Geology of Great Falls, VA

With hundreds of millennia of continental collision, mountain-building, and eroding coasts, the Eastern seaboard is full of rich geological stories. For residents of the Mid-Atlantic, many of them are visible at the Great Falls National Park that straddles the Potomac upstream of DC. Here are a few highlights of a recent trip there with the Smithsonian Paleobiology Training Program. 

Metagraywacke (shown above) of the Mather Gorge formation underlies much of the landscape here. You can read an old sedimentary pattern in the cross-bedding that bends across the stone. This metagraywacke formed deep underwater at the bottom of an abyssal plain, where debris from the continental slope roiled down in huge underwater landslides called turbidity currents. As the sediment settled out after these distinctive events, it formed the bedding you see here, which lithified over 570 millions years ago.

When Africa and Europe collided with North America, this bedrock underwent massive metamorphism, which is apparent in the tortuous patterns in the rock.

Despite the sturdy nature of the metagraywacke, it gave way before an advancing Potomac River, which cut a deep gorge here along an ancient fault line.

 This gorge, in turn, is pocketed with ecological nooks and crannies, providing habitat for a number of different species. This includes the Green Frog (Rana clamitans), enjoying the microhabitat of a water-filled pothole.

                                

But dangers lurk for unsuspecting amphibians. Only a few hundred feet away, this 5-ft long Black Racer (Coluber constrictor) was taking advantage of a rocky crevice to conceal himself.

 

 Great Falls, thanks to its status as a National Park, is full of natural gems like this. It wasn't long ago, however, when this land was prime prospecting ground for gold, which fills the quartz veins cross-cutting the bedrock. From the Civil War until the 1940's, gold was actually mined here, and it doesn't take long to see evidence of it.

 

The stream on the left is contaminated by acid mine drainage, causing high levels of iron and blooms of iron-digesting bacteria. Mitigating pollution like this can be very difficult, since it involves tracing groundwater on its convoluted path back to the contaminant origin. Knowing the geology of the area is a good place to start! Explore Callan Bentley's excellent guide for more information.

 

Geology of Theodore Roosevelt Island

A few weeks ago, I had the good fortune to accompany the Smithsonian Paleobiology department on a field trip to Theodore Roosevelt Island, a tiny bit of land in the middle of the Potomac river, between Washington DC and Virginia. The island represents a neat little microcosm of history in our nation's capital, from the time when it was housed a populous Nacotchtank fishing village (first recorded by Capt. John Smith as he sailed up the Potomac), to the days when it was cleared for the manor of John Mason (son of George Mason, the early American statesman), to the current day where it boasts a granite monument for Teddy Roosevelt. Today, most of the island's 80-odd acres are covered with forest, wetlands, and rocky outcrops.

For students of geology, the island is a great place to observe many distinctive regional features and landscape morphologies. Perhaps the most significant is the "Fall line" that bisects the island- an unconformity where hard bedrock (schist and gneiss of the Piedmont Province) stands in relief against an eroded plain of soft sediments (called the Coastal Plains Province.) This is a local expression of a much larger continental feature-the 900-mile Atlantic Seaboard Fall Line that runs from New Jersey to Georgia. On the island it is not much more than a low slope, but it serves as a great illustration of the geology, unobstructed by buildings or roads. .

(Image adapted from Google Earth Image, after Crowley, William 1976.)

The image above shows how the bedrock of the Piedmont slopes down under the Tertiary and Cretaceous sediments that constitute the Mid-Atlantic coastal plain. DC, like Theodore Roosevelt Island, is neatly divided by this line, with northwestern neighborhoods like Georgetown, Adams Morgan, and Cathedral Heights perched high on the Piedmont and the rest (including the National Mall) lying low on the Coastal Plains. 

Through out much of DC, the visible Piedmont bedrock belongs to the Sykesville Formation. On the island it appears as schist (a rock composed of metamorphosed ancient mud layers) rich in garnet and biotite mica. The garnets represent inclusions that were especially rich in iron, magnesium and aluminum.

 

Scattered among the native bedrock there are some river-rounded stones brought in for filling gravel paths. On one of these, we were lucky enough to find the fossil trace of a prehistoric burrowing worm called Skolithos.

 

Another interesting feature is the presence of ubiquitous shoreline structures like natural levees and backswamps (see below, with the office buildings of Rosslyn in the background.) As sediment washes up on shore it piles up and is anchored by tree roots, building a low, long mound along the shore. Underground seepage, however, allows river water to form marshy "backswamps" where water-logged conditions prevent the growth of trees.


It is astounding how geologically rich such a small piece of land can be. And while Theodore Roosevelt Island is certainly special, it is not necessarily unique- there are many pockets of geological knowledge to be discovered all around Washington DC.

Thanks to Dr. Ray Rye, leader of the expedition. 

Further Reading: 

Means, John (2010). Roadside Geology of Maryland, Delaware, and Washington DC. Missoula: Mountain Press Publishing Company.