This past week, I stayed in Arlington, Virginia. My dad and stepmom were in London, and I was looking after my youngest siblings (both teenagers) by staying at dad's house and serving as the Responsible Adult. It's the same house I grew up in, and it has a lot of nice memories associated with it. At the end of the street, there's a little trail which leads off into the woods and downhill towards the Potomac River along a little creek called Windy Run. ("Windy" as in the weather, not as in sinuous, though it is that, too.) At the bottom, Windy Run launches off a waterfall and tumbles down into the Potomac Gorge. On Saturday morning, I decided to go take a hike down to Windy Run and reacquaint myself with the landscape and its rocks. Here's the view from the top of the waterfall looking across the river into D.C.


Here's a view of the waterfall from the side. The big ice-rimed log at the base is about a foot and a half in diameter, to give a sense of scale:

On the way down the trail, there lies a big boulder of quartzite. This is my first rock. By that, I mean that this specific boulder is the first time I learned to put a name to a chunk of the Earth: my dad taught me that it was quartz, and I committed the name to memory. Today I would note that it's milky quartz, indicating hydrothermal deposition. (Tiny inclusions of water in the crystal lattice scatter incoming light and make it appear white.) Its upper surface is covered in black lichen. Pondering it anew on Saturday, I wondered if learning the name of this boulder in the late 1970s was the first step leading to me towards my ultimate career as a geologist. Lens cap is 5 cm in diameter.


My "first rock" lies at the base of a hill, below a linear trail of other quartz boulders. This array likely represents a subterranean vein of hydrothermal quartz, a common feature in the Virginia Piedmont.


For instance, here's a big vein of hydrothermal quartz (center) cutting across the metagraywacke host rocks at the top of the Windy Run waterfall. It's about a foot wide, and emplaced at a ~20 degree angle to the regional foliation (which strikes ~N25E). The quartz vein is oriented approximately vertically, just east of true north.


Here's some more vein quartz in the metagraywacke matrix. Foliation runs approximately left-right across this image. Note how there are large bodies of milky quartz arrayed semi-parallel to foliation: these are probably best interpreted as boudins: the results when a tabular vein of quartz was broken into chunks, and these chunks were smeared out along along the foliation during mountain-building. Boudinage (the process of producing boudins) is a somewhat brittle behavior (breaking) and somewhat ductile (smearing): under the proper combination of high temperature and directed pressure, quartz can act like pizza dough. It's capable of being molded, but also capable of separating into coherent pieces. We call these "boudins" because they resemble sausages strung out in a row ("boudin" is French for sausage). Here, only one boudin is shown, but click here for some other examples. The boudin is about 3 cm in thickness, to give a sense of scale.

There are also smaller quartz-imbued veins (white arrows, extended with dashed lines) in this rock, cutting across foliation at nearly right angles. Note how the "infusion" of quartz along these thin fractures makes them more resistant to weathering (they stand up in high relief, as seen in the lower left). This set of small quartz veins was likely emplaced at the same time the rock was being squeezed during mountain building, for reasons I explain in the next photograph.

So here's my stress interpretation of this rock. The big blue arrows represent the principal stress direction. To simplify, you could think of one blue arrow as representing Africa and the other as North America, pushing on these poor oceanic sediments caught in the middle. The yellow arrows represent extension. As the rock gets compressed in from "top" to "bottom," it gets squished outwards left to right. This deforms pre-existing quartz veins by rotating them into parallelism with foliation, and also potentially boudinaging them into chunks like the big one. The green ellipse demonstrates this overall process. One way to accommodate the rock's stretching in the yellow-arrow direction is by opening up small fractures (like the ones on the left) which get infilled with quartz.


On my walk, I saw a couple of exposures of hydrothermal quartz that strained the definition: that is, they weren't all quartz. Instead, parts of them (~5%) appeared to be granite pegmatite. In this shot, you can see several large crystals of potassium feldspar set in the quartz. Large flakes of muscovite were also semi-common. Lens cap is 5 cm in diameter.


Here's another shot of the same phenomenon seen elsewhere on the trail: large crystals of potassium feldspar and muscovite set in the "quartz vein." At what point do we stop calling these quartz veins and start calling them pegmatite dikes? Is a single crystal of non-quartz enough to change our perception of the fluid from hot mineral-rich water to wet magma? Like many things in geology, these features indicate that phenomena like dikes and veins are on a spectrum between end-members. In other words, there are shades of grey in how these things form (in addition to how we interpret them). By the way, the greenish hue is algae, not epidote. Lens cap is 5 cm in diameter.


Granite dikes (including pegmatitic ones) are reasonably common in the Virginia Piedmont. Here, as a Windy Run example, is a small granite dike I saw in a boulder on my Saturday walk. Lens cap is 5 cm in diameter.


Tomorrow, I'll explore a rockslide I saw on Windy Run, as well as the nature of the metagreywacke itself. Stay tuned, rockhounds...

Labels: geology, granite, structure, virginia