Iron poisoning from running water is extremely unlikely you're probably getting confused about Ilkley spa where punters go to get an excess of the stuff . If local stream and springs were a health hazard they would be pointed out and known , not engraved on a rock . How do orientate yourself with this “map “ where is the “you are here “ point ? What about all the other marked rocks in the area some within a matter of metres also with cups how do you know which is the right rock and are they all telling of potential danger . You mentioned comparing the rock with a map could you point out how this worked either a pic of the diagram of the springs or pointing to an online source of local maps with a similar configuration of springs to markings ? The 1:25000 scale map has no springs recorded and the nearest stream has cup and ring marked rocks even closer to it . There would have been no problem for the people who engraved the “Tree of life “ rock finding fresh water iron rich or otherwise .