Magnetic variation that is detectable within soils can often determine the nature and extent of past human activity. At British latitudes, the earth's magnetic field is approximately 50,000 nT - nanotesla is the SI unit of magnetic flux, used in gradiometry to measure magnetic variation in relation to the Earth's magnetic field.
The randomly orientated magnetic fields of ferrous compounds produce minimal magnetic variation in their natural state. However, topsoil fills of ditches and pits tend to increase soil depths, and hence magnetic strengths, relative to surrounding soils.
The application of high temperatures can enhance the magnetic properties of the ferrous components of soils. Clay, which is particularly rich in ferrous oxides, can be fired to produce ceramic materials such as brick, tile and pottery.
These, and the kilns used to produce them, can be readily detected by magnetic prospection.
40ha+ Iron Age/RB