Historical Geology/Cross-cutting relationships
Besides the principles already explained, we can determine the relative ages of various geological features by studying the way they intersect with one another: their cross-cutting relationships. In this article I shall explain how this can be done.
The principle of cross-cutting relationships
The principle of cross-cutting relationships may be stated as follows: when one geological feature cuts through another, the former is the younger and the latter is the older of the two features.
For example, consider the diagram to the right.
The brown sedimentary rocks (A) must be older than the dike (B) that cuts through the strata; the dike must be older than the erosional surface which truncates it (C) which is older (by the principle of superposition) than the gray sedimentary rocks (D) which overlie it; this rock must be older than the dike (E) cutting through the gray strata; and then this dike must be older than the fault which cuts through it (F).
One exception might occur to you: what if we have an outcrop of rock which is then covered over by sediment? The outcrop will cut through the resulting sedimentary strata, but be the older of the two. Whether or not this is really an exception to the principle depends on how you look at it; but if we consider it to be an exception, it is a recognizable exception, for the outcrop would be weathered and eroded where it formerly projected above the surface, and from this erosional surface we would therefore be able to see that it was the older component of an unconformity.
How do we know?
The reasoning behind these conclusions is actualism of the most straightforward sort. Is it really necessary to argue that an erosional surface is younger than the eroded rock, or that a fault is younger than the faulted rock? This is a matter as much of logic as of physics, for it is simply to assert that the surface of a thing cannot precede the thing.
Why must a dike or similar intrusion be younger than the rock into which it intrudes? Because a sheet of molten rock would not stand up by itself: where the magma forming a dike pierced through to the surface, then it would not go on up into the air building a dike, but rather it would ooze along the ground creating a lava flow.
Hence we can indeed use these cross-cutting relations to establish the relative ages of geological features.