My wife was born and reared in Ellis County, Texas -- the county due south of Dallas County.
In the mid-80's, we were visiting friends in Vienna, Austria, and attended a worship service at a church which ministered to English-speakers in the city (a large population there, made up of UN employees, OPEC personnel, business and diplomatic people, and -- at that time -- a considerable contingent of Christian missionaries who lived in Vienna but ministered behind the Iron Curtain).
Our host told us, "You must meet Reid and Betty. They're from Texas too." Reid and Betty had lived in Vienna over 25 years, and Reid was a Kammersanger in the Vienna State Opera. Upon being introduced, my wife's first words to Reid, "I'm very pleased to meet you." Without batting an eye, Reid responded, "You're from Ellis County, Texas."
Reid, of course, had an excellent ear for such things, needing a good ear for accents in order to do his work of singing in several Eupopean langauges. As it turns out, he grew up in North Texas, a couple of counties to the East of Ellis County. In his day, he claimed, one could recognize an Ellis County native by his accent. And my wife had it, according to Reid.
Many years later, when we settled in Ellis County, I saw a notice in the paper for a voice audition for an agency producing radio commercials. Thinking it would be fun, I showed up. I quickly learned I was exactly what they were NOT looking for. My English is ruthlessly Standard American (according to my daughter, majoring in linguistics). Headquartered in Dallas, this agency makes periodic auditioning forays into Ellis County, in order to find thick, unmistakeably rural, Texas accents for the radio commercials which require that sort of thing.
So, it seems that even perched on the edge of the Metroplex, Ellis County still preserves an abiding accent among its natives.