Even just Virginia, where I grew up, has very distinct dialect regions. We could distinguish people from just 30 m. away and particularly people to the SW of us by the quality of the letter "I" - whether it was pronounced "Oi" or "Ah", although they belonged to the same dialect region.
People to the east of us (east of the piedmont) sounded like they were central casting line ups for Gone with the Wind Part II (much better accents than from Hollywood, though). And they had that weird "ou" that sounds like nothing you've ever heard before.
Cascode, I'm afraid that's another British linguistic myth that only Britain has diverse dialects. Of course, they are probably somewhat more so than North America, but North America is the oldest British colony (early 17th c.) and so has had several centuries to develop very strong regional accents. Particularly along the eastern seaboard, there are many subtle and less subtle regional dialects. We are perhaps the only other English-speaking nation with strong regional distinctions like that, in fact. Anybody else, are there regional accents in Canada? South Africa, Australia? I don't believe so, but I may be wrong...
However, I think the post-60s generation is more or less strongly influenced by TV, so that "Sououuuuuthside Vahginnya" accent I alluded to earlier is dying out. Sadly.
Please note: There is no one "Southern" accent, but rather many. Central North Carolina has glottal stops (almost like a Cockney), Mississippi sounds entirely different from Texas, Ozark Arkansan and other parts of Arkansas are distinct, East Tennessee and West Tennessee are very different, etc.
I had a blind law professor from rural Alabama - his accent was so posh southern it almost sounded English...