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Welcome to Grandfather Mountain!
There are three points marked on the map:
1. Boone Fork Parking area. This is where we parked to begin our hike.
2. Grandfather Mountain: This is the peak we hiked to.
3. Mile High Swinging Bridge: This is at the official entrance that you see as you drive in.
We're going to take a look at how the geology between these areas change, so you should get a feel for how they're geographically related.
Grandfather Mountain is a private park, meaning it's fairly expensive for a broke college student to enter. While I do not recommend this, you can park on the Blue Ridge Parkway (Boone Fork), and backpack in (backpack- not hike. It's long). It's a beautiful trail and the rocks are very unique for this area, but it is a difficult area to backpack. Here's what we saw as we backpacked in, before we got to the park.
Grandfather Mountain has all sorts of sedimentary and metasedimentary rocks. There is everything from shale, to greywacke, to sandstone, to conglomerate with cobbles the size of my head. On this side of the mountain, we tend to see a smaller grain size, with less of the larger ones.
In the previous 2 stops, we talked about what grain shape can tell you. Here, we finally have clasts large enough that their shape is observable!
Are the clasts round or angular?
Are they well-sorted (meaning, does one rock have grains that are all the same size)?
Are the rocks clast-supported or matrix-supported (do the cobbles touch, or is there matrix between every one of them)?
There are a few quartz veins pointed out. We also noticed these at the previous stop. And at the first stop, we talked about how Bowen’s Reaction Series plays into metamorphism. Think about how the geologic history from the previous stop might tie into a hydrate melt and metamorphism.
Below: If you drove in, you'd see the Split Rock and Sphinx Rock:
Below: A view from the parking lot, between the Mile High Bridge and the trail that leads to the peak.
At this point, you've had practice making observations, without making many interpretations. Because everything you see here is related to an observation you made at a previous stop, we're going to briefly shift our focus from observations to interpretations. Please read the comments on the photos (if you put your mouse over the caption you can read the whole thing), where I'll point out an observation and ask you for an interpretation. If you have questions, PLEASE ASK. These stops are meant to build on one another to target specific skills, but if you get lost early on, you're not going to have a chance with the more complicated things, so let us help you.
Take a look at the bedding you see here. Did it form at that angle?
These two photos are just to give you a sense of what the trail looks like. Yes, there are many ropes you get use to pull yourself up (and they're even more fun when the ground is wet).
,At this point, we're getting very close to the top of the mountain!
Here are some photos from the peak!
Here are some photos from as we hiked a different trail down towards the Mile High Bridge:
Finished exploring? Great!
Even if you weren't able to answer the questions, by making these observations and starting to think about how things form, you are learning and making progress. After you're introduced to some of this material in a class (because they will give you a much deeper explanation of things) you can come back here for a real field example!
Let's talk through some of what we saw.
Back at Linville Falls, we talked about how thrust faults had led to the stratigraphic position of the Grandfather Mountain suite. The name commonly given to this area is the Grandfather Mountain Window, or Fenster (German for window). A window, or fenster, refers to a gap in the thrust fault exposing the rocks in the footwall.
This means (if you remember from the previous stop) that the rocks exposed here at Grandfather Mountain are younger than the rocks that have been thrust up.
You saw a lot of photos of different conglomerates and we asked you to make observations of the clasts and interpretations about how they formed. Let's do one together.
Observations:
The clasts are sub-angular to sub-rounded (this is a specific range; not just made up words). Generally, the clasts don't touch, instead it's matrix supported). Most of the grains aren't very spherical, they're oblong. As for size, most of them are cobbles, but there are some pebbles, which make it moderately sorted. If you were doing a sedimentary description in the field, you would also want to describe the composition (they're lithic fragments), the porosity (if you put water on them, how long does it take for the water to soak into the rock [these are not porous]), and the cement (the glue that's holding the rock together; generally, cement is quartz, calcite, or hematite).
Interpretations:
Large clasts imply a high-energy depositional environment, while small grains imply a low-energy depositional environment.
When you think of low-energy environments, think of slow moving water, far away from mountains. Mountains are the source of water, and their height creates a higher energy stream. Thus, low-energy environments can be lakes, far out in the ocean, and slow moving rivers in the Piedmont and coastal areas. Note that anywhere you have mud is being deposited is a low energy environment.
By comparison, high-energy environments are areas of fast moving water/wind. This could be a beach or dune area, a rock slide in the mountains, or a steep mountain stream.
If you see different clasts deposited on top of one another, think about how that could happen now. What would sediment deposited in a sudden flood look like? What would happen as a lake dried up?
There are so many sedimentary environments, almost all of which we can view modern-day equivalents of. Use what you know about environments you can actively observe to figure out where sedimentary rocks formed (you've been to the beach, the ocean is salty, and beach gets a lot of salt blown onto it: i.e., sandstones that form on beaches are cemented by calcite).
Here we see cross-bedding in a metasandstone (the cross-beds are highlighted in blue). Think about a modern-day, shifting sand dune. As sand grains are blown to the top, they roll down, and accumulate along the base of the sand dune. If you look at the photo above, you can see how the bottom of the cross-beds is dragged out. This tells us that the top of the cross beds is still the top today (they haven't been overturned). Also, because we can see the bottom of the dune, on the far side, building outwards, we know that the current had to be blowing from left to right (in the image above).
Tilted beds form from either brittle or ductile deformation.
To the right there are a few diagrams drawn to show you how tilting could happen. These are not exclusive. There are so so many ways that tiling could happen. There are multiple different ways thrust faults alone could induce tilting. These are just a few examples to help you visualize it.
Some sort of faulting has taken place to give us beds that dip like this.
Highlighted in blue, is the linear feature. If it was a planar feature (like a foliation, or bedding) we would see the lines continue on the other face, to the right of the back line.
Throughout Linville
Gorge and Grandfather Mountain we see a lot of these quartz veins. Some are parallel like these, while others seem to intersect at a random angle. We know that even the youngest of these rocks are 700 million years old. There have been several orogenic events since their formation. The combination of all of these factors has led to the abundance of these secondary crystallization features.
If you have a large number of silicic rocks (quartz rich) that are buried, they start to undergo burial metamorphism. As a part of this, whether it's from orogenic events, fault-related, or just the weathering of the rocks themselves, we get these silicic fluids that move around and reprecipitate along any pre-existing faults or joints.
Congratulations! Time to move on to the last stop of our seds section!
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