animals interacting with esoteric circles and spirals. They've been attributed to the mysterious Hohokam or "those who have gone," a title applied collectively to a Native American culture that virtually disappeared around the 15th century. The images are assumed to closely represent the people who left them, yet after much study, nobody can be sure they actually mean anything.

Experts have speculated over religion, ceremony, medicine and historical record as influences, but as suggested before, the figures may just be the result of boredom. Pointlessly chipping away at rocks for hours seems a little gratuitous, though, when there are chores to be done. Like finding food in the desert, for example.

On the other hand, the petroglyphs' meaning might be hidden right in the shadows. Before his death in 2004, retired technical photographer and Tucson resident Nile Root believed he had quite possibly unlocked Signal Hill's secret. After studying the symbols throughout the last years of his life, he came to the conclusion that the puzzling pile of rocks was an elaborate timepiece.

concentric circles connected by radiating lines, appears to act as a sundial. When Root realized the boulder on which it's engraved leans at 32 degrees, the site's latitude, he placed a small stone in the inner circle. The subsequent shadow interacted with the lines to track seasons and upcoming equinoxes; it disappears at exactly midday.

The most prominent symbol, a large spiral etched into Signal Hill's tallest stone, appears to link with the site in multiple ways. At noon on the equinoxes, shadow divides it in half. At sunrise on the summer solstice, a pointed silhouette meets the spiral's center just as the rest of it becomes bathed in light. On the winter solstice, if one stands at the center stone in a small, circular clearing, the spiral aligns precisely with the sunrise. Additional icons appear to correlate with the spiral and aid in the tracking of lunar events, as well.

These phenomena all would be virtually imperceptible to the chance viewer or to someone focused on interpreting the glyphs as allegorical characters. Root himself acknowledged it was all just speculation. Seeing his photos of everything working together, though, it's pretty convincing and makes you wonder if the Hohokam were actually not just tapping out doodles, after all.

To see the mechanism in action, you can view Nile Root's Web site, which has been preserved at www.niler.com.

Weird Arizona