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Welcome to Umstead Park!
Umstead Park is located on the Northwest side of Raleigh, in the Piedmont.
In the previous section, we had a bit of a break from our geologic observations, but we're going to ease back into that with some focused sections at this stop.
"The geologic history of Umstead:
Umstead park is located towards the eastern edge of the Carolina Terrane. The Carolina terrane (formerly known as the Carolina slate belt) is a major geological region in central North Carolina. It consists of lightly metamorphosed volcanic and related igneous and sedimentary rocks of Late Proterozoic to Early Paleozoic age, or approximately 620-520 million years old. These rocks were formed as part of an ancient volcanic arc on the far side of an ocean that no longer exists. Later, when earth movements caused that ocean to close, the volcanic arc collided with ancient North America, helping to form the Appalachian mountain chain.
Big Lake – Raven Rock schist
The Big Lake – Raven Rock schist is an extensive rock unit within the easternmost part of the Carolina terrane. It is named for exposures near Big Lake in Umstead State Park and others at Raven Rock to the south in Harnett County. This rock originated mainly as dacitic tuff, formed by explosive volcanic eruptions. These eruptions involved varying amounts of crystals, rock fragments, and ash that were blown into the air and accumulated on the flanks of volcanoes. Lithic tuff features rock fragments, crystal tuff features phenocrysts, or crystals, of quartz and feldspar; crystal-lithic tuff has both.
These volcanic features have been somewhat obscured by later geological events, namely deformation and metamorphism that came as the result of the Appalachian mountain-building events later in the Paleozoic era. This modified the volcanic rocks into metamorphic phyllite and schist with the growth and alignment of the platy minerals mica and chlorite. The direction of the lined-up minerals is the foliation of the metamorphic rock.
At Oak Rock, the foliation is steeply dipping, nearly vertical. As you explore the park’s trails, you will quickly see that the foliation is not consistent. The reason for the variation is that these rocks have been bent into folds by the collision between plates that formed the Appalachians.
If you look closely, and ignore the lichen on the rock, you may see patches of the rock that have slightly differing color. These are the flattened rock fragments of this metamorphosed lithic tuff."
(From "NC Greenways Geology" )
Take a look around! Not every picture shows something unique —some are just there to give another perspective. When you’re out in the field, you're attention won't always be drawn to something special; you'll have to figure out what observations are important. Once you've explored, scroll down and you'll find some questions.
View 1: Just a view to show the start of the trail. Our stop is 0.25 miles down it.
View 2: And here we are! 0.25 miles down the trail. Start taking a look at the rocks in the creek.
View 3: Some rocks to the right of the bridge shown before.
View 4: An area further to the right of the initial bridge.
View 5:
View 6:
View 7:
View 8: This first image is from one of the veins.
The two on the right are close-ups of the host rock.
Note that all images are 1.5cm x 2.5cm.
Done exploring?
Did you notice a clear cleavage plane? Do you think this is from bedding (sedimentary)? Jointing (rocks breaking)? Foliation (metamorphism)? Has tilting taken place?
If you wanted to take a strike/dip, which face would you use? Look at the photos in View 5. What are you taking a strike and dip of?
Did you notice the veins? Do they follow the other cleavage plane (the plane the rock breaks on) you noticed?
It's difficult to determine from pictures, but looking at the rock up close (View 7), what kind of rock do you think this is? Igneous, metamorphic, or sedimentary? How can you tell?
Based on the mineral grains that you can see, what would you name this rock as? (Look at size and shape. See your field guide for the needed diagrams!)
Let's take a look at this together!
It can be difficult to distinguish sedimentary rocks from volcaniclastics, and it's even more difficult to do from pictures. But, generally speaking, your best bet is to look for glass. If you can find glass, you know you’re dealing with a volcanic rock and not a sedimentary unit. When volcanos erupt, in addition to spewing crystals and rock fragments, they also spew some of the liquid magma itself. Being a liquid magma, it's at least several hundred degrees Celsius, and after being thrown into surface level temperatures, it cools very rapidly. This rapid cooling doesn't allow for crystals to form, and we get glass (such as obsidian). While it's unlikely you'll see an inch-big glob of glass, if you notice a shiny reflection, you can generally start to see little bits of glass with a hand lens.
Let's start by determining what kind of rock we’re looking at. At the beginning, you were told that we’re looking at volcanic rocks in the Carolina Terrain. Now, let’s try to name that rock. Looking particularly at the photos in Views 7 and 8, you should be able to see clearly defined crystals.
In the photo to the left, quartz grains are outlined in green, plagioclase in red, and k-spar in yellow, while a mixture of metamorphosed tuff matrix is outlined in blue. Note that the minerals are clastic and cemented together by this tuff mixture.
Now we look at our ternary diagram of tufts. This rock doesn’t have much glass or lithic fragments, so we’d call this a crystal tuff (a volcanic tuff with large crystals present).
These ternary diagrams are read just like the previous ones we have looked at on this trip, they just have different characteristics that we use to define them. To reach the determination stated above, we used to diagram to the right.
Next, let's discuss the tilted cleavage you were seeing. If we look at a close-up of the rocks, you can see an alignment of biotite grains. This biotite grew during its metamorphic phase, and thus reflects some of the stress that it was under, defining a foliation plane along the X-Y axis.
Consider the rotation of clays that we talked about in your field guide. We know that clays (in this case, biotite) will rotate to be perpendicular to the direction of the greatest force. Thus, if we know which direction the grains align, we know in which direction it was being squished.
In addition to showing up on the individual mineral grain level, we see this same foliation reflected in the tilted cleavage planes in the stream bed. The cleavage planes are highlighted with different colors.
The foliations of individual mineral grains occur on a microscopic scale, while the tilted cleavage planes of the outcrop occur on a macroscopic scale. If you are able to identify signs of this foliation on a hand sample, it would be an mesoscopic observation.
As previously mentioned, this foliation is not consistent throughout the park, but it is fairly consistent in the areas shown in the photos. The strike/dip of the northernmost point shown here was measured to be 225°/31° NE, while the strike/dip at the southernmost point is 220°/29° NE. The two locations of where these measurements were taken are about 200 meters apart.
Looking at the left photo, which side should you take a strike and dip of?
The red one! The red plane is the one along the foliation. It’s a little difficult to see in the photo; look back at the 360° photo in View 5. The blue plane is the result of jointing and erosion. When taking a strike/dip, make sure that you are actually measuring a bed or foliation, and not a cut face!
Let’s make some observations of what you see in the photo to the left.
Highlighted in red is a quartz vein (trending 027°).
The black lines mark the foliation. You may notice that the observed trend doesn't follow the previous stated strike (225°). Keep in mind what a strike is: the line made when a horizontal plane intersects a dipping plane. Since what we're looking at isn't a horizontal or even surface, the projection of the strike/trend is going to appear distorted.
In green are joints. They're fun to look at, and fun to measure, but don't really tell us much. To be clear, a fault is a fracture that has displacement along it, resulting from the stresses applied to the rock. A joint is a localized, tensile fracture, which is not the same as a fault. The surfaces of tensile fractures move away from each other, while the surfaces in faults move past each other The surfaces of tensile fractures move apart in the direction of least resistance (the weakest applied force).
Who would have thought NC had volcanic rocks. I told you the piedmont wouldn't disappoint! Let's see some more.
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