[Nataku X]

I've forgotten everything I learned in Geology 101, but I think that some rocks carry a "signature" of the Earth's magnetic field from the time that they were formed. Do a Google search on palaeomagnetism. Basically, the Earth's magnetic field will go through periods of strengthening-weakening and also will reverse their polarity.

[FactQuest]

paleomagnetism

Thanks, that's what I was looking for.