So just what is an angular unconformity?
Well paddle on over here to the posted coordinates and take a look at a prime example of one. Makes sense yet? If not, then let me explain a few things that might help.
Technically speaking, “An angular unconformity is an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers.”
Yea right, that helps, doesn’t it.
A little more help would likely be useful; instead of some geologic gobbley gook about strata and discordance and whatever else was mentioned above. Hey, you’re looking at one if you are at the coordinates, and it a pretty cool thing to look at. So let’s try a more basic way of explaining what took place for this impressive rock formation here along Blue Creek.
First, a few rules or laws about sedimentary rocks. (The rocks on this cliff you are looking at are sedimentary, nothing igneous or metamorphic here)
1) The Principle of Superposition.
“Any sedimentary succession that has not been overturned, the oldest strata occur on the bottom.”
Okay, in words we can fully understand; if the whole rock cliff you are looking as has not been turned upside down, the oldest rock layers are at the bottom and the youngest are on top. Not too hard but important to what happened to form this angular unconformity. Here, the rock cliff has not been turned upside down; what you are looking at is right side up, so the rocks near the water are the old ones and the rocks at the top of the cliff are the youngest ones.
I knew you could handle that, so on to the next law;
2) The Principle of Original Horizontality.
“Sedimentary layers are originally deposited as horizontal sheets.”
Okay now, for a displaced Canuck that has learned to enjoy the most awesome pleasure of a hot humid afternoon, a quenching sip of sweet iced tea, I can create for my fellow southerners a mental image that demonstrates this law. Think of those accommodating souls that brew up unsweetened tea, trying to be kind to those who cannot partake of the true and proper way of sipping their iced tea sweetened while still warm from brewing with an overdose of sugar); and then we, the true connoisseurs of this refreshment, are forced to sweeten for ourselves an already chilled glass of tea. When you add the required spoonfuls of sugar to the chilled tea, all does not dissolve. Some settles out on the bottom of the sweating glass; look closely and you can see it laying over the bottom of the glass. Notice that is has settled flat or horizontal across the glass; there are no hills or valleys of dissolved sugar, just a flat layer across the bottom. That my friends is sugar deposited as horizontal sheets.
Okay, now for the third rule,
3) The principle of original lateral continuity.
“Sedimentary layers are originally deposited as laterally continuous sheets that naturally terminate against basin margins or barriers.”
Now that one is a mouthful of words that one has to study a bit to understand. But hey, how about another mental image that would help you to understand.
When it comes to family gatherings in the south, a basic and never violated social law is that food is provided; no exceptions. And when this happens, there is a special person assigned to making the layered Jell-O desert. Usually it is someone’s blue haired aunt; but hey, she knows how to make layered Jell-O desert (If she is really good she knows how to include some fruit layers as well). 
Now picture someone’s blue haired aunt, who is depicted above, making this concoction in her kitchen. Into the Jell-O mold goes the first mixed and colored liquid, often green Jell-O. This emerald green mixture is poured in and it fills the mold to a certain depth; the top surface of the Jell-O is flat or horizontal, and the liquid conforms to the shape of the mold; which are the barriers or margins in geology. After is gells, another layer of Jell-O liquid is poured into the mold (I prefer yellow next) it too extends out to the edges of the mold with the top surface of the yellow Jell-O (no that has a ring to it) flat or horizontal. This continues until the mold is full or the cook tires of mixing Jell-O. End result is layers of Jell-O in the mold all of which are flat on top and all extend out to the edge of the mold. Someone’s blue haired aunt has managed to create a kid popular desert for dinner on the grounds while inadvertently assisting in a lesson on geology of the canyon walls of Blue Creek.
Many millions of years ago, this area where you now float in Blue Creek, was once the bottom of a body of water. Sediment suspended in the water was dropped out, the grains of sand falling to the bottom and forming horizontal layers. Layer upon layer of sediment accumulated, each layer on top of the older layer under it.
After the layers of sediment had been deposited there was an event of uplifting that pushed the rocks around and tilted the layers of strata. How you may ask, did these rocks get pushed around? Simply put, tectonic forces … and the subject matter of another lesson from another earthcache. For now, accept it; tectonic forces. The result being that the nice horizontal and level layers of sediment that formed the sandstone were pushed and moved so that they ended up oriented differently; the layers were no longer level.
Here our sandstone tilted. And it dipped. The tilt is easy to see as you sit in your boat and stare at the cliff. For a point of reference, the surface of the water that is floating your boat is level. Compare the water surface level with that of the layers of rock. Pay attention, test question here.
Not quite as noticeable as the tilt is the dip of the strata. To get a good perspective of the dip, get close to the rock face in a place where you can observe the top of the exposed tilting layer. Notice than not only does it tilt along the edge of the creek, it also dips relative to the water. One side of the top layer of rock, that is either the side closest to the water or the side farther away from the water, is higher than the other. This is the dip.
Now after the tectonic forces had had their way with the layers of sandstone, getting out of their sedimentary comfort zone of being flat and level, the erosive forces of nature worked on the exposed surface. With the passage of time, a whole bunch of time, these erosive forces removed some of the exposed sandstone. This area was once again submersed, and the process of sedimentation started again. As before, horizontal layers of sediment were deposited on the bottom of a body of water, and as before these layers built up and over time formed sandstone.
Remember, back at the beginning of this narrative the subject of this whole earthcache is an angular unconformity. This exposed rock face is that angular unconformity, in particular the joint between the lower tilted and dipping rock and the upper horizontal layers of rock.
So to get credit for visiting this earth, drop me, the CO, and email answering these questions:
1. Are the older layers of rock, those being the ones closest to the water level, tilted so that the upstream side is higher or lower than the downstream side?
2. Estimate how many feet above the water level is the angular unconformity? That is where the horizontal and tilted layers of rock meet.
3. There is a large piece of the cliff that has cracked and shifted away from the face of the cliff. Look at the layers of rock in both the main cliff and in the separated section. Do the layers of strata line up; that is, has the separated section settled relative to the main cliff.
4. Why do you think we can see this angular unconformity here?
