What it means
Stand at any trailhead in the Preserve and look at the boulders. The granite is pale, but the faces longest in the sun are a deep brown-black, as if stained. That is desert varnish, and when a local says a rock is “varnished” they mean it has been sitting there, undisturbed, for a very long time. Petroglyphs are what happens when people notice the same thing. Peck through the dark skin with a harder stone and the pale rock underneath shows through, and a design made that way can last for thousands of years.
Around Scottsdale the two words travel together. The McDowell Sonoran Preserve and the county’s McDowell Mountain Regional Park hold dozens of sites, most made by the people archaeologists call the Hohokam, some by far older hunter-gatherers, and some by the Yavapai, who lived in these mountains into the 1800s. The ancient desert era chapter carries the long story; this page is about the two words.
Where you’ll meet it
On Conservancy-led hikes, where stewards point out varnish and will not tell you where the glyphs are. At Brown’s Ranch, whose rock shelter has been used since Archaic times. And on every north Scottsdale hillside, once you know to look.
Where the word came from
Nobody has claimed the term “desert varnish.” It is a plain description that geologists and travelers were using for the shiny coating on desert rock long before anyone knew what it was, and the literature still uses it alongside the drier “rock varnish.” The argument about what it is has not finished. It is mostly clay, blown in on the wind and stuck to the rock, cemented by oxides of manganese and iron. Black varnish contains 50 to 60 times the manganese found in ordinary crustal rock; orange varnish is manganese-poor and iron-rich. Whether bacteria concentrate the manganese, as many researchers have argued, or whether the chemistry works on its own, as a 2008 microscopy study proposed, is still debated. What is not debated is the pace: micrometers per thousand years, on surfaces that stay dry, stable and unbroken.
“Petroglyph” is from the Greek for rock and carving, distinct from a pictograph, which is painted. The McDowells have petroglyphs, made by pecking with a hammerstone, incising with a sharp edge, or abrading a pecked shape smooth.
The local numbers come from a volunteer project. In 2006 and 2007 members of the Arizona Rock Art Coalition approached the city and spent about 600 hours walking the mountains, recording 42 petroglyph sites in the Preserve and the county park. Their forms and photographs now sit in the city’s database. The coalition sorted the designs by the standard Southwestern style names: Western Archaic (roughly 5000 BC to 200 AD) for the grids, rakes, chained circles and rows of dots; Hohokam, or Gila style (700 to 1100 AD), for the spirals, bull’s-eyes, lizards, birds and stick figures; and a Yavapai style (1500 to 1700 AD) that is hard to separate from the rest. The surveyors also proposed a local sub-style: about 50 abstract human figures shaped, in their words, like a beer mug with two curved handles, concentrated at three larger sites and possibly dating to a reoccupation after 1150.
What it is not
Desert varnish is not paint, lichen or burn scarring, and a petroglyph is not a pictograph. The cowboy initials and dates cut into McDowell boulders after 1800 are rock art of a kind, but they are the historic layer, not the ancient one.
The fine print
Dating rock art is harder than dating a pit house, because there is nothing organic to test. The McDowell inventory leaned on two indirect clues: style, and re-patination, the slow regrowth of varnish inside the pecked lines. A glyph that has darkened almost back to the color of its background is old; one still bright is recent. That is why touching matters: skin oil darkens the exposed rock and speeds the fade, and a few seasons of hands can undo centuries of weather. The National Park Service’s rule at Saguaro National Park applies here too: take photographs, leave footprints, nothing else.
Why the McDowells? The canal builders who watered the land Scottsdale now covers used these mountains for hunting, gathering and, apparently, marking. Agricultural features and geometric and animal petroglyphs turn up across the range, and the Brown’s Ranch rock shelter was used from Archaic times through the Hohokam and Yavapai periods. What the designs meant is unknown. The city’s presentation lists the candidates honestly: boundaries, maps, clan signs, hunting scenes, solstice markers, ceremony, visions, or doodling.
The record has holes. Forty-two is the count of sites the volunteers reached, not a total, and later surveys suggest hundreds of panels remain unrecorded. Locations are withheld on purpose. To read glyphs in the open, go to the Deer Valley Petroglyph Preserve, run by Arizona State University in north Phoenix: more than 1,500 of them on one hillside, with a museum attached. In the McDowells the varnish is the exhibit, and the art is left to the patient.