Like manson1972, I’m a native speaker of the marry/Mary/merry merger, which is true of most of the United States.
When I was in grade school, they showed us an educational film with a cartoon of two police detectives interrogating a suspect who wouldn’t answer the question of where he was from. So they had him pronounce words and used phonetic isoglosses on the map to narrow him down to his specific hometown.
The first pronunciation they tried was “marry, Mary, merry,” the guy pronounced them all differently, and they said “Aha! East of the Alleghenies,” and drew a line on a map. In the film the voice actor enunciated the words very slowly and clearly, to help even those of us who thought they were all the same to hear them distinctly. It’s probably the best-known and most definitive pronunciation isogloss in American English.
That film helped me along as a budding young linguist, and ever since then the phrase “east of the Alleghenies” has stayed with me. Later, when I researched the history of my pioneer ancestors who moved west of the Alleghenies in 1788, I realized the significance of that big escarpment as a barrier to movement was when there were no roads over it in Pennsylvania (the only road over it went through Cumberland, Maryland), while my ancestors had to climb up it at what would become Altoona.
Then it became plain to see how a different dialect was bound to develop there in the early days, when it was a big pain in the ass to cross the Alleghenies at all, let alone hauling cargo. Which was the major reason contributing to the Whiskey Rebellion. Western Pennsylvania’s commerce was more closely connected (via rivers) with New Orleans than with Philadelphia.