This restricts the contexts that can be dated to in situ features, such as hearths, kilns, moats, ditches etc.
The Earth's magnetic field also changes in terms of the strength/intensity of the field over time and can also be used to assign a date to a feature.
This includes most materials of geological origin, such as rocks and clay.
These differences in magnetic orientation, which can give us an accurate date, are compared to a known curve of the movement of the earth's magnetic pole.
This is not always a straightforward process - the curve sometimes doubles back on itself in a wave pattern reflecting the back and forth movements of the pole over time - therefore a sample can apparently give two or more separate dates, sometimes centuries apart.
This will results in a 'mixed' signal where two different positions of the Earth's magnetic field are recorded.
The types of features that record a TRM signal include hearths, kilns, ovens and furnaces.