A technique for the location of archaeological features adapted from techniques used in geological surveying. It is based on the fact that features with thermo-remanent magnetism, like hearths or kilns, or features with a high humus content, like pits or ditches, and iron objects, distort the earth's magnetic field from the normal. Instruments such as the proton magnetometer or the differential fluxgate gradiometer are used to measure those disturbances, and by plotting the results, a map of the features can be built. The ways in which the different types of feature distort the magnetic field vary, though they can all be picked up on the same instrument. Hematite or magnetic, present in most clays, have a small magnetic effect when unburnt, since the grains point in random directions and cancel each other out. Once heated to about 700? C or more, the grains line up, increasing the magnetic effect and causing an anomaly in the magnetic field. This thermo-remanent magnetism is also the basis for magnetic dating. The presence of modern iron as in wire fences can cause problems with this technique of location; if the area to be surveyed is clearly crossed with power lines or fenced with iron posts, a resistivity survey may be more suitable. The method of surveying used requires a grid to be measured out on the site and readings to be taken at regular intervals. The nature of the site may prevent such a grid being laid out, for instance if it is heavily wooded, and magnetic survey may not be possible on these sites. It is one of the most commonly used geophysical surveying methods.