When a design is scratched through the surface, the original color of the rock is exposed. Naturally occurring water stains and lichen growth also add color. As time goes on, the exposed surfaces develop a patina, which is caused by the presence of black or brown mineral stains. The patina becomes darker as it ages, so that it is possible to determine more recent rock art by its lighter patina. Another method of dating is by examining the vertical placement of rock art styles on a cliff face or in a rock shelter.
Erosion happens from the top downwards, so the figures of later artists are lower.
It's also possible to date rock art by comparing styles with the figures used on pottery, clay pipes, or walls in Indian ruins. Or dates may be provided by the subjects of the art. For example, if a horse is represented, the rock art is from the 16th century or later, because Spaniards imported horses to the New World beginning in the 1500s. Some rock art even depicts specific historic events, for example the Spanish massacre of Navajos in an expedition into Canyon de Chelly in 1805. Pictographs are found most often in the mountains of southwestern New Mexico because of its predominantly light-colored rock.