The Geology of Washington, DC

Like most of you, I walk around this city day in and day out thinking about what’s next. I think about that next presentation I have to give, the report I’m stressed about, or a wheezing cough I swear I heard from my daughter just as I slipped out the door this morning and that if I miss another day of work to take care of that wheeze, my deadlines are shot.

And of course, here in DC, we’re all working on some “big issue”—gun control, terrorism, Ammon Bundy, the Trumps or global warming. However, I rarely have enough time to take a deep breath and think about anything beyond the next ping from my inbox.

However, on the particularly deep breath that was the snowed-in weekend and the Blizzard of 2016, I thought a little deeper—literally. What are we standing on? Seriously? Little did I know that this question was a doozy…

For a guy whose only real exposure to geology was one undergraduate class more than a decade ago, this was no trivial thought experiment. I really wanted to understand what rocks and stuff made up this neck of the woods.

The more I looked at “metagraywacke” and similarly alien lingo across websites, the more I realized there was no one to explain what the hell was going on under DC—at least no one that any mortal being could understand.

This is my best attempt at being that indelible link:

According to the great knowledge-pedia of Wik, Washington DC can be divided into two broad classes of geologic formations. The Northwest quadrant of DC and the surrounding Montgomery County, MD belong to the Appalachian Piedmont. The Southern and Northeast quadrants, along with most of Northern Virginia and the eastern counties of Maryland, are part of the mid-Atlantic Coastal Plain.

The importance of this is that DC was strategically positioned on the fall line—literally where the first impassable waterfalls are on the Potomac. This meant it was the farthest upstream they could build the city and still get ships to serve its ports.

Roughly speaking, the division between the Piedmont and the Atlantic Coastal Plain is demarcated by the continuous fall line represented by the hundreds of falls along rivers and stream inland of the Atlantic.

The Piedmont, itself, is just a hilly plateau between that fall line (the Atlantic Coastal Plain) and the Appalachian Mountains, and the Blue Ridge Mountains in particular. The name comes from the Italian region of the same name abutting the Alps, ultimately from the Latin pedemontium or “the foot of the mountains.”

The region is a hodgepodge of actual rock types. Most are metamorphic formations, which usually come from uplift and the smashing and crashing of continental tectonic plates—i.e. mountain formation. That means that the area more closely resembles the geologic makeup of the mountains than it does the neighboring silt-developed coastal plains. Thus the Piedmont is considered a “physiographic province of the Appalachians.”

Most of these metamorphic rocks were formed between the Ordovinian and Devonian ages. This rock is old—as old as 485 million years ago, right after the Cambrian explosion when trilobites starting making their mark on the world’s high seas.

The newer rock dates back to the formation of Pangaea 300 million years ago. The breakup of that famed land-mass split the then-massive full Appalachian chain and left the Old and New Worlds looking like long-lost spooning partners.

Geological_time_spiralEast of Rock Creek, things are a bit different. Most of the Atlantic Coastal Plain is exactly what it sounds like—flat land from North to South and extending inland to the fall line (which we can now define). It’s largely sedimentary rock from the Tertiary and Quaternary periods.

That’s super new on a geologic time scale and covers everything from 66 million years ago to the present, also known as the Paleogene and Neogene—the two “genes” of the current Cenozoic Era. By this point, mammals were already dominating the landscape. Heck, some of the “rock” of the Atlantic Coastal Plain is non-lithified, which means it is literally just sediment, and not even technically considered a rock yet.

The oddest thing is that hills in different parts of town might represent hundreds of millions of years of difference on the timeline. There are hills and high land formations in the South and East of DC—especially East of the Anacostia. But instead of being Appalachian formation, like the Piedmont, they’re really just sedimentary rock that’s been carved out by the wavering riverbanks of the Anacostia and Potomac.

In contrast, Georgetown, the DC Palisades, the Chain Bridge and the Falls are built on much older metamorphisized rock from a few hundred million years ago. And that brings us to metagraywacke…

Metagraywacke, or meta-graywacke starts as greywacke, which is immature sandstone of irregular granularity (i.e. varying sized pieces). From what I gather, the metamorphosis lends it the prefix “meta.”

In either case metagraywacke makes up the majority of the DC-area’s bedrock, especially north and west of Rock Creek—which comes to my mind as the informal dividing point (correct me if I’m wrong—umm, anyone who knows…)

This “gray” matter is often packed full of other odds and ends known as clasts. (Of course) before metamorphosis rocks were twisted into their mountain-forming state, they were sedimentary rocks. These particular ones seem to have also collected chunks of all sorts of other things, including quartz, granite and garnets.

Garnets were once clay minerals deposited between the sedimentary layers. When smashed during metamorphosis, they become unstable, and turn into mineral garnets—usually rich in iron. They often stand up to the test of time as little red knots sticking out of otherwise weathered rock.

So, I hope that gives at least the basic (and perhaps decent?) overview of the geology of DC. It’s split right down the middle—oddly enough mimicking other divisions in the city… but I’ll leave that for another blog!

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