The third principle is that of cross-cutting discontinuities.
This means that if we have horizontal beds, and
then they get traversed by a lava dyke or a fault,
then the bed must be older than the event that led to this cross-cutting.
So for instance, here is a satellite image, and
you can see some brightly colored rocks on the surface here,
and then a similar set of brightly colored rocks over there.
And we recognize that there is a fault here that has displaced one
section of this towards the north, relative to this other section.
So again, using the principle of cross-cutting discontinuities,
we recognize that this, these sediments,
these rocks must have been laid down first, and then the fault
led to the northward relative movement of one to the other.