[ansel12]

live in a region where 90% of the people they encounter are in the same denomination as them.

You keep using that figure, what are your sources for that, what states or regions do you mean?

[muawiyah]

New Canaan Connecticut maybe?

I thought I was pretty familiar with this country, and I'll be doggone I can identify even a single county where 90% of the population belong to one denomination,

Provo perhaps?

We need some names of places!