Perched on a lonesome bluff above the dusty San Pedro River, about 30 miles east of Tucson, the ancient stone ruin archaeologists call the Davis Ranch Site doesn’t seem to fit in. Staring back from the opposite bank, the tumbled walls of Reeve Ruin are just as surprising. Some 700 years ago, as part of a vast migration, a people called the Anasazi, driven by God knows what, wandered from the north to form settlements like these, stamping the land with their own unique style. “Salado polychrome,” says a visiting archaeologist turning over a shard of broken pottery. Reddish on...