Taking rock away, or adding colour
Translate the two words and you have the difference. Petroglyph joins the Greek petra, rock, and glyphein, to carve. The image is made by removing stone: a sharp flake incises a narrow line, repeated cuts deepen it, a hammerstone pecks out a figure one dent at a time, and rubbing smooths a patch into a shallow field. The fresh surface is usually paler than the weathered skin of the rock, and that contrast is what you read.
Pictograph joins the Latin pictus, painted, and the Greek graphein, to draw. Here colour is added: ochre for reds and yellows, charcoal or manganese for black, put on with fingers, pads, brushes or a spray. Paint sits on the surface, so it lasts best in caves and under overhangs, out of the rain. The details of how painters prepared their colours changed from site to site.
Three checks you can do on site
First, the light. Step to one side of the panel and let low light cross the surface: grooves and pits throw tiny shadows, a painted line stays flat and shows its colour best in soft, even light.
Second, the texture. A pecked figure is built from hundreds of small dents, a bit like a newspaper photo made of dots, while paint has soft or brushed edges and sometimes visible grains of pigment.
Third, the overlaps, which give you the order of events. Paint lying over a groove came later; a groove cutting through paint came after the painting. Many panels mix both, so describe what you see before deciding what it means, and keep your hands off the rock while you do it, because skin oils darken stone and lift pigment.
Desert varnish, a suntan on the rock
In deserts, exposed rock slowly grows a dark skin called desert varnish: a thin layer of clay rich in manganese and iron oxides, laid down over a very long time with the help of rain and bacteria. Peck through it and the paler stone underneath shows, which is how most petroglyphs of the American Southwest were made.
Newspaper Rock, in Utah, is the textbook case. More than 650 figures crowd a single sandstone face, added over perhaps 2,000 years by Ancestral Puebloan and Fremont people, then by Ute and Navajo artists, and finally by settlers. The varnish keeps growing inside the marks, so a pale, crisp figure is usually younger than a dark, soft one. Usually: dust, lichen and later scratching can even things out.
Paint ages differently. It goes first at exposed edges and survives in hollows; in caves, a skin of calcite can veil a painting in milky white.
Dating: easy for paint, hard for grooves
Charcoal can be dated by radiocarbon, as at Chauvet, where black drawings go back about 37,000 to 33,500 years. A calcite crust that grew over a painting can be dated by uranium series, which gives a minimum age: the red disk at El Castillo in Spain is at least 40,800 years old, a hand stencil on Muna Island in Indonesia at least 67,800. Not everyone trusts the oldest results. In 2026 Georges Sauvet argued that uranium can leak out of the crust and make it look too old; the method’s supporters answer that careful sampling avoids the problem.
A groove contains nothing to date. Researchers work from the varnish inside it, from the style, or, with luck, from earth that later buried the rock. That luck struck at Fariseu, in Portugal’s Côa Valley, where an engraved rock face turned up under layers left by people; a date of about 18,300 years for those layers gives the engravings above them a minimum age.
Which motifs get carved, and which painted?
Technique nudges the subject. Straight and repeated lines suit a hammerstone, so spirals, grids and ladders turn up often as petroglyphs, along with cupmarks, small round hollows pecked into the stone. Paint handles tone and soft edges, so hand stencils, shaded animals and fields of dots are usually pictographs. Both families, though, include almost every motif somewhere.
Photographing without harm
For petroglyphs, wait for low sun early or late in the day. For pictographs, an overcast sky keeps the colours honest. Never chalk, wet or trace a panel, use flash only where it is allowed, and follow the rules of the local and Indigenous communities who look after many sites, including their requests not to post the exact location of fragile ones.
Sources
- Bureau of Land Management, “Newspaper Rock” visitor brochure (Monticello Field Office, Utah).
- D. S. Whitley (ed.), Handbook of Rock Art Research (AltaMira Press, 2001).
- T. Aubry et al., studies of the Fariseu rock and its buried engravings, Côa Valley (Fundação Côa Parque).
- A. Quiles et al., “A high-precision chronological model for the decorated Upper Paleolithic cave of Chauvet-Pont d’Arc”, PNAS 113 (2016).
- A. W. G. Pike et al., “U-series dating of Paleolithic art in 11 caves in Spain”, Science 336 (2012); A. A. Oktaviana et al., Nature (2026).
- G. Sauvet, critique of uranium-thorium dates (2026), reported in Archaeology Magazine, 26 May 2026.
