Winding Stair Gap

Welcome to the first real stop of your virtual field trip! Can't you just feel that damp, humid, cold mountain air, after it rained for two weeks?

Here we have metamorphism at its finest: Winding Stair Gap! It's located in the Blue Ridge physiographic province, near Franklin, NC.

Winding Stair Gap exhibits very high-grade metamorphism. It’s been dated to 460-450 Ma, attributing it to the Taconic orogeny.

Take look around this map, and zoom out to get a feel of where this is located.

Take a look around. You can scroll and move the 360° photos, but most of what you should observe here is visible in the hand sample photos, and the ones taken with my poor-quality microscope below.

Once you think you've finished looking around, you should be able to answer these questions (I'll then explain them at the bottom of the page). Feel free to click and see them as you're ready!

What minerals do you see? You should be able to name 5. There is an additional 6th one, but these aren't the best samples, so we'll tell you there is sillimanite in there too.
Are those minerals grouped in any way?
After you name the minerals, using your field guide, what metamorphic zones are present?
Based on the minerals present, what temperature/pressure conditions did these rocks form under?

(I strongly recommend looking at your field guide!)

Here we have what we call a road cut. As roads are built through mountains, it's often easier for engineers to blast open those mountains — and we geologists get to reap the benefits! Those long straight vertical lines you see are residual from this blasting process, and not a feature of the rocks themselves.

The measurements on this board are in inches:

The 6 images below are all 1.5cm x 2.5cm

Let's take a look together, and discuss what we see! Please expand the text below as you're ready.

Bowen's Reaction Series:

Here we have Bowen's Reaction Series. We classify the light minerals (the ones lower on the series) as felsic. While we classify the dark minerals, the higher ones on the series, as mafic.

You should notice that in some of the samples, the minerals seem to be grouped into light-colored sections (Lecuosome), and dark-colored sections (Melanosome). This alignment of like-minerals is a product of metamorphism. The size of the banding here makes these sections gneissic, and should give you a hint that we're looking at some sort of high-grade metamorphism.

What mineral is this?

Circled in green is biotite. We can recognize biotite by its dark color (differentiating it from muscovite) and it's platy habit.

The white minerals are likely quartz.

Deformation feature:

Here, you can notice the compositional banding — making this at least a gneiss. If you look at the band in the center, you may notice that there are thicker sections and thinner sections. This feature is called a boudin (or boudinage), and we'll discuss it more at later stops.

Three more Minerals:

Circled in red are garnets. Circled in blue are individual quartz grains. Circled in yellow are little pieces of biotite, and there is also some feldspars.

What Mineral is this?

A very large, square garnet (roughly 1 in x 1 in).

What Mineral is this?

Here you can see some pyroxenes.

Based on the minerals identified above and this diagram from your field guide (to the right), we should be able to determine which are metamorphic and define the zones.

You should see the minerals you named above, and find the areas that they overlap — this tells us that we're looking at high grade rocks.

So, with our knowledge of the minerals and a general estimate of the conditions it formed under, we can start to figure out its metamorphic history.

We know that the minerals present require a high temperature and pressure.

Based on this, we can estimate these rocks fall somewhere between either the high temperature side of the amphibolite facies, or in the granulite facies.

I'll go ahead and tell you that these rocks have been studied and determined to have formed in the granulite facies, and we geology students are able to figure that out from just looking at pictures of them and using our diagrams!

There were no thin sections required, no drawing skills requiring technology. The “microscope” images given to you are the same mineral assemblages you could see with a hand lens. The point being is that you can 100% do this in the field. All that it required was making observations, and having the right diagrams with you (which, are provided for you in your field guide: that will be with you on all trips). You just did field geology!

Stop 3

You've now learned to make "micro"-scale observations. Ready for something new? Let's go to Stop 3!

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