Sentence:  The southern borders of these states are keyed to the same horizontal projection, one surveyed by the frontier planter William Byrd in 1728, while the rivers forming their northern extents fall back just opposite each other from the flanks of the Appalachian <WSD>massif</WSD>. 
Meaning of the word marked with <WSD> tags:  A block of the earth's crust bounded by faults or flexures and displaced as a unit without internal change; normally consists of gneisses and schists