It appears the queen's English ain't wot it used to be. A scientific study of Queen Elizabeth II's accent has found her vowels moving steadily downmarket. The study published in Nature magazine found there was a drift in the queen's accent toward one "characteristic of speakers who are...

It's a very thorough map of North American English dialects, based on pronunciation patterns.

The American accent began to develop quickly after America won the Revolutionary war against the British. Until that time, both parties spoke with a rhotic British accent similar to the modern day American Accent.