Perched on a lonesome bluff above the San Pedro River in Arizona, the ancient stone ruin that archaeologists call the Davis Ranch Site seems out of place.

Staring back from the opposite bank, the tumbled walls of Reeve Ruin are just as surprising.

About 700 years ago, as part of a vast migration, a people called the Anasazi wandered from the north to form settlements like these, stamping the land with their unique style.

"Salado polychrome," says a visiting archaeologist, turning over a shard of broken pottery. Reddish on the outside and patterned black and white on the inside, it stands out from the plainer ware made by the Hohokam, whose territory the wanderers had come to occupy.

Archaeologists have traced the Anasazi (or Ancient Puebloans, as they are increasingly called) to the mesas and canyons around Kayenta, Ariz., not far from the Hopi reservation. They were distinctive in other ways.

They liked to build with stone; the Hohokam used sticks and mud. Their kivas, or chambers, like those they left in their homeland, are unmistakable: rectangular instead of round, with a stone bench along the inside perimeter, a central hearth and a sipapu, or spirit hole, symbolizing the passage through which the first people emerged from Mother Earth.

"You could move this up to Hopi and not tell the difference," said John Ware, the archaeologist leading a field trip examining a Davis Ranch kiva.

Finding it at this site is a little like stumbling across a pagoda on the African veldt.

The migration raises the most vexing and persistent question in Southwestern archaeology: Why, in the late 13th century, did thousands of Anasazi abandon Kayenta, Mesa Verde and the other magnificent settlements of the Colorado Plateau and move south into Arizona and New Mexico?

Scientists once thought the answer lay in impersonal factors such as the onset of a great drought or a little ice age. But as evidence accumulates, those explanations have come to seem too pat.

Looking beyond climate change, some archaeologists are studying the effects of warfare and the increasing complexity of Anasazi society. They're looking deeper into ancient artifacts and finding hints of an ideological struggle, clues to what was going through the Anasazi mind.

"The late 1200s was a time of substantial social, political and religious ferment and experimentation," said William Lipe, a Washington State University archaeologist.

When scientists examine the varying width of tree rings, they indeed see a pernicious dry spell gripping the Southwest during the last quarter of the 13th century, around the height of the abandonment. But there had been severe droughts before, and even in the worst of times, major waterways kept flowing.

"Climate probably explains a lot," said James Allison, a Brigham Young University archaeologist. "But there are places where people could have stayed and farmed and chose not to."

Hopi was far from an anomaly.

"The whole abandonment of the Four Corners, at least in Arizona, is people moving to where it's even worse," said Jeffrey Dean, an archaeologist at the University of Arizona's Laboratory of Tree-Ring Research.

Soon after the abandonment, the drought lifted. Though the rains returned, the people never did.

"Why didn't they come back?" said Catherine Cameron, a University of Colorado archaeologist. "Why didn't anyone come back to the northern San Juan? It was a fine place, and apparently by 1300 it was very fine."

Ultimately the motivation for the abandonments may lie beyond fossils and artifacts, in the realm of ideology. Imagine trying to explain the 19th century Mormon migration to Utah with only tree rings and pollen counts.

By studying changes in ceremonial architecture and pottery styles, Donna Glowacki, a University of Notre Dame archaeologist, is charting the rise of what may have been a new puebloan religion. For more than a century, the established faith was distinguished by multistory "great houses," with small interior kivas, and by much larger "great kivas" -- round, mostly subterranean and covered with a sturdy roof.

Originating at Chaco Canyon in northwest New Mexico, the formidable temples seem designed to limit access to all but a priestly few. But by the mid-1200s, a different style also was taking hold, with plazas and kivas that were uncovered, like amphitheaters -- hints, perhaps, of a new openness.

In an effort to draw together the skein of causes and effects of the Anasazi migration, Timothy Kohler of Washington State University and members of the Village Ecodynamics Project are collaborating with archaeologists at Crow Canyon on a computer simulation of population changes in southwest Colorado from 600 to about 1300.

Juxtaposing data on rainfall, temperature, soil productivity, human metabolic needs and diet, gleaned from an analysis of trash heaps and human waste, the model suggests a sobering conclusion: As Anasazi society became more complex, it also became more fragile.

Corn was domesticated and then wild turkeys, an important protein source. With more to eat, the populations grew and aggregated into villages. Religious and political institutions sprung up.

When crops began dying and violence increased, the inhabitants clustered even closer. By the time the drought of 1275 hit, the Anasazi had become far more dependent on agriculture than during earlier droughts. They also had become more dependent on each other.

Amid the swirl of competing explanations, one thing is clear: The pueblo people didn't just dry up and blow away like so much parched corn. They restructured their societies and tried to adapt. When all else failed, they moved on.