Every now and again, we wonder when we Americans started sounding like Americans instead of like English folks. People who move here from other places learn to speak English and within a generation or two most traces of their old country's accent or speech patterns are gone. That goes for folks from England as well, because even though we both speak English, we speak it differently than they do, and not just in vocabulary terms.
Obviously, we've only been able to record speech for the last hundred years or so, which means we don't know when our great-grandfathers and great-grandmothers stopped speaking the King's (or Queen's) English and started speakin' 'Mercan English. But it seems that one of the real differences in accents came not from changes over here, but back over in the Merry Olde digs.
According to the article, both British English and American English for a large part of our shared history were "rhotic," which means we both pronounced the "r" sound in words like "hard." Our "general American" accent -- the way most newscasters and folks from the midwest speak -- is still rhotic, but in the 19th century non-rhotic pronunciation took hold in England as a way of denoting the speaker's culture and breeding. In other words, the educated folks and the people who wanted to imitate them said "hahd," while the general low-life no-accounts (say hello, my Irish ancestors) kept saying "hard."
It sort of spread over the ocean, as the cultured classes in the former colonies also adopted non-rhotic pronunciations. Remnants of our non-rhotic accents can be seen in the drawl of the Southeast U.S. and the clipped tones of the Bostonian and other New Englanders, although each of those has its own regional intonations. The influx of non-English speakers had their own influences on our pronunciations, which gave different regions their own accents rather unlike anything that ever bloomed amidst the tea and crumpets (Siddown, Brooklyn).
None of which, of course, explains the Valley Girl.