Why is there such a sharp political divide between Chester and Lancaster Counties PA?

[Sol]

Chester County also has demographic patterns which are somewhat conditioned by the historic Pennsylvania Main Line--not just the towns commonly referred to as such, but also places like Downington, Coatesville, etc. These communities look a bit more like closer-in suburbs because they're older and in some cases industrial, and come close to the border with Lancaster, which makes it seem like a much more dramatic divide than it actually is. If you look at the southern (ignoring Lincoln U) and northern part of the county, the transition is more gradual.