Can Prog Be Prog Without Drums?

I guess what would make you think that playing just acoustic is not rock? Again, I mentioned The Beatles' "Eleanor Rigby", which is devoid of all guitars, bass, keys and drums, and yet it is considered one of the finest rocks songs ever composed. Also, if one listens to the hundreds of folk-rock bands that were spawned from Bob Dylan's fertile acoustic womb, there are thousands of acoustic-based rock songs.

The divergence from blues or rhythm and blues-based rock and roll that occurred when Dylan strapped on an electric at Monterrey was at first a folk/rock synthesis (as employed at the same time by The Byrds, Simon & Garfunkel and The Beatles), so "acoustic rock" has been a staple rock genre since the mid-60s, predating prog elements, but only by a short time (The Beatles were to incorporate acoustic guitar folk rock with strings on "Yesterday", and became even more adventurous on the Stockhausen-influenced strident and staccato strings on "Eleanor Rigby" and Eastern sitar on "Hide Your Love Away"). Humorously enough, an acoustic version of Simon & Garfunkel's "The Sound of Silence" appears on the album Wednesday Morning, 3 AM, and the electric version with drums is on the next album Sounds of Silence. Each has virtually the same vocal tracks, so is one rock and one is not? 

These early classical, Eastern and folk influences in rock then merged with the burgeoning psychedelic scene and psych folk was born (bands like the It's a Beautiful Day, Donovan, Incredible String Band, and Tim Buckley, along with Jefferson Airplane and the Moody Blues on occasion). 

By the late 60s/early 70s, true progressive folk-rock ran parallel with the primarily British 2nd Folk Revival (with folks like Renbourn, Jansch, Martin Carthy, Nick Drake and folk-rock bands like Fairport Convention and Steeleye Span). Prog folk would include Comus, Jan Dukes de Gray, Jethro Tull, and Renaissance, who combined both folk and classical motifs. 

Roy Harper had a foot in both camps, but an album like Stormcock  is definitely progressive folk rock. Please listen to the song I provided earlier, "The Same Old Rock". It has multiple time signature changes, dense lyrics with a heightened sort of poetry that was a hallmark of folk rock since Dylan, classical influences, difficult chording and one hell of an instrumental section towards the end.

Another instance from around the same time, Led Zeppelin's "Battle of Evermore" has no electric guitar, bass or drums. There is Jimmy Page on mandolin, John Paul Jones plays acoustic guitar and recorder (and an EMS VCS 3 that is used as a voice effect), and Robert Plant and Sandy Denny of Fairport are singing. Not a person here would say it was not a "rock song" because there are no electrics or drums.

Jethro Tull has countless songs that employ just acoustic guitar and strings, like "Reasons for Waiting", just acoustic guitar and organ, such as "Sossity", long acoustic passages in Thick as a Brick and A Passion Play, or simply straight acoustic rock songs like "Solitaire", "One White Duck / 0¹⁰ = Nothing at All", and "Salamander" that exhibits classic influences, blues, folk and rock in one concise 3 minute progressive package: