It has long been noted that on some rock art panels , particularly those
employing duct/tails that the tails either join up with other motifs or end up in a crack or fissure . Andy Jones took this a step furthr with his notes on some Kilmartin panels and their superimopsition and juxtaposition . Ethnography shows that shaman in various cultures believed that access to the spirits /ancestors / etc can be gained by cracks in rocks hence another aspect to the geology .Whether the engravers believed the natural markings cracks etc. were made by the ancestors /gods is another point too.
That the natural markings have an impact on some design is not doubted .