Dig reveals ancient fields

The discovery of a prehistoric irrigation system in the Marana desert is giving archaeologists a deeper glimpse into one of the first groups of people to farm in the Tucson basin.

"What we're looking at is, perhaps, the earliest sedentary village life in the Southwest with people depending on agriculture as a primary food source," said project director Jim Vint.

For more than 3,000 years, an elaborate ancient irrigation system has remained hidden deep beneath the sand in Marana.

In January, excavation at the Pima County Regional Wastewater Reclamation Facility at Ina Road and Interstate 10 revealed the ancient irrigation system. It is said to be the most intricate system of its kind uncovered in North America.

"We've uncovered dozens of these fields. We can see the actual holes where they planted the corn in many instances" geologist Fred Nials said. "We can completely reconstruct their irrigation system."

The $6.8 million project at a site — called "Las Capas," or "The Layers" — is part of an expansion of the Pima County Regional Wastewater Reclamation Facility. The excavation, by Desert Archaeology Inc., complies with state and county regulations requiring that a site be excavated before development of land that may contain historical artifacts.

What the archaeologists found was more than they ever expected, said Desert Archaeology President Bill Doelle.

"Usually what people have found when digging in the flood plain is the main irrigation ditch that diverts water out of the river; they'll just see that ditch," he said. "We had no idea that whole field systems are also preserved in that flood plain sediment."

The field system, which spans about 60 to 80 acres, is just downstream from where the Cañada del Oro and Rillito join the Santa Cruz River. The site was revealed by scraping away thin layers in broad 7-foot sections using a backhoe.

"If you take that really thin scrape, it's really amazing how features can just appear from a couple inches of soil being removed," said Loy Neff, program manager at the Pima County Cultural Resources and Historic Preservation Office.

The archaeologists were able to recognize the outlines of fields, canals, pits and housing sites by different compositions that appeared in the dirt. For instance, the flow of river water through canals and into fields left sediment that could be seen as soil was scraped away.

"What you get is differences in color and texture in the soil that form patterns," Neff said. "When you scrape, suddenly you get a rectangular pattern or a linear pattern; they just pop up."

The group has now discovered more than 200 individual maize fields and more than 170 canals of various sizes throughout six major layers of sediment. The topmost or most recent layer dates to 800 B.C., and the sixth layer, which is about 13 feet underground, dates to about 1200 B.C.

These "layers," produced by periodic flooding of the river, reveal the story of one of the first groups of people to recognize the enormous potential of the Santa Cruz River and agriculture. These pre-Hohokam peoples, thought to be ancestors of today's Tohono O'odham Indians, depended on the river for their livelihood. ...

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