THE "FALLS LINE" is an approximately 900-mile long boundary between physiographic provenances along the Atlantic Seaboard of the US, running north-northeast from Georgia to New Jersey.  It is expressed as a long series of waterfalls on the predominantly west-to-east flowing rivers that drain the Appalachian Mountains and piedmont into the Atlantic Ocean.

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It is the demarcation of the boundary between the Piedmont to the west and the Atlantic Coastal Plain to the east.  First documented in the early 1600s, the falls were created as rivers transition from flowing over the hard crystalline igneous and metamorphic rocks of the Piedmont, to the much softer and more easily eroded sedimentary deposits that lie between the falls and the Atlantic coast.

A couple hundred million years ago, the predecessors of the current Appalachian Mountains were created as the result of a titanic collision between Africa and North America.  They soared to heights estimated to be in excess of 20,000 feet.  Time and the elements, the great levelers, have since eroded and worn most of that enormous mass of material away.  Rivers have cut down through the lower areas, creating "gaps" in what remains and carrying vast amounts of sediment away, depositing it along the coastal plain to the east as well as the midcountry of Ohio, western Kentucky and Tennessee, etc. all the way to the Mississippi River.  

To give you a sense of how much sediment there is, consider the image below (swiped from the same source as that above):

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The falls line boundary has had tremendous impact on how people, particularly European settlers and their descendants, have interacted with the land.  Cities, towns, and villages tended to concentrate at the falls, which provided cheap power to operate mills.  

The falls are the natural "head of navigation" for the rivers; below them, sailing vessels and steamships could operate easily, but they could not pass beyond.  Towns along the fall line naturally tended to become inland ports, places where incoming trade goods could be delivered by vessel and outgoing materials such as crops, furs, timber, and other items could be picked up by the same vessels.

Fords across rivers above the falls tended to be located in places where the river bottoms expose and flow directly over the hard crystalline rock of the Piedmont; it's much easier for people, horses, and wagons to cross at such places than  where the river bottom is deep, slippery, clinging mud.  Several fords at or near the falls line played important roles in, among other things, key Civil War battles including Fredericksburg VA, Gettysburg, Antietam, as well as many other related operations.

On the coastal plain, rivers tend to be crossed by bridges and ferries rather than by fords.  Roads are built much more easily on the relatively flat, low-lying, unconsolidated alluvial soil of the coastal plain than in the tortuous, steep, winding hills of the Piedmont.  Naturally, that coincides with the establishment of many more settlements along and to the east of the falls line than to the west.  Wikipedia will happily tell you much more, should you care to learn.  

The short version is that the East Coast Greenway appears to follow the falls line for much of its (the ECG's) length as it traverses the mid-Atlantic region.

I happen to live within ten miles of the fall line where it crosses the Potomac River (at Great Falls, in Virginia and Maryland).  I have in-laws in Charlotte NC, and there's relatively inexpensive bicycle-capable Amtrak service between there and Washington DC.  My plan is to take the train down to Charlotte then set off riding back north until I get home.  I'll intersect the ECG somewhere east of Greensboro NC, then use that and U.S. Bike Route 1 to work my way home.  Starting somewhere well south of Richmond I expect to be on or near the falls line pretty much the rest of the way.

Rather than camp, my overnight accommodations will be a mix of Warm Showers hosts, an AirBnB, a couple commercial hotels, and (I hope) a friend's house.  That will lighten and shrink my load considerably, eliminating the need to carry my tent and all the related camping impedimentia.

It's just over a month before all of this is slated to happen, and thus far I've logged 25 miles of riding for the year.  Guess I'd best get started  changing that.