(S:01|E:02): In the first episode of America’s Neighborhood, we discussed the origin of the word “Potomac,” tracing it back to the Native Americans and maps drawn up by Captain John Smith in the 1600s.
Today, we look back a half billion years to investigate the geological formation of the lands known as the Palisades along with the DC area’s age and development, modern changes like the Potomac River, and new features like Great Falls.
How the Palisades were Made
Washington, DC has long been a place where two opposing sides knock against each other changing the landscape. But a little more than a half a billion years ago, it wasn’t the opposing forces of conservatives and liberals; it was the future continents of North America and Africa on a collision course in areas that would become Pangea.
At this point, there were no Palisades perched above the Potomac River or even a Potomac River for that matter. In fact, much of the lands we tread had yet to rise from the world’s oceans.
But little by little, things were about to change. Before becoming one huge land mass of Pangea, these future continents were shifting on their own independent tectonic plates, which these days typically move about one to two inches per year. Basically, the speed at which fingernails grow.
So with this slow acceleration, you would expect the force of continental collision between North America and Africa to be small, until you include the other variable from Newton’s second law of motion: mass. These slow-moving plates were hauling some serious mass—literally continents worth.
“This was a major collision in earth’s history,” said Vincent L. Santucci, Senior Geologist & Paleontologist for the National Park Service. “Think of the size of these two land masses
colliding. That the Appalachian Mountains, which are heavily eroded over the past 300 million years of time, once stood higher than they do today.”
To set the scene for this crash, we have to go back hundreds of million of years and travel below the equator to where our neighborhood was at the time. If this area sounds shockingly south to you, it is nothing compared to the then-location of the eastern parts of DC. (For more information, see the fascinating work of scientist Christopher Scotese.)
Say that your office was downtown in Mount Vernon Square, Farragut North, or Capitol Hill, you might need to build some more time into your commute. That’s because those areas of the district, along with a decent section of the East Coast, were very close to the South Pole at the time, and nowhere near the rest of North America.
Much of those parts of town were under the ocean and had really yet to form at that point.
Now, instead of going back in time, if you put things into forward motion, a few inches per year, you can see how plate tectonics is enough to change DC and the world. The convection of these plates into each other is what made the Palisades and launched the Appalachian Mountains some 10,000 feet into the air, making them comparable in height to ranges like the alps and Himalayas.
The force of this continental crash was also rough enough to eject most of DC in that large wedge of America from between the future continents of Africa and South America
“When you look at the map of the Washington DC area, it’s extraordinarily colorful given its complex nature,” Santucci said, “Given a very complicated history.”
The DC area still shows the signs from this elongated crash. Its pushes, rips and pulls have left a kind of scar tissue on the earth’s crust where we live.
Some of its resulting provinces and land formations include the Appalachian Plateaus, Valley and Ridge Province and the Great Valley Section. But of the three that are nearby, the Blue Ridge province is the oldest, with parts of it forming some 1.1 billion years ago predating the super continents of Rodinia and Pangea.
The Piedmont province, where we live as Palisadians, was next up locally with two spans of construction, when Rodina broke up, between 700 and 550 million years ago, and later around 500 to 450 million years ago.
The elevated lands of the Piedmont province stretch from the Palisades and parts of Rock Creek Park and Arlington toward the basins at the foot of the Blue Ridge.
Finally, during the last 100 million years, you have the Coastal Plain, which runs from sections of Georgetown, Columbia Heights and Fort Totten all the way eastward toward the Atlantic Ocean.
“So, the exposures that have been referred to as the Palisades are areas of very very old rock,” Santucci said. “The Sykesville Formation was laid down almost a half a billion years ago and they’re now preserved at the surface.”
It was this long collision between the future continents of North America and Africa that made the Palisades, which are the oldest lands in Washington DC. The word “palisades” means a line of steep cliffs especially along waterways like a river or an ocean.
The original name of our neighborhood, “The Palisades of the Potomac” represents a half-billion year span of earth’s history within its short 28 characters. While the lands of the Palisades may be ancient, approximately 500 million years old, scientists say the waters of the Potomac and the features they have carved are quite new.
Much of the shaping of our area occurred during the last Ice Age, which began about 1.5 million years ago, as sea levels changed and ice sheets advanced and retreated in areas of North America.
"The Potomac River is a very recent story," Santucci said. "It dates back about 3.5 million years ago, where drainages from what would become the District of Columbia and four other states (part of Pennsylvania, West Virginia, Virginia and Maryland) were draining water (river way systems) down through this channel that was making its way through fault crest crevices and fissures, eventually eroding through that area, widening to a full river channel and we see that river today making its way all the way to Chesapeake Bay."
The Earth’s crust in the region was lifted and changes in sea level caused the Potomac and other rivers to engrave, etch and carve their courses into the scenery, chiseling through mountain ridges as sea levels fell during the last ice age.
While the Potomac River is just 3.5 million years old, some of its most noticeable
landmarks are vastly younger still. Mather Gorge and Great Falls, were only formed during the last 30,000 years.
Locally, they are the most recent of decorations from physics and mother nature to a landscape that has continued to evolve over many eras and eons.