As soon as patina is removed, bacteria start to create patina again on the new surface.
Roughly, we can say that darker petroglyphs are older, because they have been exposed longer to formation of patina.
But this works only roughly, because surfaces exposed in different directions and to different weather will form patina at different rates. Though they most often use mineral pigments, they may use an organic “binder” to attach the pigment to the rock surface so it will stay there for a long time.
There can be different interpretations of the relations of the phenomena.
The Linnaean system for setting up divisions in biology is an ordering that was only later found to be in accord with biological evolution.
For this reason classifications can be only a preliminary step in the study of variables, for they cannot deal elegantly with transitional situations in which variables are to be expected.
The more gradual the change, the fewer are the distinctive features upon which to define natural classes and the more difficult it becomes to draw a line between classes. When the problem is simply that of ordering unconditioned phenomena, it is difficult to distinguish typologies from classifications.
Stone is not directly datable in the time frame of human history. It is created by bacteria living on the rock and attaching clay particles to the rock.