Oriented paleomagnetic core samples are collected in the field ; mudstones , siltstones , and very fine - grained sandstones are the preferred lithologies .
The magnetic grains are finer and more likely to orient with the ambient field during deposition .
Oriented paleomagnetic core samples are collected in the field ; mudstones , siltstones , and very fine - grained sandstones are the preferred lithologies because the magnetic grains are finer and more likely to orient with the ambient field during deposition .