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.