by Rosemary Jann
Although we may not be aware of it, we live in a region of borderlands. Of course, our area is politically shaped by the explicit borders of the District, Maryland, and Virginia. But the diversity of our plant and animal life derives in part from the fact that the southern limit for many northern species overlaps here with the northern limit for many southern species [https://www.dcr.virginia.gov/natural-heritage/natural-communities/document/ncoverviewphys-veg.pdf, p.23]. We also straddle a third significant geographic borderland, which is responsible for some of the most dramatic features of our region, like the Great Falls of the Potomac depicted below.
This border is known as the Atlantic Seaboard Fall Line, formed by the points on eastward-flowing rivers at which navigation becomes impossible because of rapids and waterfalls. This line runs down the mid-Atlantic, as seen in the diagram below, and has shaped the development of this area since its earliest history. The barriers to upstream navigation hindered inland migration by Europeans and spawned cities to their east to facilitate the transfer of goods and people: Georgetown and Alexandria in our area, but also Fredericksburg, Richmond, and Petersburg. Today, Interstate 95 roughly parallels the Fall Line in the mid-Atlantic region.
The Fall Line forms along the border between two physiographic provinces that meet in our area. A physiographic province is a geographic region with distinctive soils, topography, and vegetation. The Piedmont province is underlain by hard, crystalline bedrock. In the Coastal Plain, bedrock is deeply covered by softer sedimentary soils. As rivers flowing to the Atlantic over resistant bedrock meet the Coastal Plain, they begin to move more quickly and to cut down through those softer soils, creating waterfalls and rapids as the river descends to sea level [http://www.virginiaplaces.org/regions/fallshape.html].
The Great Falls of the Potomac, where the river drops 77 feet in less than a mile [https://www.nps.gov/grfa/learn/nature/naturalfeaturesandecosystems.htm], is the Fall Line’s most dramatic local manifestation. However, the rapids that originally blocked upstream navigation for European immigrants are actually located at Little Falls, slightly north of the Chain Bridge [https://en.wikipedia.org/wiki/Little_Falls_(Potomac_River)]. Over time, the energy of the Potomac has continued to erode the bedrock it flows over, causing the more dramatic waterfalls to migrate westward over millions of years, leaving Great Falls today approximately 14 miles upstream from Washington, DC.
That fact helps underscore the point that it is more accurate to talk about a Fall Zone than a Fall Line. Rather than a knife-edge transition from bedrock to sedimentary soils, the harder rocks of the Piedmont intrude into the Coastal Plain in irregular outcroppings over an area approximately 10 miles wide, creating a ragged boundary with patches of bedrock upthrust into Coastal Plain soils. We can witness this irregular border at Theodore Roosevelt Island, described by the National Park Service as the last bedrock island in the Potomac as it flows eastward to the Chesapeake Bay: “The island thus marks the Fall Line with bedrock exposures on the northern shoreline (Piedmont) and swamp and tidal marshes on the southern shoreline (Atlantic Coastal Plain)” [http://npshistory.com/publications/gwmp/nrr-2009-128.pdf, p. 14]. Both are illustrated in the photographs below.
When we add the plant diversity that we gain from having vegetation characteristic of both these physiographic provinces in our area, we can fully appreciate the richness of our Fall Zone borderland: it offers a living map to our geological history, serves as a reminder of how our cultural history has been shaped by natural forces, and helps enrich the biodiversity we enjoy as residents of northern Virginia.