ABSTRACT
Iron constitutes about 6 per cent of the Earth’s crust, but little of it is readily apparent. Most of it is dispersed through soils, clays and rocks as chemical compounds which are very weakly magnetic. Man’s activities in the past have redistributed some of these compounds and changed others into more magnetic forms, creating telltale patterns of anomalies in the Earth’s magnetic field, invisible to a compass but detectable with sensitive magnetometers.
