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).
One of the coolest examples of what seems to be (but really is not) a violation of this Law of Geology is a thrust fault. A thrust fault is essentially a horizontal fault (as opposed to the more common vertical fault) in which rock from one area slides completely over another area. When this happens, the rock at the base of the upper unit (the one that slid over the other rock) is older than the rock on which it rests.
The fault isn’t really horizontal. but it’s horizontal enough for this to happen. And, in fact, the whole thrust-faulting thing is actually fairly complicated, and there are different processes that cause a similar effect. But in the end, you get an older layer sitting on top of a younger layer.
The reason this is not really a violation of superimposition is this: The older rock was actually deposited on top of the younger rock later in time than the formation of the younger rock. In a way, this is not much different than an ancient mountain made of ancient stuff eroding and generating sand that flows downstream and covers some pre-existing sediment. The fact that the grains of sand were formed a long time ago does not make that recently formed sand deposit old. It is young. But it is a young deposit made of old stuff. A thrust fault is the same thing but instead of there being a zillion tiny grains of sand deposited on some earlier sediment, it’s all one big giant piece!
Here’s a couple of photographs of the Keystone Thrust fault. Tell me if it looks familiar to you:
The dark grayish rock is the older rock thrusted upon the red and yellow rock.
If you go to Las Vegas and gaze westwards you’ll see a ridge in the distance. Some people say that part of the ridge resembles a man lying on his back. That man’s outline is the upper edge of this thrust fault, which is exposed here in Red Rock Canyon reserve, a BLM property where you can go and hike, climb rocks, and observe interesting geology and wildlife.
Technically,the Keystone is a Reverse Fault with a shallow dip. The gray rock is Cambrian limestone (Bonanza King Formation) and it rests on top of the Jurassic Aztec Sandstone that gives Red Rock Canyon its name.
People do argue that the thing in this photograph is actually part of different thrust fault system related to the Keystone. I’m not sure if that argument is settled yet. Either way, it’s a good example of a thrust fault.