Main mound at the University Indian Ruin from the southeast during 1940 excavations, a Civilian Conservation Corps project directed by Julian Hayden. The Santa Catalina Mountains are visible in the background. (Photo Credit: Emil W. Haury, Arizona State Museum, University of Arizona)

Tree-Ring Dates for Charcoal from Ancient Indian Ruin Bolster Tucson Basin Archaeology

By Lori Stiles
December 04, 2003

University of Arizona tree-ring scientists have used charcoal collected between 1930 and 1940 to date a Hohokam Indian site called University Indian Ruin. The site is near Sabino Canyon and Tanque Verde Roads, seven miles from downtown Tucson.

Dating the prehistoric site is a major breakthrough in desert archaeology. University Indian Ruin is only the fourth prehistoric desert site dated by tree-rings, and the first firmly placed in the Tucson Late Classic phase.

Charcoal dating shows that Hohokam lived at University Indian Ruin during or shortly after the winter of A.D. 1371 and until after A.D. 1375, said UA Tree-Ring Lab archaeological Research Specialist David J. Street and UA dendrochronology Professor Jeffrey S. Dean.

That's 125 years after Hohokam were known to have occupied three other tree-ring dated sites near Tucson. The other sites are Gibbon Springs, Whiptail Ruin, and Marana Mound.

University Indian Ruin is a 656-by-427-foot area containing at least three separate room blocks and a platform mound. UA archaeologists partially excavated the site between 1930 and 1940. Wood samples collected at the site were numbered and archived at Tree-Ring Lab during the 1960s.

Main mound at the University Indian Ruin from the southeast during 1940 excavations, a Civilian Conservation Corps project directed by Julian Hayden. The Santa Catalina Mountains are visible in the background. (Photo Credit: Emil W. Haury, Arizona State Museum, UA)

Tree-ring scientists weren't able to date the samples then, but they now have developed better local tree-ring chronologies and better dating techniques.

In October, Street reanalyzed 47 tree-ring samples collected at the ruin about 60 years ago, and was able to date eight charcoal pieces.

"These dates provide the first absolute and independent dating controls that are precise enough to provide a firm anchor in time for this site," Street said.

The Hohokam (ho-ho-kam), ancestors of today's Pima and Tohono O'odham people, built villages in the Tucson Basin from about A.D. 300 A.D. to A.D. 1500.

Archaeologists can place the sites within a 50- to 200-year range through radiocarbon dating and by studying ceramics and architectural styles. But only tree-ring dates are accurate enough to tie occupation to a calendar year.

Dating Tucson-Basin tree rings has been difficult because desert trees often don't grow a single ring each year as do Southwestern trees living at higher elevations. Southern Arizona's climate produces trees that grow one or more false rings annually, or sometimes miss rings altogether, as they cope with an arid climate.

A strong local tree-ring chronology is vital to dating short-lived and difficult trees, Street said. The samples dated at University Indian Ruin had 35 to 65 rings and could be dated only because they overlapped in time and shared a strong climate signal. All the samples were from young trees — some with only 5 to 20 rings — because the Hohokam cut young trees in the lower Santa Catalina Mountains.

It doesn't look like much, but the small piece of charcoal (left) yielded some of the best tree-ring dates from University Indian Ruin.

Arizona State Museum archaeologists Paul Fish and Suzanne Fish plan to excavate University Indian Ruin in 2005 during an anthropology department archaeological field school.

"The new tree-ring dates are exciting," Paul Fish said. "The very late tree-ring dates suggest that University Indian Ruin occupation may extend into the 15th century. One of our research objectives is to learn more about the final phase of the Hohokam, and to learn more precisely when it ends."

Some archaeologists argue that Hohokam culture vanished before Europeans came to southern Arizona. But others think Hohokam were still in the Tucson Basin, Suzanne Fish said. New excavations at University Indian Ruin might confirm contact between the Hohokam and Spanish explorers, she said.

Tree-ring dates also will give archaeologists clues to migration within the Tucson Basin, Paul Fish said. "Some sites appear to have been abandoned after A.D. 1300, while others continued to be occupied and probably even grew in population."

Until recently, archaeologists weren't interested in collecting wood samples in the Tucson Basin and surrounding areas because they believed dating them wasn't possible or not worth the effort, Street and Dean explained. This has left tree-ring scientists with too few samples now that chances of dating desert wood have increased dramatically.

During the past decade, the UA Tree-Ring Lab has made significant strides in dating southern Arizona's prehistoric wood samples, and now need more Tucson-Basin specimens to strengthen the local tree-ring chronology.

The dating breakthrough resulted from prolonged, steady efforts.

First, tree-ring scientists dated historic Tucson structures, proving that if they have enough good material and a long chronology they can date prehistoric desert wood.

Next, in the mid 1990s, Dean and others reviewed 1,664 previously collected samples, extracting 21 dates from prehistoric Sonoran desert sites for the first time. They dated samples from the Gibbon Springs and Whiptail Ruin sites (both near the University Indian Ruin), and from the Marana Mound site. The three sites were simultaneously occupied and abandoned by A.D. 1250.

Street (left) and Wright review collected wood that they hope will strengthen the Santa Catalinas tree-ring chronology.

Then in the late 1990s, Christopher Baisan strengthened a tree-ring chronology compiled by Henri Grissino-Mayer from southeastern Arizona's Pinaleno Mountains. This chronology now extends back to A.D. 882, and Street used it to date University Indian Ruin.

In another development, Ed Wright, a Tree-Ring Lab student who since has completed his doctorate, led wood-collecting expeditions to the Santa Catalina Mountains in the 1990s. Street is now analyzing these samples and attempting to extend and strengthen the Santa Catalina tree-ring chronology. If he succeeds, the chronology may help date more Hohokam sites.

"At the moment, this chronology is deep back to about 1400, just a little after where we need it to be," Street said. "Dating this material is still extremely difficult, but we expect further success at dating these sites in the future," Street added.

Copyright 2004 by Arizona Board of Regents