You might quibble about your version of the dates, but Joe Boyd gets it pretty right when he writes: The sixties began in the summer of 1956, ended in October of 1973 and peaked just before dawn on 1 July, 1967 during a set by Tomorrow at the UFO Club in London (White Bicycles: Making Music in the 1960s, p. 1).
So, in Britain we had skiffle survivors, art school and a fusion of influences from Across the Pond as, the source of a phenomenon that swept the world in 1964 and 1965. It was a development that was always going to produce, and it produced plenty of them. The one that interests me the most is the one in the stateside music industry, which was shaped by a number of factors.
One was the fact that the British Invasion ran straight into an entrenched and quite lucrative, music industry. That came complete with executives, producers, songwriters and performers, all of whom needed to adapt to the new environment.
Those players had a few avenues that could shape the adaption. One was a well-established base of songwriters who could churn out a product that was at least as good as anything most of the English groups could produce themselves. When you look at the hits from the sixties that end up being recycled over and over, it's hard to avoid the impression that most of them came out of the Brill Building.
Across the pond, while things tended to concentrate in New York and Los Angeles, there were significant regional centres with distinctive music scenes. One could probably go beyond the obvious names (Chicago, Nashville, Memphis and New Orleans), but even without doing so, that quartet is enough to make, and probably underline, the point.
Outside them, there were also a number of regional styles and genres, which could throw additional elements into the mix.
Folkies and Folk-Rockers East and West Coasters San Francisco The Tide Recedes (And Flows Back In)