The solid blue line on Map 2 represents LANE's border between Eastern and Western New England. The English settlement of the area east of the blue line began at Boston and Plymouth and other settlements along the Atlantic coast, and moved west and north from there; west of the line, settlements began along Long Island Sound and moved north. The area of TLN corresponds roughly to Western New England, with notable gaps in the Hartford and Springfield areas and in the southwestern corner of Massachusetts. The lines weave in and out of each other in Vermont, but the E/W border is less secure there as Vermont was settled later than the rest of New England and included populations from both the West and East regions.
The string of red spots in western Connecticut follows the Housatonic River, which was at one time one of the main routes of settlement and travel in Western New England. This suggests that full neutralization of marry-merry-Mary may, in this area, have originated in Bridgeport, at the mouth of the river, and spread northward along it.
The gap in the TLN region in southwestern Massachusetts suggests that TLN was not an original feature of the Western New England dialect that spread with settlement; if that were the case, TLN would have to have been originally present in Hartford and in southwestern Mass. and then reversed by influence from the east. This is unlikely because mergers are notoriously hard to undo. On Maps 3 and 4, we look at other phonological features native to western New England and how their interaction with TLN could have stopped TLN from spreading into these anomalous areas.