East of I-95, the soil is sandy. It is light-colored (sometimes almost yellow or even white), and there are few hills. West of the interstate, the plowed fields expose red clay and the land rises steadily to the Blue Ridge, the eastern edge of the Valley and Ridge. The interstate highway is a rough guide to the location of a geologic boundary known as the Fall Line. As portrayed so elegantly on the UGGS "A Tapestry of Time and Terrain" site, the Fall Line is the boundary separating the soft Coastal Plain from the hard Piedmont.
The Piedmont rock was once the soft sediments of the Outer Continental Shelf in the Iapetus Ocean, located offshore from the ancient shoreline of Virginia before the Iapetus Ocean closed and the continents collided. When Africa (and Europe) bumped into the North American continent and created the Appalachian Mountains, those soft sediments were scrunched up from the ocean bottom and pushed onto the North American continent. You can push icing on a cake and "scrunch" it with a fork onto the last piece of cake in the same way.
In the process of being scrunched, the sediments from the Iapetus Ocean floor were squeezed and baked into the hard metamorphic rock that now underlies much of Virginia between I-95 and the Blue Ridge. The metamorphosed sediments, formerly the Outer Continental Shelf in the Iapetus Ocean before being shaked and baked, are exposed at Great Falls on the Potomac River just upstream from Washington, DC.
After Africa and North America collided and pushed up the Appalachians, the combined continents then split apart. The crack where they split filled with salty seawater and formed the Atlantic Ocean.
When the Atlantic Ocean formed initially, the Virginia shoreline was at the eastern edge of the Piedmont (now roughly the route of I-95). Since then, sediments have washed down as the mountains eroded. Those sediments have accumulated east of the Piedmont to form the Coastal Plain. Virginia's land area has expanded to the east, as the North American tectonic plate has drifted to the west and the Atlantic Ocean has widened.
In the last 150 million years, the water levels in the Atlantic Ocean have risen at times. When water covered the eastern edge of Virginia, sediments were deposited onto the Coastal Plain. Those relatively recent Coastal Plain sediments were formed by deposition when ocean levels were higher, and as freshwater rivers eroded the modern Appalachian Mountains and carried debris to the edge of the continent. Sediments deposited after the continents started to split apart and the Atlantic Ocean formed have not been baked and squeezed tight, so those Coastal Plain sediments are not hardened like the metamorphosed Piedmont bedrock.
When today's Virginia rivers flow eastward from the Piedmont onto the Coastal Plain, they leave the Piedmont where the riverbottom is hard rock. When they encounter easier-to-erode soft Coastal Plain sediments... they etch into those rocks. The energy of the water carves a deeper channel in the softer sediments, creating waterfalls.
The edge of the Piedmont/Coastal Plain is marked by a line of waterfalls (the Fall Line) where various rivers move from harder to softer bedrock. The waterfalls are most obvious at Great Falls on the Potomac River, on the Rappahannock River at Fredericksburg (look westward from the I-95 bridge), and on the James River near downtown Richmond (look westward from the Lee Bridge). The waterfall on the Occoquan River near Lorton has been "dried out" by the construction of a dam, trapping the water in the Occoquan Reservoir. On the rare occasion that Fairfax County Water Authority opens the gate, you can see the exposed rocks at the Fall Line by walking upstream from the town of Occoquan.
The Fall Line is a zone - sometimes several miles wide, rather than just a narrow line - between the Coastal Plain and the Piedmont physiographic provinces. The bedrock in the Piedmont is the hard crystalline rock that you can see at Great Falls on the Potomac River. The eastern edge of that hard rock is downstream on Teddy Roosevelt Island, at the end of I-66 where it crosses the Potomac River on the Teddy Roosevelt Bridge (connecting Rosslyn with the District of Columbia at the Kennedy Center). Great Falls is several miles upstream, showing how the Potomac River has etched its way upstream and carved out Mather Gorge in the crystalline bedrock.
