LICHEN OVERGROWTH  --  The lichen grows over both the light and dark surfaces of this rock.  We know that the petroglyph was made before the lichen grew over the top of it.  Lichen grows slowly, but predictably.  By measuring the diameter of lichen clumps we can arrive at a minimum age for the petroglyph.

CONTENT ANALYSIS  --  This petroglyph depicts two men about to launch arrows at each other.  Because the bow and arrow began to be widely used in the Americas about 1,500 B. P. we recognize this image as being no older than the bow and arrow.