Me'n Zogg have been pondering this problem ever since I heard a lecture -- the informal kind -- about geomagnetic dating.
The banjo player that I regularly jam with is archeology professor at nearby ND. As we were driving back from a three-hour jam, he started telling me the concept behind this kind of dating.
He thought I'd be interested because I'm a potter and one of the things that comes up when discussing this kind of dating is the possibility of dating fire pits.
Seems that the iron particles are "freed" up when heated enough. When this happens, they point toward wherever magnetic north may happen to be at the time.
Thus, he said, if one could be aware of the orientation of one of my pots as it sat in the kiln, one could deduce by this manner of dating, when the pot was made.