Petroglyphs, Lighting, and Magnetism
The passage of a lightning discharge over the surface of a rock is shown to produce two roughly parallel loci of maximum anomalous magnetization, having opposite polarities, marking the edges of the discharge path. For an electron flow from sky to earth, the north-seeking end of a compass needle is attracted to the right-hand locus. Perpendicular to the path, the magnetization varies like that of the field induced in and around a long, straight cylindrical conductor by the flow of an electrical current. Magnetization due to lightning has been observed on the surfaces of rocks in the Providence Mountains of southern California. Petroglyphs on one of these rocks appear to be related to the magnetic anomalies. The present study suggests that a lodestone may have been used to detect and mark certain of these anomalies, rather than observing the actual lightning strikes or their physical traces on the surfaces of the rock.