....US English has involved much more slowly than UK English and a lot of 'Americanisms' are in fact 'Old Englishisms'. The classic example is 'Vermin' and 'Varmint' - Shakespeare would have said 'Varmint' and evidence suggests that some Elizabethan English would have had a distinctly mid Atlantic drawl......
Very true in the mid Atlantic region you referenced. In other parts of the country though, Americanisms evolved (and are still evolving) from languages other than English. Remember, when we first adpted the constitution in 1787, they almost made German the official language as there werte more German speaking colonists than English speaking ones. Then over the followig 60 years or so they added huge regions of Spanish speaking and French speaking peoples.
After that, the military began to bring back phrases from around the world (much the same way the British military did there)