(1) A hoodoo hat, or...
(2) As used by trail builders, this term means: A rock or course (layer) of rocks laid as the finishing touch on top of a structure such as a stone retaining wall. This layer may consist of larger stones than those below, and may overhang them a bit, simulating a railing if this layer is carefully made and continuous, or those stones may be just a collection of rocks sitting on top.
Cap rock as used by recreational geologists: In recreational geology a cap rock is one that is sitting on top of another rock just for your amusement.
Sometimes things really do work out this way. The rocks involved in this sort of relationship aren't any two ordinary rocks that happen to find themselves together. No indeed. They are special. In fact the thing that is being sat upon may not be a rock at all. How crazy is that?
But the cap rock is always a rock, and it's always on top, like a cap.
Thing as we use it here to denote
the thing that is being sat upon is a general term, but not too general. This thing is a thing that can stand on its own, so it's got some backbone, and if it is stone then it has lots of backbone, but not too much, because.
Because normally the cap rock is a hard stone, in the sense that it doesn't weather quickly, while the stuff under it (which might be only a firm sort of soil) is relatively susceptible to weathering. And that there is your secret.
This relationship is one that blossoms only in dry climates which see episodes of infrequent but sometimes intense weathering. The cap rock was once loafing around innocently like any ordinary rock on the surface, and the surface was one great piece of level land, and then along comes time. Lots and lots of time. Ever so much time.
And time brings its close friend weathering. After much time elapses what we see is what's left of the landscape. All of the level land that went on almost forever, remember that? Well it weathers away and washes down the drain, and the only part left is that rock and the stuff under it that the rock has protected from the elements.
This one piece of landscape that's left often takes the form of a pillar wearing a cap. The cap rock is that cap. No one actually knows what
recreational geology is, so take this all with a pinch of dust.
Cap rock as used by petroleum geologists: In grownup, boring geology a caprock (i.e., our familiar little cap rock) is an impervious layer of rock that lies over an oil or gas deposit deep underground.
This is important because the caprock keeps all that oil and gas from shooting upward and getting all over your nice clothes if you're an urbanite, or your cows if you're a rancher, or into your bowl of noodles if you're a backpacker who finally got a chance to sit down and prepare lunch.
Keep in mind that caprocks are under a lot of pressure, even though technically they sit on top of the pressure rather than actually being under it. Figure of speech. But pressure. It's pressure all the same. A lot of pressure. You have no idea how much pressure. Only baffling quantities of complicated numbers can even come close to describing it.
In fact it's only because this is all deep underground and there are thousands and thousands of feet of rock and dirt and groundwater and accumulated gopher poop above the caprock layer that the whole arrangement doesn't spontaneously explode and blow itself all over the landscape.
That's where you come in.
See, all these layers got laid down little by little over eons and it was only later, long after all these random layers drifted in that all the ratty old dead dinosaurs and creepy fallen giant tree ferns and big and little things with and without eyes or feelers or chlorophyll gradually squished together under it all in a big mooshy mess and had their juices pool and ferment into oil and gas and built up insane amounts of bubbly pressure.
This is dirt after all, along with a few rocks, and none of it was designed to work in a high pressure environment so generally it's pretty fragile, and all it takes is one idiot on his day off, stomping around and yelling or even playing his radio too loud and that might be enough, just that one little extra tickle of vibration, it might be just enough to set the whole thing off, so be respectfully quiet out there, walk softly, and chew those noodles carefully.
Innocent bystander spotted along the Maah Daah Hey trail.