Rocks that Talk

You've probably heard of nature preserves, mountain preserves and animal preserves, but did you know there are rock preserves?  Preserved rocks are not your average rock specimens, but rather special stones etched with images known as petroglyphs that once served as canvases for expressing and preserving ideas.  Due to the enduring properties of rock, these ancient works of art can still be found in arid regions all over the world, but can be easily missed if you haven't got your best tracking eyes wide open.

At first glance, petroglyphs appear to be rock paintings with a lighter medium brushed over a dark substrate.  In reality, light rock is hidden under a dark outer layer known as desert varnish.  The dark varnish is a combination of clay, dust, and other minerals cemented to the rock by bacteria as it slowly builds up on the lighter surface of volcanic and sedimentary rocks.  Desert varnish can be black, brown or reddish, depending on mineral content. Interestingly, scientists still do not understand exactly how it forms.


Ancient people pecked, scraped and pounded pictures into the hard surface, revealing the lighter colored rock just below the varnish.  After surviving thousands of years of weathering, we still have this cryptic symbol language dotting caves, hillsides and shorelines. They are truly treasures and cause us to wonder about the role of rocks in the past.

In the Phoenix area, one collection of over 1,500 petroglyphs can still be viewed undisturbed on the rocky hillside just south of the Adobe Dam at the Deer Valley Petroglyph Preserve. Visitors learn about the history of the area and the people who spent time there over the centuries, then proceed to the actual rock strewn hillside where dramatic images appear to glow from the jumble of huge basalt boulders. When you go, plan enough time to stand before the rocks from different angles and let your eyes adjust to the ancient language before you.

Wonder Questions

• Are all the rocks in the preserve the same color?  Do they have the same amount of shine?
• What makes these rocks good for creating pictures?
• What pictures do you see?
• What do you think they mean?
• Are some petroglyphs older than others?  How can you tell?
• Do you think the same person created all petroglyphs in this area? 
• If you could scratch a picture into desert varnish, what would you create?

References and Resources
Deer Valley Petroglyph Preserve
National Park Service - Desert Varnish
Live Science - Mysterious Desert Varnish

Images - Sharon Pegany