“Theme for an Imaginary Western” is a song written by Jack Bruce and Pete Brown. The song is sometimes referred to as “Theme from an Imaginary Western”. The song originally appeared on Bruce’s Songs for a Tailor album in 1969. The following year, “Theme” appeared on Mountain’s Climbing! album. Mountain bassist/vocalist Felix Pappalardi, who sang the song with Mountain, had helped produce Bruce’s album and brought the song to guitarist/vocalist Leslie West’s attention for their album. Mountain performed the song at the Woodstock Festival in 1969; this version appeared on the Woodstock 2 album. (More from Wikipedia)
After Mountain, the solo album by Leslie West was released, the group then went on the road using the band name Mountain, bringing along Steve Knight on keyboards after Norman Landsberg and Ken Janick left to start the band Hammer. Their fourth concert as a working band was at the 1969 Woodstock festival; though they were not included on the first album or the concert film, their performances of “Blood of the Sun” (from the Leslie West solo album, Mountain) and “Theme for an Imaginary Western” (written by Jack Bruce and Pete Brown) were included on the double album Woodstock 2 that came out in 1971.
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The other member of Cream, Jack Bruce released his debut solo album, Songs for a Tailor in the U.K. at almost the same moment that Mountain was performing one of the songs on the album, “Theme for an Imaginary Western” at Woodstock.