There is no particular rhyme or reason to the location of petroglyphs, they mostly take advantage of the type of rock surface. In the desert, some rocks very slowly become coated with manganese oxide, the black coating you can see on the rocks. Small traces of manganese are dissolved in rain water. By chipping away the black coating, the lighter color of the original rock shows through an makes a semi-permanent surface for drawing pictures. You could walk right by these petroglyphs and unless you were attuned to the desert, you would never see them. Petroglyphs give you a direct connection with the ancients who carved them into the rocks. I doubt my writing will last as long.