Between 1940 and 1970, over 5 million African Americans moved north across the Mason Dixon line, leaving behind their southern homes for the promise of increased prosperity. Despite the major social and political significance, this mass migration went largely undocumented for years. It is only recently that scholars have begun to examine closely this phenomenon.
Below is an example of the impact migration had on Northern cities during the first half of the 20th Century (when the story of Rainbow Beach begins). The chart shows the increased numbers of black residents in the following cities:

1900 1910 1940  
New York 60,666 91,709 458,444
Washington 86,702 94,446 187,266
Chicago 30,150 44,103 277,731
Detroit 4,111 5,741 149,119

By 1950, only 40% of the Black population lived on farms and the number of acres operated declined by 37% to 25.7 million acres. Moreover, in 1950 the United States Census Bureau reported that for the "non-white" population - 95% of which was Black - only 18.4% were employed as farm workers, with 38% as "blue collar workers" (mainly industrial) and 34% as "service workers." This transformation of the social form of the Black community - from a
pre dominantly agricultural labouring class in the rural South to an integral sector of the industrial proletariat more concentrated in the urban North - is one of the most significant social transformations in the history of the United States.

picture from - www.blackpast.org