I'm confident of 'fall line', also called 'break line' - see examples below:

The fall line marks the geologic boundary of hard metamorphosed terrain—the product of the Taconic orogeny—and the sandy, relatively flat outwash plain of ...
en.wikipedia.org/wiki/Fall_line

... meaning at a right angle to the downhill line **(the break or fall line)**... is going to be at exactly 90 degrees to the downhill break line. ...
www.probablegolfinstruction.com/break-master-read-putting-g...

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.
www.virginiaplaces.org/regions/fallshape.html

A fall line is a geomorphologic unconformity between an upland region of continental ... Terrain or relief is an essential aspect of physical geography, ...
www.absoluteastronomy.com/topics/Terrain

Cartographic relief presentation - Google Books Result
Eduard Imhof - 2007 - Technology & Engineering - 388 pages
... laws in this way he would improve the cartographic portrayal of the terrain. ... an important form-revealing element, the fall line, is lost. ...
books.google.com/books?isbn=1589480260...

Sep 15, 2010 ... Relief map illustrating the Eastern Seaboard fall Line ... fall line marks the geologic boundary of hard metamorphosed terrain—the product ...
www.ask.com/wiki/Fall_Line

Mountains are areas of high relief having distinct changes in terrain slope and ..... in representing break-line features characteristic of mountain areas. ...
www.gisdevelopment.net/application/nrm/.../mount0008pf.htm

... times) around terrain break lines to render locally the relief morphology, ... (c) all triangles overlapping the break line are removed and (d) the ...
books.google.com/books?isbn=0748409440...