Thrust Fault | ScienceBlogs
The law of superimposition says that stuff found on top is younger than stuff found lower down, in a geological or archaeological column. This is generally true, but there are exceptions, mostly trivial and easily understood. If a cave forms in a rock formation, the stuff that later ends up in that cave is younger in depositional age than the rock underneath which it rests (the rock in the roof of the cave, and above).
Source: scienceblogs.com