Blue Jay Point

Welcome to Blue Jay Point! It's in the Piedmont Physiographic region, on the Wake County side of Durham, NC.

This is a pretty exciting stop! It’s a mere 5-minute drive from where I grew up, and I came here every weekend when I was little, all the while not knowing what joys were a few meters away.

This is truly a meta-ultramafic terrain at its finest.

Geologists split igneous rocks into 4 groups: felsic, intermediate, mafic, and ultramafic. We are able to associate these 4 groups with different origins (the rock's provenance): felsic rocks = continental crust, mafic rocks = oceanic crust, and ultramafic rocks = the mantle. If we see an ultramafic rock in North Carolina, then we know that the mantle somehow made it to the surface.

As an area experiences continental collision, it starts with oceanic crust subducting, pulling the continents closer together before the collision itself takes place. When the continents get really close, they can pinch some of the oceanic crust and underlying, forcing them up on to the surface. These form ophiolite sequences. Ophiolites are sections of oceanic crust and mantle with a specific order that are exposed on land. The most famous of these are in Oman.

What we see here in NC hasn't been officially classified as an ophiolite sequence (it's very weathered and discontinuous); it is called the Falls Lake Mélange. However, there are several signs that point to the possibility that outcrop is an ophiolite, including the presence of a sequence of columnar basalt about ~5 miles away.

Note that none of these pictures do these rocks and minerals justice. Unfortunately, the camera just doesn't match up to the human eye, but these rocks really are awesome nonetheless.

Let's set the scene. To get to where we’re going, you can either park in the park itself, or along this road pull off, but a word of caution: It's scary to get back on the road from the pull off. It's right next to a bridge and is a blind curve from the other direction; you’ve been warned.

From the parking lot, you'll want to go to the area by the visitor’s center and follow a trail with white dots. There are many very short trails, so not much time is lost if you take the wrong one. It will quickly dump you out at the lake (which is worth visiting if you have the time).

We’re no longer in the mountains here — which means our chances of coming across a nice road cut are slim — so where do we look for rocks? Stream valleys! In physical geology, you learn the Rule of V’s, and how streams will cut back into the surrounding rock, leaving it exposed. In many of our stops along the Piedmont, you’ll notice most of them are in valleys, such as the one below.

The rocks that we see here and above are not meta-ultramafics; instead, they're the surrounding biotite-muscovite schists. But take a look at these photos; you might be able to spot the quartz vein (it's difficult to see since all the rocks are wet).

In the stream valley, we get a great view of the schist that makes up most of this area, but when we get to the top of a hill (not a large climb — say 20 feet higher), instead of seeing cobbles of schist, we see cobbles of quartz, and plagioclase.

An image of the schist. Those shinny grains are mica!

One of the many quartz/plagioclase cobbles found at the top of the hill.

An example of the schist found in the stream bed.

Now, you were previously cautioned about making assumptions about detached rocks — and yes, they have to be approached carefully. But in this case, their abundance and size, both on and off the trail, lends them to be good enough to make this point.

So, if you have an outcrop of schist that's been intruded by quartz veins, that undergoes erosion and weathering, why would the quartz be what’s left behind? (You may want to check out Figure 19 in your field guide.)

Let’s now talk about these 360° views. If you look at this, compared to the two other streams showed before, you should notice that the shape and amount of exposed rocks make the stream a different shape. We’re no longer in the schist and have moved into the meta-ultramafic terrain. What are some reasons that a stream cutting through meta-ultramafic rocks would be wider, more down cut (i.e., have less of a gradient at this point), and have fewer exposed rocks than one that cuts through a quartz-intruded schist?

There's not a single right answer; there are several possible reasons, just think about it.

To the right are photos of what we can find in the field in this area. If it's hard to see things in these photos, don't worry! There are some clean samples in better lighting below.

Unrelated to geology, but still interesting: in one photo, you can see a downed tree. In Florida, trees are typically blown over by hurricane winds. In North Carolina, most downed trees are the result of ice storms. When freezing rain falls, it instantly freezes wherever it lands (like on branches and leaves, etc.). As this ice accumulates, it becomes extremely heavy, and if there is any prior lean to the tree, the heavy ice can easily tip them over. Sometimes, roots will remain in the ground and, rather than dying, the tree can turn its upward facing branches into trunks, all stemming from the same base. Nature sure is neat!

Okay! Let’s try our hand at identifying some of these very cool and somewhat unusual minerals!

This is difficult to do with a just photo, so you'll be provided with as much information as possible.

It's definitely worth your while to get used to how you might see these minerals in the field. It is important to note that these are extremely well-developed mineral samples — usually they won’t look anywhere near as nice. But even now, these aren’t the perfectly hexagonal quartz crystals that you’re asked to ID in mineralogy lab.

One final note: The pictures make the colors almost always seem to appear distorted. Please trust the color written, even if it looks different than what the image shows.

Mineral 1:

Color: Greenish-grey (it looks blue in these photos, but it’s not)

Luster: Vitreous

Hardness: scratches glass; quartz scratches it

Habit: Bladed, fibrous

Steak: Colorless

Cleavage: Prismatic

Fracture: Splintery

Density: Not unusually dense

Magnetism: Not magnetic

Listed here are examples of this mineral in two different rocks