The internal structure of Earth is layered in spherical shells: an outer silicate solid crust, a highly viscous asthenosphere and mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core.
A fall line is an upland region of the coastal plains meets and the term states that lots of rivers fall from this Piedmont to the coastal plain making rapids with waterfalls.
These falls are important as for the settlement as they block the ocean going ships from entering in any further upstream and also provided water power to run the region industries.