I’ve touched on paleomagnetism a little bit before, both as a technique for tying rocks in to the geologic timescale, and as something which can be found by using a fluxgate magnetometer. It’s a pretty interesting set of techniques and uses some cool science tools, so I thought I’d explain a little bit more.
Magnetism from the Earth’s magnetic field can be retained by individual layers of rocks, at least under some circumstances. If you have a bunch of layers stacked on top of each other like pancakes, the different layers (beds) can have different magnetic directions.
As you might expect, the equipment needed to make sensitive measurements of the magnetic field are not particularly portable (and may be a topic for another post). Samples need to be collected in the field and brought back to the lab, and the sample orientation must be marked and recorded in such a way that the measured magnetic field can be related back to the magnetic field in the rock itself.*
To do that, paleomagnetists (or paleomagicians) will drill a small (1″ diameter by a few inches long) annular hole into the rock, leaving a plug of rock in the center. That will become the sample. Before it can be removed from the hole, a mark is made on the top of the plug with a brass rod. The direction of the hole is determined with a compass (or a sun compass when conditions allow), as is the angle away from vertical of the core (the hade).
When the plug is freed from the rock, the down-hole direction is marked with arrows along the mark using a permanent marker. The samples (several from each bed) are then placed into sample bags, labelled appropriately, and carefully transported back to the lab.
Are you irresistibly attracted to such a magnetic field of study? This is probably the best place to go for more information, and is freely accessible online.
 Tauxe, L., Banerjee, S.K., Butler, R.F. and van der Voo R, Essentials of Paleomagnetism, 3rd Web Edition, 2014. [accessed Aug. 27, 2015]
* The field magnetic field?