the natural downward course between two points on a slope
2.
the edge of a plateau
Fall Line in British English
noun
a natural junction, running parallel to the E coast of the US, between the hard rocks of the Appalachians and the softer coastal plain, along which rivers form falls and rapids
Fall Line in American English
the line east of the Appalachian Mountains, marking the end of the coastal plains and the beginning of the Piedmont Plateau
fall line in American English
US
1.
the geographical line indicating the beginning of a plateau, usually marked by manywaterfalls and rapids