Raven Ridge

Welcome Raven Ridge, one of the greatest places to hike! This is a section of the NC Mountain-to-Sea Trail that begins just off of Ravens Ridge Road. This stop is in the Piedmont physiographic region, and is in North Raleigh, NC, towards Durham.

Ravensridge intro.mp4

Ravensridge-MtS.mp4

While you're here, let’s talk a little bit about some of the differences between the Blue Ridge and the Piedmont physiographic regions. In the Blue Ridge, at Stop 1, you were introduced to road cuts. They are a geologist’s dream, where we literally have a mountain blasted open and a fresh face (a non-weathered surface) exposed. But, in the Piedmont, the chance of us having a rock big enough to blast open is slim. So, where do geologists look for exposed rock in the Piedmont? Stream valleys!

Streams don't just flow over a surface — they actively down cut (this is why we have the Rule of V's!). This down-cutting can be through the many feet of thick Carolina clay, or even into rock itself. Unless you've got a very, very, young stream (within a human lifetime), there’s a good chance that you have some sort of rock exposed in the stream valley.

Ravensridge 2.mp4

ravensridge flooded.mp4

Unfortunately, it was a little too flooded the first trip to see some of the better outcrops, so a second trip was made to get some pictures. (Another luxury of virtual field trips! During in-person field trips, there is rarely a chance to go back to a stop if nature doesn't allow it at the planned time.)

Without further delay, here is some pretty awesome structural geology. Take a look at the 360° photos and see what you notice. Then, take a look at the following pictures of some interesting geology. Once you've jotted down your observations in your field book, we'll look at some photos (taken with a Brunton to show direction) and talk about what's going on here.

Things that stood out:

ravensridge outcrop1.mp4

RavensRidge(ice).mp4

If you've taken a look around, feel free to read on. Here are some more pictures with a Brunton, and we'll discuss what the directions of these features tell us.

Here are some photos taken on a different day with a Brunton. Unfortunately, it was a muddy day and some of the features are much less clear, so refer to the above pictures if needed!

Also, keep in mind that in all pictures, the Brunton is oriented so that the arm points north (the needle may not look like it, but that’s because it's on an uneven surface)

Let’s discuss what we're seeing and what it can tell us, starting with this photo on the right:

First, note that these are deformed dikes. A dike is an igneous intrusion (a little, vertical body of magma). In this case, they are felsic dikes, but you can absolutely have mafic dikes too. There are also quartz veins in this outcrop, but the intrusions shown in the picture to the right are dikes.

The deformation of these dikes is called boudinage (boudin is a type of sausage in French). It is an extensional feature. If you were to take a roll of Play-Doh and start to pull it apart, it would stretch and thin more in some parts, while others would thin less, or not at all. That's happened to the amphibolite layer here.

Ravensridge last.mp4

Ravensridge boudins.mp4

If we look at the neck of the boudinage (outlined in blue), its orientation tells us information about which way it's being pulled.

Here is another example of boudinage, with the necks outlines in blue. We can see that they're pretty much orientated the same at the one above (which is what we would expect, as this isn't a very large outcrop).

Below: Here you can see the foliation of the rock. Comparing this with the orientation of the boudin necks above, you can see they're in a similar direction. This tells us that the stresses that deformed the dikes, also formed the most recent foliation on the country rocks (basement rocks).

Above: Here we see two tension gashes. Notice how on both ends they rotate to the same direction as the boudin necks. (You'll learn about tension gashes in structural geology. If you want to know more about them, ask in our live session!)

Left: Notice the little 'gashes'. They're all facing the same direction, but don't connect (yet). We call this en echelon (which means steps of a ladder in French) and we can observe this same behavior in faults on a regional scale.

Above: Here we have a rotating grain, like we talked about at Stop 8.

Ravensridge last real.mp4

Hardfreeze ravensridge.mp4

You made it to the end of our structural topic!

Topic 4: An Overview of Igneous Geology in the Field

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