Why are archaeological soils magnetic?
Oxidation and reduction of iron rich minerals form ferri-magnetic iron oxides in the topsoil. Burning generally causes this process but natural inorganic and bacterial processes also contribute. In the case of ditches that are cut into the subsoil, they become filled with the more magnetic topsoil. This added with any burnt occupational material, which may have been dumped into the ditch over time, makes them more magnetic than the surrounding soil and detectable as magnetic anomalies. Burning is a key factor in the detection of archaeological occupational remains. When a material is burnt magnetic grains in iron rich minerals move freely and align with the ambient magnetic field. As the minerals cool down the grains become “fixed” in the same direction as the ambient field, thus becoming magnetic, this is known as the Le Borgne effect. These minerals are ferri – magnetic. Some archaeological features, such as hearths and kilns, will have sustained high temperatures during use and are
