Sentence:  Old attitudes are held more tenaciously in the Tidewater than the Piedmont ; so that a line running down the length of the South marking the <WSD>upper</WSD> limits of tidewater would roughly divide the Old South from the new , but with , of course , important minority enclaves . 
Meaning of the word marked with <WSD> tags:  the limit on the upper (or northernmost) side of something