SCIENCE : At Arizona’s New Art Center, the Pictures Frame the Museum : And the petroglyphs frame the questions, as researchers study ancient puzzles of culture, creativity and time.

They passed this way for thousands of years, pausing at a desolate hillside to leave their marks.

Crude drawings of four-legged animals and other creatures of their experience were pecked into the volcanic rocks of the Hedgpeth Hills for reasons that are not fully understood. Little is known about who they were or why they came. But the petroglyphs created over scores of generations shout one message quite clearly.

“The one thing we know about this place is that it was important for easily 1,000 years, and possibly for thousands of years,” says Peter H. Welsh, assistant professor of anthropology at Arizona State University and director of the university’s new Deer Valley Rock Art Center.

Standing at the foot of a hill that harbors at least 1,500 petroglyphs, Welsh adds: “If there’s a message to be learned, it’s that people were here. Right here. For thousands of years.”

No one knows for sure why they chose this spot. Perhaps a subterranean stream, now known as Skunk Creek, rose to the surface when it reached the hard rock formation of the Hedgpeth Hills, providing an oasis for weary travelers. For whatever reason, they chipped into the rocks along the hills.

The site now is being set aside so that others can try to understand more about those who came this way so long ago. The story is told partly in their artwork. The hundreds of petroglyphs that dot the hillside above the art center provide the centerpiece for a unique museum.

The center, which opened Dec. 10, is different in that it combines a public showcase of ancient rock art with a tightly focused research mission. Visitors can walk along a short trail to the foot of the hills, where petroglyphs can be seen in their natural setting.

The 7,000-square-foot center was designed by noted Phoenix architect Will Bruder to blend into the setting. It will provide scholars with access to archives and research papers dealing with the interpretation of these mysterious symbols chipped into the rocks.

Petroglyphs are not rare in the Southwest, but in most cases they are in hard-to-reach sites that require considerable dedication just to view them. Most of those that are more accessible have been vandalized and looted.

Except for an occasional bit of modern graffiti, the Hedgpeth Hills site has remained relatively untouched--despite the fact that sprawling subdivisions in Phoenix’s most rapidly growing area are now only a few hundred yards away.

“It’s an important site,” says Ken Hedges, a rock art scholar and chief curator of San Diego’s Museum of Man. “My feeling is that the bulk of the art is quite old.”

The site would undoubtedly have been lost had it not been for a series of devastating floods in the 1970s. That led to an extensive flood control project in north Phoenix, including the construction of the Adobe Dam, designed to channel floodwaters away from populated areas. During the construction of the earthen dam, the Army Corps of Engineers catalogued a number of petroglyphs.

Although archeologists and some locals had known of the petroglyphs, as evidenced by the date 1933 painted on one of the rocks, the richness of the site had not been fully appreciated until the corps began an inventory.

Archeologists counted 1,500 petroglyphs and identified a number of sites along the hills where prehistoric people quarried rock, Welsh says.

Some of the petroglyphs were believed to be the work of Hohokam Indians, known to occupy this region from about 700 to 1050, according to Welsh. A Hohokam pit house, found in the area where the dam eventually was constructed, was the only sign of occupation other than the petroglyphs. The fact that it was discovered in a flood control basin is fitting, since the Hohokams are known for the extensive irrigation systems they developed throughout this parched region.

But when archeologists studied the petroglyphs further, they made a surprising discovery, Welsh says. Some of the rock art is archaic in style, indicating it was created by clans that dated back to 5000 B.C.

“Archaic rock art in that part of Arizona is little studied and little understood,” Hedges says. “The archaic stuff tends to get overlooked partly because it’s older and more patinated and less visible.”

Welsh believes this particular site had been used by many clans over thousands of years. Thus the site was singled out by peoples of various cultural backgrounds, and they returned to it over and over again.

The rock art center was funded chiefly by the Corps of Engineers in keeping with federal laws designed to protect archeologically significant sites.

“We see our mission as being a gateway to the study of petroglyphs and rock art,” Welsh says, for both the general public and scholars.

That dual role makes the center somewhat unusual, although students of rock art in the Southwest might find the examples here a bit disappointing. Some of the petroglyphs have dimmed so with age that the rocks must be studied carefully even to see them. They are not as spectacular as those along the San Juan River near the Four Corners area, for example. But what sets these apart is their apparent age and their accessibility.

The rock art is subject to wide interpretation, and anthropologists emphasize that only the foolish try to be too explicit in their statements about the meanings of various designs. Among the most prominent are four-legged creatures that resemble deer or elk. Others include simple stick figures of humans, a wide range of quadrupeds, snakes and star-like objects.

Are the petroglyphs a form of ancient graffiti, or high art?

No one knows, Welsh says. What is clear is that this 47-acre reserve was so important that many chose to leave their mark. Perhaps they were establishing their claim on the territory, or perhaps they simply wanted others to know they had passed this way.

Petroglyphs have captured the imagination of the public, copied on everything from wallpaper to T-shirts, but they have been a source of frustration to archeologists.

Chief among the goals of any archeologist is the desire to establish the date when an artifact was created and thus what it can tell us about cultural evolution. But that has proved particularly difficult for petroglyphs.

Since they are literally chipped into stone, the only way to estimate their age has been through cultural association--and that can lead to gross errors and a wide range of interpretation. Thus no one is certain just how old the Hedgpeth Hills art really is.

“Without a clear means of getting dates, petroglyphs and rock art have not fallen into the mainstream of archeology,” says Welsh, former chief curator of Phoenix’s Heard Museum, a centerpiece in the study of Native American cultures.

But one researcher who is working with the new center may be onto a process that would yield better age estimates.

Ronald I. Dorn, an associate professor of geography at Arizona State University in nearby Tempe, is developing a technique that is based on independent scientific analysis, and it could move petroglyphs back into the mainstream of archeological research.

Exposed rocks are coated with something called desert varnish, a thin film that makes the outer surface of the rock darker than the rock itself. Unlike pictographs, where the image was painted on the rock, petroglyphs were made by chipping away the varnish with a harder rock, literally carving the image into the surface.

The varnish is caused by bacteria that grow on the rock, leaving a sticky substance that concentrates various elements from the rocks as well as from the blowing dust. Dorn is working on several techniques, the most direct of which involves extracting organic materials that accumulated on the petroglyph after the ancient artist chipped away the varnish to create the image. New varnish traps and preserves the organic material, which can be carbon dated.

“So we know the rock art has to be at least that old,” Dorn said. His results so far have been mixed, not always agreeing with well-established dates based on cultural association, but he is still refining his techniques. He can shed little light on the actual age of the Hedgpeth Hills artwork, but in time he may be able to provide a definitive answer, according to several experts familiar with his work.