Green’s Dictionary of Slang

(US black) that area of Harlem, NYC running from 7th avenue to the Harlem river.

1927		F.M. Thrasher Gang 14: In the southern portion of the Ghetto is an area known as the ‘Valley"—[. . .] the present territory of the notorious ‘Forty-Twos’.
1957		C. Himes Rage in Harlem [ebook] East of Seventh Avenue to the Harlem River is called The Valley. Tenements thick with teeming life spread in dismal squalor. Rats and cockroaches compete with the mangy dogs and cats for the man-gnawed bones.