"Epic" compositions worth giving a spin

Not concept albums, but a single LP side consisting of a work with some sort of unity/continuity.  Possibly with one or more recurring musical themes or ideas.  Could be vocal or pure instrumental and I would think in the 20-30 minute duration.  What made me think of this is "Supper's Ready," by Genesis, which I've listened to a few times recently.  It seems challenging to put together a piece that has repetition and variety such as this and I think they pulled it off nicely.  I especially like the concert version with the added visual interest of Peter Gabriel's costumes, gestures and mime routines, but for me this one works well with just the music.  Does anyone have a favorite one-side long-form piece to throw out?
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Shine On You Crazy Diamond.by Pink Floyd on Wish You Were Here.
Technically Parts I-V and VI-X are on different sides with a couple of songs sprinkled in between, but it works for me.

If sticking to the one side only theme I’ll go with Echoes from Pink Floyd’s Meddle LP.

For a live LP I’d go with Dazed and Confused by Led Zeppelin's The Song Remains The Same.
Meshuggah - "Catch ThirtyThree" 
"Get Ready"- Rare Earth Live
"Gates of Delirium"- Yes
"Anesthetize" Porcupine Tree
"Utopia"- Todd Rungren's Utopia
"Singring and the Glass Guitar"is another interesting one from Utopia.
"A Plague of Lighthouse Keepers". Van Der Graff Generator
"Remember the Future" and "A Tab in the Ocean". Nektar
"Eruption". Focus
"Close to the Edge" Yes
"Arriving Somewhere But Not Here". Porcupine Tree
"Dabsong Concerto". And "Nine Feet Underground" Caravan
"The Snow Goose" Camel
"The Magicians Birthday".  Uriah Heep
"Child in Time" and "Space Truckin"  from Deep Purple "Made in Japan"
"Tarkus" and "Karn  Evil 9" by ELP
"The Soft Parade" The Doors
"Terrapin Station". Grateful Dead
"Thick as a Brick" and Minstrel in the Gallery". Jethro Tull
"Dogs" Pink Floyd

2112- Rush
Change of Seasons- Dream Theater
Universal Migrator-Ayreon
I am working my way through the symphonies of Sir Arnold Bax on the Lyrita label. They certainly qualify!
How about Miles Davis:  "Bitches Brew" (the track, on the LP of the same name) and "Right Off" (side 1 of "A Tribute to Jack Johnson"). 
Rodney Crowell: The Houston Kid; Rosanne Cash: The List; Iris Dement: The Trackless Woods; Loudon Wainwright III: Last Man on Earth; The Fraternal Order of the All (Andrew Gold and Graham Gouldman): Greetings From Planet Love; Bill Frisell: The Space Age!; Evan Johns: Moontan; The Ventures: In Space; Bob Dylan: Blood On The Tracks and/or Self Portrait; The Band: Music From Big Pink; The Band: The Band; The Beach Boys: Pet Sounds and/or Smile; Van Dyke Parks: Song Cycle; Randy Newman: Good Old Boys; Neil Young: Harvest; The Traveling Wilburys: Vol.1; The Rutles: The Rutles.
Looks like a lot of "whole albums" and at least one double album as opposed to long songs listed there (although many are favorites of mine).

Oh, okay, NOW I understand the premise! I’m probably less a fan of the "long-form" song than anyone else here, but there have been a couple I liked. Brian Wilson’s "Elements Suite" would have been a classic had it not gone down in flames, both figuratively and literally, with the rest of The Beach Boys Smile album. "In Held ’Twas I", the second side of Procol Harum’s Shine On Brightly album is a good one. I liked The Who’s Tommy when it was released (and saw the tour in support of), but that is spread over four LP sides. I guess each side could be considered a "movement" of the mini-Opera. A rock Group referencing their music to Opera about says it all, doesn’t it? Oy.

For the generation in High School in the early-to-mid 70’s, Progressive music was what was happening. I’ve played music with guys that age, and it has always amazed me how completely British influenced they are, knowing so little about American "Roots" music. The British-lead Prog Rock Bands discarded American Rock ’n Roll for Classical and Jazz as the ingredients for their music making. I was not and am not alone in finding the music made by these self-proclaimed and self-congratulatory "Classically trained" musicians pompous, pretentious, and shallow---an exercise in displays of empty virtuosity. "Pictures At An Exhibition" as performed by a 3-piece Rock Group? What an astoundingly bad idea! It’s execution is even worse. I saw Emerson’s pre-ELP Group The Nice live (and had their album), and nice they were not.

Around the same time The Kinks were able to come back to America, and I saw them twice in a year. Now THERE was a great British Band! They have two albums that qualify for inclusion in this discussion---The Kinks Are The Village Green Preservation Society, and Arthur (or the Decline and Fall of the British Empire). Great albums! I don’t know how they did it, not having any classical training.

I don't know how I overlooked In-A-Gadda-Da-Vida by Iron Butterfly.
"The British-lead Prog Rock Bands discarded American Rock ’n Roll for Classical and Jazz as the ingredients for their music making."

No they didn’t. 

 "I was not and am not alone in finding the music made by these self-proclaimed and self-congratulatory "Classically trained" musicians pompous, pretentious, and shallow---an exercise in displays of empty virtuosity."

Are you referring to all British prog artists during that period? Do you retain the same belief for contemporary progressive music?
bdp24, I liked the Kinks (more 60's and early 70's than later, but some good things happened later on) and the British prog-rock bands, to a degree.  I liked Yes through Close to the Edge and Genesis up until The Lamb Lies Down on Broadway.  DIdn't like them after Gabriel left and Phil took over.  I felt King Crimson was usually interesting and good no matter what the lineup.  For a less indulgent and more rock-based sound try the Yes album, my favorite of theirs.
As far as the thread prescription goes, I think any side of Yes' Topographic Oceans fits better than a side of Tommy.

Timeless - John Abercrombie and Jan Hammer. Simply mezmerizing...

I always felt that U2's Joshua Tree had great continuity and flowed very well.

good thread--reminds me of the period after tommy where every technically-proficient band in england made their "concept albums." looking through some of the above-mentioned, it seems that most were more notable for their ambition than their listenability, the kinks (of course) excluded.
two that i'd toss in:
1. side one of "ogden's nut gone flake" (small faces)--i could never quite grasp what the story's about, but it's funny as hell and periodically rocks like the dickens.
2. richard buckner, the wall--an album length rendering of the poet edgar lee masters' "spoon river" done by my favorite dour folkie. really, really good.stuff.
I'm going to disagree with Bdp re: "The Kinks have two that fit".  They have (at least) three; adding in "Schoolboys in Disgrace".

As to his view that prog rockers "discarded American rock n roll", there's much more than a little truth to that.  Putting aside the technical specifics for a moment, the spirit of early rock n roll was primitivistic and reductionist.  Prog rock went 180 degrees the other way.  OTOH, many "non-prog" bands had already done that years earlier.  Long guitar solos (and any band that employs them) can also be reasonably described as anathema to that early r'n'r spirit.  That covers a lot of ground beyond prog.

While I was a prog fan in the '70s, I'm much less so, now.  My view is less harsh than Bdp's, but I do find that it usually leaves me cold.  Still love a flashy guitar solo, tho.
I like good music. Some of it happens to be prog rock. Also yes it was/is the furthest thing with any rock elements from original rock and roll of Chuck Berry and company pretty much by definition. Most rock music falls in between the two somewhere. So its a continuum of styles ie all shades of grey from rock and roll to prog rock, not black and white.

Neither extreme forms are all that popular these days. Things have head off in many different directions since the term prog rock was originally coined.

Viva la music!
Personal favorites listed below, some already mentioned. You can add to this list any one of 100s of symphonies and tone poems but the likes of Bruckner, Dvorak, Strauss, Mahler, Sibelius, Elgar, Shostakovich, etc....the list goes on and on.

Camel: Snow Goose….
ELP: Pictures at an Exhibition, Tarkus and Karn Evil 9
Pink Floyd: Dogs, Echoes, The Wall, Atom Heart Mother Suite
Jethro Tull: Thick as a Brick
RUSH: 2112, Cygnus X-1 Book I: The Voyage and Book II: Hemispheres,
RUSH: La Villa Strangiato, The Fountain of Lamneth, Xanadu
Genesis: In This Cage->Afterglow, Supper’s Ready
Genesis: Cinema Show, The Battle of Epping Forest
YES: Gates of Delirium, The Revealing Science of God, Close to the Edge:
Dream Theater: Six Degrees of Inner Turbulence, Change of Season, Octavarium
Dream Theater: In the Presence of Enemies, Metropolis Pt. 2
Supertramp: Fool’s Overture
Spock’s Beard: The Great Nothing, The Water
Green Carnation: Light of Day/Day of Nothing
Keith Jarrett: The Vienna Concert, The Kohn Concert, etc…
master of long improvisations on themes spanning entire concerts
Pat Metheny Group: The Way Up:
SOLARIS: Marsbéli Krónikák (Martian Chronicles)
SOLARIS: Nostradamus: Próféciák könyve (Nostradamus: Book of Prophecies)
Forgot to include the following;

ELOY: Land of No Body, The Light from Deep Darkness, Poseidon's Creation, 
"Atlantis' Agony at June 5th - 8498, 13 p.m. Gregorian Earthtime", The Apocalypse,
Reflections from Spheres Beyond, The Answer
Do Spocks Beard "Snow" and the Who "Quadraphenia" count?
Nutty, the original idea was one LP side, more or less.

I'm impressed with the number of contributions (some of which are actually longer than described, but it's still fun, right?).  Glad to see Procul Harum's "In Held 'Twas in I" get a mention.  Anyway, I'm grateful for more leads than I could ever hope to follow up.
Miles Davis - Pangaea-
Allman Brothers "You Don't Love Me", "Whipping Post"
Tedeschi Trucks Band "Uptight"
Built To Spill "Cortez The Killer"

Van Halen-

Running w/ the Devil- Eruption- You really got Me!

When I first saw the topic Selling England by the Pound came right to the top, as I read the posts and your replies you focused on 'sides' or extended songs; my vote...Anthony Phillips-The Geese and the Ghost.. (has distinct Genesis roots if you don't already know, thx for the topic
Rattlesnake Shake off Fleetwood Mac's Live at the Boston Tea Party album (Yea I know, but still a good song). Dark Star off the Grateful Dead's Live Dead album. King Crimson Red. Various Bardo Pond sides (they love long songs). Frank Zappa's Billy the mountain. Side 2 of My Morning Jacket's album that looks like crap hit the fan (clear with various sized brown spots splattered about / You get the picture).