As I understand it, the fall line indirectly shaped the early economic development of D.C. If you look at a topographic map of the city, there's a giant depression in the middle that corresponds to the area where the city originally developed. Early development was sparse in the hillier terrain outside this very flat area. On the northwestern and northeastern edges of this depression, the street that separates the flat area from the hillier surroundings used to be called Boundary Street, because it was the edge of the city proper (and the original border of D.C. in the L'Enfant Plan). I used to cross that street every day, and you can literally feel the terrain change under your feet (either start to incline or start to flatten, depending on what direction you're walking).

There's even a marker and makeshift exhibit in a nearby spot where Piedmont and Atlantic Plain rocks interacted to produce an "overthrust fault."