granaries crammed with enough surplus food to last for years. Such a center marked the beginnings of a complex, stratified society. The big cities with platform mounds alongside the irrigation canals housed the priests and the elites. Meanwhile, a network of connected rural settlements housed extended families who were loosely associated with the platform mounds and who harvested the wild resources of the Sonoran Desert, including prickly pear, cholla, agave, and saguaro fruit.
Arizona State University archaeologist Glen Rice, a mild-mannered man with a boyish shock of hair, a neatly trimmed beard, and an air of dogged obsession, sold the Bureau of Reclamation on a project of nearly unprecedented scope. Bespectacled and intense, Rice is an archaeologist of the new school: He loves computers and designed a project in which every artifact has been logged into a computer, including more than 123,000 fragments of pots and 44,000 stone tools. Machines that analyze the trace elements in bones provided clues to diets, including everything from meat content to water sources. Chemical analysis of pot shards even revealed the source of the clays and pigments and helped reconstruct trade networks linking the Salado to other cultures stretching from Mexico to Colorado. Machines that measured different types of carbon dated wood samples, enabling scientists to construct a chronology from bits of charcoal in ancient hearths. Chemical analysis of things like obsidian and turquoise pinpointed sources of precious minerals and so helped reconstruct trade networks. Microscopic analysis of pollen grains and other plant remains helped reconstruct ancient diets, gauging the relative importance of irrigated crops and wild foods. Tree-ring analysis allowed the researchers to reconstruct stream flow year by year. Gathering such evidence requires hundreds of hours spent shifting dirt through screens and digging with dental tools to find delicate artifacts. An archaeological dig in progress simmers with slow-motion excitement. Someone might find the intact rim of a gigantic storage pot, triggering happy shouts. Then the painstaking excavation continues for hours, or days, as the treasure emerges, inch by inch.
The people who do the dirty work are a varied bunch of scientific vagabonds. Most have degrees in archaeology and constantly move around the country. Most read voraciously, think nothing of a 500-mile (805 km) weekend drive to visit another dig, and have sacrificed marriages, money, and a normal suburban lifestyle to search for the past on hands and knees on some remote, sun-seared slope. Some, like Jacobs, remain quietly out of place, ironic, relentlessly curious, vibrating with life and its possibilities, but tinged with an ancient skepticism. They are discovery junkies, hooked on the thrill of discarded mysteries and the rush of insight. They treasure the moments of contact with a vanished culture. ASU's Glen Rice recalls the day he found a perfect palm print on one of those enigmatic pillars. “There was a Salado hand looking at you, from 900 years ago. That is the thing that is quasimystical. We are not just studying pots and mud and stuff.”