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Welcome to Linville Gorge! Located in the Blue Ridge physiographic region in North Carolina, most of Linville Gorge is lightly metamorphosed sandstone.
Take a look at these rocks. This is a road cut that is exposed at the entrance to the park. Sandstones can be difficult to photograph, as much of the observations you make are on a microscopic scale, but:
What mineral would you say these sandstones are made of? Recall that we define the composition of a sandstone by the amount of quartz, feldspar, and lithics (rock fragments). It's difficult to determine how much of each is there without a thin section, but these sandstones are primarily one of the three constituents.
Would you describe these sandstones as finely bedded or massively bedded?
Because you're not holding these rocks, you can't actually do this, but what observations could you make about sand grains to help determine where they came from? If the grains were angular what would that tell you? What about if they were oblong or poorly-sorted?
These photos are taken from an outcrop, viewing the falls. The rocks we're standing on here are the same sandstones we saw in the road cut and that are alongside the falls.
Yes, you are at a distance here, but sometimes you don't have the opportunity to get up close to a rock. Even if you were here in person, you're still unable to get close to the falls. Actually, doing this virtually at least allows you to zoom in on some of the photos. We already told you that these are sandstones. From a distance, what are some observations that stand out to you and tell you that this is a sandstone? What could tell you that this is the same unit you saw above?
These photos were taken right above the falls:
Take a look at these rocks and make observations. We'll then talk about how what you see relates to the geologic history of the area.
If you've finished making observations and doing the best rock description you can (we know its hard with just photos) let's talk about them!
To start with, you were asked what mineral they were made of, and hopefully you saw quartz. The colorless shine on the right-hand lens photo should have been your hint. Minerals are always difficult to identify from photos, but we're sure you did a great job!
Next, did you think that they were massively or finely bedded? There's not a right answer to this one! How you view this is based on the distance (scale) you look at them from. From afar, they look as though they are fairly massive. Up close, however, they could be described as having a fine bedding
Lastly, what interpretations were you able to draw from the grains? Generally, angular grains indicate that they were deposited near the source (where they were eroded from), or are from glacial deposits (but there are other signs for that!). Oblong grains indicate the amount of textural maturity (see Figure 18 in your field guide), as does the amount of sorting.
This may look like a really, really scary diagram, but trust us — you can handle it! We'll talk you through this in the order of mineralogy, petrology, and finally sedimentary geology.
By the time we finish, you'll know how to use this diagram!
In mineralogy, you learn how to use a basic ternary diagram and how to apply that to specific minerals (such as the plagioclase k-spar diagram). If you have not taken have mineralogy, please proceed with caution! If you want to know it and you don't experience in mineralogy, please reach out to us! We'd love to talk you through it during our session time. Stop 10 has a more basic ternary diagram. If you would prefer, you can look at the steps explained in that stop before jumping into this one. If you'd prefer not to jump ahead, below is a detailed explanation of how this works.
Let's break this down:
Here we have the basic, first ternary diagram. See — it's not that bad! At the top, we have 100% quartz (yellow). On the left, we have 100% feldspar (green). And on the right, we have 100% rock fragments.
So what do you do if you observe a mineral that is not listed here? You should include it in your initial estimate, and then you normalize it out (though, other than calcite, it's unlikely you'll see many additional minerals).
Not bad at all, right? The different quadrants (which are labeled) define different rocks. If it's >95% quartz, it is a quartz arenite. If it's >75% quartz, and has more feldspar than lithics, it's a subarkose. If it has <75% quartz, it's an arkose.
Now, in petrology, we have 3D ternary diagrams that form a pyramid. This is a 3D ternary diagram that forms a triangular prism. So, let's talk about that z-axis.
Looking at this z-axis, this tells us is that the rock names are defined by 4 characteristics. We saw above, the quartz, feldspar and lithics. So, on the z-axis, we have % matrix.
Matrix is the tiny grains that are too small for the mineral to be determined, even in thin section. Generally speaking, the rule is that the components of a matrix are less than 30 microns in diameter.
The amount of matrix in a rock tells you which ternary diagram you should look at. If it has ≤15% matrix, look at the closest ternary diagram to determine what kind of arenite it is. If it has 15- 75% matrix, look at the ternary in the middle to determine what kind of wacke it is. Finally, if it has greater than 75% matrix, it's a mudrock.
Putting this all together, you can use the amount of matrix present to determine which diagram you look at and what type of rock it generally is. Then, you can further name the rock by estimating its percent quartz, feldspar, and lithics.
Next, how do you determine from a distance that a rock is a sandstone? What we want to try to show you here is that even in the field, you don't always have the ability to walk right up to a rock. Sometimes you have to figure things out from a distance. How could you tell that these are, at the very least, sedimentary rocks?
Do you see beds? Beds are layers formed by the horizontal deposition of grains.
Left: An example of bedding in shales from Big Bend National Park, TX. These beds are obvious, but there was still clear bedding in the photos above!
2. Do you see any crossbedding? Crossbedding is formed by the deposition of grains as they're deposited on a certain side of a dune (either a sand dune, like in a desert, or a ripple that could form in moving water). They can be used to determine the direction of paleocurrents.
Left: Cross-bedding at Tallulah Falls in Georgia.
3. Can you see the ripple marks? Ripple marks show the preservation of fluvial sediment structures. Like crossbedding, they can also be used to determine the direction of paleocurrents. (Cross-bedding is really just ripple marks that are cut vertically)
Left: Modern-day ripple marks that are left behind as the tide goes out on Jekyll Island, GA.
Now let's talk a little bit about what’s going on in this area — especially as it connects to our next stop. The rocks exposed here (and north, in the Grandfather mountain suite) are ~700 Ma sandstones, while the rocks just south of here are Grenville in age (~1.2 Ga). These older, southern rocks are sitting stratigraphically higher than the younger sandstones. So how does that happen?
As shown in the diagram to the left, we have Unit C somehow brought to the level of Unit A. This happens because of thrust faulting. When continents collide, not only do rocks bend (fold) to release the stress, but they also break (fault). Thrust faults are when large amounts of crust are pushed up onto other crust, which causes younger rocks to be put on top of older rocks
This area is home to the Linville Falls thrust belt, which is a series of thrust faults that formed during the Alleghenian orogeny. If you look closely, you may actually be able to see one of the faults, as it cuts through this area.
There you have it! You now have a brief introduction to sedimentary rocks, as well as your first dose of the geologic history of the Grandfather Mountain Window!
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