From Rolling Stone:
Groupies and “The Lord of the Rings” seemed to provide the inspiration for “Ramble On,” recorded in 1969 in New York on Led Zeppelin‘s first U.S. tour. Over Page’s acoustic guitars, Plant wails, “In the darkest depths of Mordor/I met a girl so fair.” Middle-Earth influenced more than just the band’s music: “After reading Tolkien,” Page said, “I knew I had to move to the country.” Legend has it that John Bonham plays a plastic garbage can on the song.
The opening line (“Leaves are falling all around”) is probably a paraphrase of the opening line of [J. R. R.] Tolkien‘s poem “Namárië“. The poem may also be the inspiration for the entire first verse.
The Tolkien references later in the song refer to the adventures of either Frodo Baggins as he travels to Mordor, or that of Aragorn as he has to choose between staying with his love Arwen (Elrond‘s daughter) or going to destroy the Ring in Mordor:
Mine’s a tale that can’t be told,
My freedom I hold dear;
How years ago in days of old
When magic filled the air,
T’was in the darkest depths of Mordor
I met a girl so fair.
But Gollum, and the evil one crept up
And slipped away with her.
The guitar’s jangly introduction employs a classic Jimmy Page technique: using regular open chords superimposed higher on the fretboard.
The song also serves as an illustration of the tight interplay between bassist John Paul Jones and drummer John Bonham. Jones’ light, melodic bass phrases give way to an ascending motif which follows Bonham’s bass drum.