The song is 5 minutes 15 seconds long. It was written by Roger Waters and performed with David Gilmour (vocals, guitar) and Richard Wright (organ). The song has a hallucinogenic, pastoral quality, with prominent organ and bird sound effects, like those that later that year featured on the Ummagumma track "Grantchester Meadows". It was also included on Pink Floyd's compilation album Relics. The song features no drums, which creates a rather unusual feeling. The Hammond and Farfisa organ coda is similar to that found on the "Celestial Voices" section of "A Saucerful of Secrets". While the Hammond provides a stately foundation with an Em-Bm-D-A-G-D-B sequence, about 1/4 way into the coda Wright introduces the Farfisa which, run through a Binson Echorecplatter echo, produces the swirly, trembly, echoey sound that hovers over the Hammond.
The opening birdsong is from a 1961 recording entitled "Dawn Chorus" and the single bird featured over the organ part is a nightingale also from 1961. Both featured on an HMV sound effects single (together with a recording of owls) but presumably the band just borrowed the originals from the EMI sound effects library as EMI owned HMV.
"Cirrus Minor" has an unusual chord sequence: E minor, E flat augmented, G major, C# minor 7, C major 7, C minor 7 and B 7. The chords are built around the chromatically descending bass line. The B 7, C major 7 and G major chords are the only chords which fit into the functional context of the E minor key. This chord sequence gives the song a very surreal atmosphere.
Equally surreal is the stately and pastoral Hammond/Farfisa coda. It is one of the best examples of how and why Wright was so crucial to the band's early sounds and textures.