Before the show begins, Karen from the tie-dye stall tries to explain how the Dead crowd became factionalised in the mid-’80s, when an influx of unruly younger fans brought new levels of chaos and disruption to the band’s shows and generally “pissed off everyone”. “We weren’t here to get wasted,” she says. “We were here to see the shows and the whole LSD thing was part of the experience. We didn’t make people mad in small towns. We knew not to sleep downtown, not to panhandle, not to drag our dirty puppies through restaurants.

“The best quote I ever heard was, ‘It used to be a zoo of a kind, now it’s kind of a zoo.’ It was said by one of my friends, tripping in the middle of the night. I think,” she laughs, “his name was Wind.”

The day after, a couple of miles from the stadium, I meet a dishevelled twentysomething who is a kind of millennial equivalent of the people Karen described. He has only grubby socks on his feet, and is begging because he is “desperate for a pair of shoes for tonight’s show.” The events that befell him after the previous night’s concert are, to say the least, unclear, and he has no idea where his friends are or where he is meant to be staying. Throughout it all, though, he has held on to his skateboard, and to a cache of “really good LSD I’m trying to get rid of”. With a very old solution to a very modern problem, he offers acid in exchange for the use of a cellphone charger.

It would be churlish, of course, to see his behaviour as dishonouring the drug etiquette of vintage Deadheads. Tom, from Idaho, is a veteran of 50-odd shows by the Dead and their offshoots, and first saw the band at the massive Watkins Glen show in 1973, when they shared a bill with the Allman Brothers and The Band in front of 600,000 people. Tom drove down to Watkins Glen from Boston, and remembers it as “one of the wildest times of my life. I was doing meth and peyote. I was 18, with nine friends, we were nuts. By the time we went home there were only three of us left, the rest of them we lost. They couldn’t handle it, they had to get the fuck out. It was mayhem; overwhelming.”

At 9.40, the atmosphere for the evening’s second set is sensational. Traditionally, the latter half of Dead shows privileged the more expansive and exploratory side of their art, and tonight is no exception. After a prologue of “Cryptical Envelopment” – quaveringly sung by Phil Lesh in a voice that, for all its weathered charisma, remains mostly unsuitable for fronting a rock band – the literal and metaphorical fireworks begin. “Dark Star” was established as the Dead’s signature jam in 1968, and tonight’s 30-minute version is a bracing reiteration of the band’s uncanny chemistry. In So Many Roads, David Browne describes a “Dark Star” from November 1969. He writes of how Lesh, with his wildly ambulatory basslines, briefly appeared to be “wrestling for control of the song,” before “Garcia would take command but then retreat back into the song’s haze… Lesh wouldn’t be steering ‘Dark Star’ for long,” Browne observes. “In fact, no one would.”

Trey Anastasio is, as discussed, no Jerry Garcia. As the 2015 “Dark Star” unravels, however, his discretion acts as a useful analogue for the slippery ways in which Garcia subverted his role of lead guitarist. Like the best Dead jams, all the elements – Anastasio’s sculpted leads and Weir’s impressionistic rhythm guitar flecks, the two percolating drummers, the two elegantly interpretive keyboardists – seem to be on their own trajectories: following orbits which only occasionally intersect; untethered by anything remotely resembling an orthodox bassline. Its success seems more down to chance than traditional musical logic and, if Anastasio succeeds at anything tonight, it is in insinuating himself into such a mystifyingly intuitive unit.

At times like this, the Dead’s ability to hold the attention of nearly 100,000 people while playing radically weird music, remains astonishing. The traditional “Drums > Space” jam is an especial challenge, beginning with Kreutzmann and Hart methodically working round several dozen percussion instruments, and progressing through a kind of tribal techno that is, perhaps unwittingly, reminiscent of a Spiral Tribe or Megadog rave in the early ’90s. When their bandmates return for the “Space” improvisation, Anastasio finally slips in some of the prog quackery that makes much of the Phish canon a hard sell to those averse to such whimsical excess.

It is, though, a rare indulgence on Anastasio’s part. Discipline isn’t something traditionally associated with continuous jams that last for nearly two and a half hours, but this is the prevailing mood of the second set, and its two magnificent highlights. First there is “St Stephen”, euphorically punching its way out of the skronk. Then, at the end, there is Bonnie Dobson’s “Morning Dew”, originally attempted by the band on their debut album, and here essayed with a bluesy grace and spaciousness that feels restrained even when Anastasio lets his notes hang and arc in the air, and Bob Weir, lugubrious in trademark shorts and sandals, pushes his hearty but limited voice as far as it will possibly go.

Finally, Hornsby leads them through one more of the homespun singalongs from Workingman’s Blues, “Casey Jones”. Apposite quotes for most occasions leap out of the discography of any band with a long career. The Grateful Dead are no exception, with a rich catalogue – mostly written by Garcia’s old friend, Robert Hunter – that presents risk, fate and mortality as part of some picaresque outlaw adventure. The refrain of “Casey Jones”, though – “Trouble ahead, trouble behind” – feels particularly useful. For all the stage-managed finality of Fare Thee Well, this is very much an ongoing story.

 Heads up! Next month’s Uncut – on sale July 28 in the UK – comes with a FREE GRATEFUL DEAD CD: our historic attempt to piece together the album that should have followed “Workingman’s Dead” and “American Beauty”…