[Ads-l] Source(s) on history of alveolar tap/flap in English dialects?

[Neal Whitman]

Can someone point me to a relevant source or sources for the history or 
origin of AmE, NZE and AustrE alveolar flapping/tapping? I've found 
plenty of sources describing the phonetic environments in which it 
occurs, and the dialects that have it, but nothing yet on a time frame.

Thanks!

Neal

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