More might be written, but you might like to see some existing discussions in the "Accents and dialects" section of the FAQ. There are four discussions there that seem applicable, by /u/keyilan , /u/YuunOfYork , /u/LordMayorOfReddit , and tropical_chancer (despite the title of the last, "since the Civil War", it talks about before the war too).

This is not to discourage discussion. More questions, data, and debate are welcome.