Julius Hemphill's "Dogon A.D." — the 15-minute piece, and the album that's named for it — was one of the startling jazz recordings of the 1970s, a rethinking of possibilities open to the avant-garde. In the 1960s, free jazz was mostly loud and bashing, until some Chicagoans began playing a more open, quieter improvised music. That inspired St. Louis players like Hemphill, who also had ties to heartland rhythm-and-blues scenes. Hemphill's genius was to combine the Chicagoans' dramatically spare sound with a heavy backbeat. His new urban music smacked of old country blues.