In January of 1969, British power blues quintet
Fleetwood Mac came to Chess Records studios to jam with the likes of
Willie Dixon,
S.P. Leary, Honeyboy Edwards, and longtime
Muddy Waters' pianist
Otis Spann. The sessions were so rich and fruitful that three-fifths of the Mac (specifically bassist
John McVie and guitarists
Peter Green and Danny Kirwin) impressed
Spann enough to cut a record with them at the same sessions. While the classic "Country Girl" and a seven-minute "Someday Soon Baby" (which features a lengthy intro from
Green on which
Spann can be heard barely off mic telling the rest of the band to "let him play on") ended up on the Mac's Blues Jam at Chess double set: remaining cuts included "Dig You" and "Walkin'" and are a near perfect match of
Spann's exciting, emotive singing and the Mac's youthful muscle. The Biggest Thing Since Colossus was released on Mac manager/producer/strongman
Mike Vernon's London-based Blue Horizon label.