I think it depends on what part of the country you are talking about. Back East where there are more big cities and the distances between are less than those out West, I saw more of the clustering BUT didn't feel any prejudice toward me or others not of that particular group.

Out West, people had to rely on one another much more; the distances were much greater and goods only available in the towns and few cities. To be sure, there were still enclaves and there were places designated by their ethnic/cultural identity; some have become designated historical neighbourhoods.

For an idea of how prejudice could perpetuate in a very rural area, even in the 20th century, I would recommend a movie we just watched called "Sweet Land." Despite the portrayal of a closed, small-minded community, it also uplifts when people open their hearts.