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In the previous "Stop", one of the first things we talk about is scale. You've now seen a stop (Winding Stair Gap) that focused on hand-sample (mesoscopic) and microscopic observations. We're now going to work on looking at outcrop-scale observations (macroscopic) and comparing them on a regional scale.
Neither of these scales are any less or more important than the other. Ideally, you need to be able to keep them all in mind to create a cohesive understanding. In this folder, you'll find way too many stops as we make our way up the NC section of the Blue Ridge Parkway.
That's right — we're doing a mapping exercise that stretches the entire length of the state!
At each stop, look at the photos, you can make any outcrop scale, hand-sample scale, or microscale observations you want. You are provided with more information than you could ever need. But, in the end figure out how to name that rock based on the descriptions below.
There's nothing fancy going on here (on a regional scale) and we’re assuming constant elevation.
Here is the blank map we're going to be filling out.
Each stop is labeled with a letter. You'll look at the photos for each stop and based on the units described, you will decide which rock corresponds to which letter. We'll walk you through it, not to worry!
As you go through this, you might say, "Gosh, why are there so many stops, is this really necessary??" Please trust me! Everything that is shown to you has a point, even if you don't know what it is...yet! We are not just dumping a million photos on you for nothing. Keep in mind, this is well over 100 miles you're mapping, and we are showing you 14 outcrops.... Yes, those 14 outcrops are important!
So what rocks we might (wink, wink) see:
These are the descriptions of them given on the state geologic map, as well as an image of what they could look like:
Coweeta Group; Purple
Biotite Gneiss: 1600-541 Ma, Migmatitic; interlayered and gradational with biotite-garnet gneiss and amphibolite; locally abundant quartz and alumino-silicates.
Let’s go through what this description is telling us. In stop one, we talked about what a gneiss is (Compositional banding). When it says, "Quartz and alumino-silicates", that’s the white band in the photo to the right. The white is the quartz, and if you look right in the center, you can see a piece of muscovite (which is an alumino-silicate). All of this is telling you that this unit is going to be a grey, high grade rock (gneiss to migmatite), with white veins running through it.
Ashe Metamorphic Suite and Tallulah Falls Formation: Pink
720-686 Ma, Ashe Metamorphic Suite consists of mixed assemblage of clastic sediments and metavolcanic rocks. In Sandymush and Canton quads, dominant rock types include two-mica schist, metagraywacke, and two-mica gneiss; minor rock types include amphibolite and calc-silicate granofels.
There's a lot of words here! But we should be able to easily break this down. We're going to ignore the first sentence of the description for now (we'll come back to it at a future stop). But the second part is just telling us that well see a schist, a metagraywacke, and a gneiss in this unit (We'll be mapping them all as one unit). You should also be able to tell from the two photos that this unit tends to be reddish)
An example of the gneiss
An example of the schist
Alligator Back Formation: Blue
717.45-669 Ma, Light-gray, medium- to coarse-grained porphyroblastic garnet-mica gneiss; contains interbeds of dark-gray graphitic mica schist, calc-gneiss, mica gneiss, feldspathic quartzite with blue quartz granule beds, and garnet-hornblende schist. Has a strong foliation.
So what this description tells us, is that this unit is going to be mostly gneiss, with some possible schist. I'll also tell you, that blue is going to be a common defining feature of this unit.
Here are a few general tips for mapping. Even though this is a introductory, virtual mapping exercise, these tips still apply, and they are generally good to keep in mind as you do more mapping assignments (courtesy of Nikita):
Have a plan of attack, figure out what you will be doing for the first 5 min, for the next 30 min, to the last hour.
When you map, be sure to mark exactly what you see on the page. You can use geologic principles (cross cutting relationships, horizontality) to aid in understanding of what you are mapping, but save the interpretations for when you’re out of the field! Whatever you see in front of you is there.
Establish your scale: understanding your scale is half the battle, otherwise you will get lost and confused. You will have points on your map that you can easily identify.
Remember what type of rock you are dealing with, as this will help you understand the structure of what you are trying to map.
Record all of your thoughts and include informed speculations; these show your brain was working in the field and you were not just on autopilot — this is what the examiners will be looking for. Even if this means sitting for a few minutes, jotting down your thoughts, and not getting moving with the mapping, this could be extremely beneficial. Trust us, you won't remember those numbers, what that symbol means, or an unmarked picture after you leave an area. Label everything clearly!
Let's Start!
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