Bill Doelle groans every time he reads a newspaper headline that says “Archeology Find Halts Construction Project.” As the president of Tucson-based Desert Archaeology Inc.—consultant to Pima County on the cultural preservation efforts during the Regional Wastewater Reclamation Department’s $720-million Regional Optimization Plan—Doelle and his team were just as attentive to the present day needs of construction crews as they were to investigating the past.

“The county has been developing for 25 years a pretty standardized procedure of assessing what’s in the ground and taking care of the big issues before construction gets underway,” Doelle says.

History is literally underfoot everywhere in Tucson, so it’s no wonder that the county and its residents have been so active in protecting and preserving cultural resources since the 1970’s. To date, more than 8,500 archeological and historical sites have been discovered, shedding light on the history of humans that have lived in the area continuously for 12,000 years, according to the Pima County Cultural Resources & Historic Preservation Office.

But it was during the optimization plan’s expansion of the Ina Rd. Wastewater Treatment Facility that one of the most extraordinary discoveries was made. Dubbed Las Capas, the discovery is “the earliest complicated and sophisticated agricultural irrigation system discovered in North America so far,” says Loy Neff, program manager for the county’s historic preservation office. The extensive fields and irrigation trenches date back to the San Pedro Period, 1250 B.C. to 750 B.C. 

The system was uncovered during archeological excavation begun in 2008, and was rated one of the 10 top discoveries of 2009 worldwide by Archaeology Magazine. Work will continue off and on through 2013 as construction progresses. 

Neff says the findings are significant to archeologists and historians because the ultimate question of how prehistoric populations developed and what technologies they used to produce food–along with the resulting changes in social organization—are critically important in understanding the whole prehistory of North America. 

Cultural Excavation

Working long before the planned start date of the plant’s expansion, archeologists began the investigation by digging a series of trenches up to 10 ft deep using a standard 2-ft-wide backhoe bucket. The trenches allowed archeologists to view the various soil layers laid down over millennia by the region’s seasonal flood cycle and choose areas for further investigation. 

Next, an excavation subcontractor stripped away the top layer of dirt, including up to 5 ft of fill placed during previous construction at the site. 

Desert Archaeology then brought in their own skilled backhoe operators, who used specialized 7-ft-wide blades, referred to as “giant trowels.” The operators could pull off as little as a half-centimeter of dirt at a time. “If you work with a backhoe long enough, you get an almost fingertip-like sensitivity in what that metal blade scraping over the ground surface is telling you,” Doelle says.

It was during this time that the team noticed differences in the dirt coloration and pattern, indicating berms surrounding planting fields.