“Mountain Jam,” The Allman Brothers Band.

The Allman Brothers Band formed in 1969, when a group of veteran performers coalesced around the ferocious slide guitar of Duane Allman and the eerie, haunted vocals of his younger brother Greg. They were equally at home with soulful pop (before guesting on Eric Clapton’s “Layla,” Duane established himself as a session guitarist at Muscle Shoals, recording with Wilson Pickett, Aretha Franklin, Laura Nyro, etc.), straight ahead blues, and Eastern-tinged jazzy improvisations. Collectively, ABB crackled and rumbled like a thunderstorm in an earthquake.

The apex of the ABB’s recorded career was a set of live recordings from the Fillmore East in New York City, taped in 1971. The Rolling Stone Record Guide, not known for praising extended psychedelic improvisations, noted that on this album of extended tunes there were “no pointless jams, no wasted notes”: this from an album with two songs clocking in at over 19 minutes. But as great as At the Fillmore East is, the ABB’s greatest moment came on a leftover track from these concerts, “Mountain Jam,” which was included on their follow-up, the otherwise studio LP Eat a Peach.

Built on a piece of psychedelic fluff from the British singer, Donovan, “Mountain Jam” is thirty-four minutes of percolating jams. Duane Allman solos like a man possessed, improvising gorgeous motifs and then turning them inside out with altered passing tones. Berry Oakley’s bass solo and the drum duel that follows are also monumental. Then Duane takes over again, calming the band, floating through space in a freeform sequence worthy of the Grateful Dead, and then . . .

. . . then (at 27.20) Duane drops the band into a perfect, gently grooving instrumental rendition of the Southern Gospel standard, “Will the Circle Be Unbroken?” It is a moment of electric, hair-raising beauty. Two choruses later, the opening theme returns, and the song (and concert) come to a thunderous conclusion.

“Mountain Jam” is essentially the epitaph for both Duane Allman and Berry Oakley. On 29 October 1971, before Eat a Peach was released, Duane was back in his hometown on a break from recording. Allman was driving his motorcycle and collided with a flatbed truck, loaded with lumber. He was taken by ambulance to a local hospital, where he died of his injuries. Thirteen months later, Berry Oakley’s motorcycle collided with a Macon city bus; this collision took place less than a quarter mile from Duane’s fatal collision. Oakley died of severe head trauma later that day.