Where is the "natural" boundary between North and South?

[Swan Station]

Again to the op topic, the western edge of the South is always a real interesting question, as it seemed to move northward later with Mizzou... if the civil war had come later, might it have continued northwest?

It's tough to say... the abolitionists were winning in Kansas as of a couple of years before the Civil War, but if Bleeding Kansas goes differently, you'll change a lot of the background of the War.

But, I think the different climatic conditions in the Great Plains would pretty much transform Southern Culture into something different.

 

[jackalope]

A pre 1900 assessment, but assumed post 1865?

My guess is that Mason Dixon Line to Kentucky, to Arkansas, to Oklahoma, to Texas to Mexico.

People forget that small numbers of slaves made a great difference since it was the rich people who had those slaves and rich people usually shape society. Society is then copied by the lower rungs. In southern New Jersey and Indiana, strong feelings were still felt, apparently, by those years, or so say books on the KKK regarding a few years later. By this I mean that the KKK hit upon the southern model, as the northern one was different, it seems.

The Civil war rounded out matters on both sides. The Tuckahoe accent took over German language areas of the Shenadoah (last Lutheran Church changed over to English about 1905, Mennonites might still have some). This was because Union troops thought them as Southerners and got robbed blind, rather than the somewhat more lenient attitudes of the Confederate Army. Southern Indianans might still grit their teeth some, but the advantages were too great so to not pay lip service to methods and attitudes.

Slaves were in many states, Utah having 10 in 1860, but when it got to plantation levels, societies shifted in ways still detectable. For a general rule of thumb, though, the Mason Dixon line is fine. Delaware is a wild card, however, as mentioned by others. Missouri would have certainly been southern were it not for the massive german immigration, and TR, as a historian not as president, reluctantly agreed they tipped the balance despite his dislike of Germans since being beaten up by two in teenage years.