Hi there,

Answers here are not discouraged, but you might have a better chance over at /r/linguistics.

Things mentioned in most of the answers about American accents: there were and are lots of British accents. People from different regions came to the US and sometimes local British features were retained in the US. British accents continued to change over time and place, such as non-rhotic speech increasing.

Mentioning previous answers is intended to provide quick pointers to existing answers, not to discourage further questions. If you have more questions or disputes, please feel free to post them.

A recent answer by /u/YuunofYork , a linguist, was in "How close was George Washington's accent to the modern American accent?" under https://www.reddit.com/r/AskHistorians/comments/7w4ldq/how_close_was_george_washingtons_accent_to_the/ . It does look a bit at Washington in particular, but just to aver ignorance about Westmooreland, Virginia, and "Washington was a 4th generation native Virginian, so we can assume his speech followed suit with his peers, but it's difficult to say with certainty what features his peers had for several important reasons". So they mostly talk about what features might or might not have existed then.

The FAQ has a section on English accents and dialects at https://www.reddit.com/r/AskHistorians/wiki/language#wiki_accents_and_dialects . Specifically:

Despite the title, this article discusses pre-Civil War accents too: "How has the American Southern accent changed since the Civil War? Before the war, was it perceived as prestigious in the North and elsewhere?" at https://www.reddit.com/r/AskHistorians/comments/34pux1/how_has_the_american_southern_accent_changed/cqx5nsc/ , /u/tropical_chancer .

"What caused Americans to develop a different accent than the British?" at https://www.reddit.com/r/AskHistorians/comments/39i1k4/what_caused_americans_to_develop_a_different/cs4370z/ has a relatively short comment by /u/keyilan on the general phenomenon, and a link to a comment by chocolatepot.

"Why do Australians speak without the American rhotic R sound?" at https://www.reddit.com/r/AskHistorians/comments/6tf3py/why_do_australians_speak_without_the_american/ , by /u/hillsonghoods

Outside the FAQ, and again looking at the US South:

"How did the "Southern" accent develop on the U.S and what accounts for its variation across the region?" at https://www.reddit.com/r/linguistics/comments/5xn77e/how_did_the_southern_accent_develop_on_the_us_and/ is a bit loosey-goosey, despite being only 11 months old, and it depends on links -- there's no good definite chunk to point to. But there's some Appalachian stuff, and zabulistan quotes their words about more recent Southern dialect development which may inform you of what you consider "a southern accent".

Speaking of all over the place, Thesleeperhold asked on /r/linguistics , with "How did the "Southern" accent develop on the U.S and what accounts for its variation across the region?" at https://www.reddit.com/r/linguistics/comments/5xn77e/how_did_the_southern_accent_develop_on_the_us_and/

For Canada accents, plural, I find this in /r/linguistics: "Why are there so few strong regional accents in Canada?", https://www.reddit.com/r/linguistics/comments/27ulkz/why_are_there_so_few_strong_regional_accents_in/ , answer by Soldado4lyf .