Folk-pop met shaggy rock 'n' roll onstage at Mountain Jam today, as the Avett Brothers hosted Bob Weir, the storied singer and guitar player from the Grateful Dead, during their set at the Upstate New York music festival, going on this weekend.

The Avetts were tearing through a 90-minute set when the jam-band icon -- clad in a T-shirt, cargo shorts and an appropriately wild beard -- came onstage. Weir joined the band for a few songs, including the Avetts' 'Go to Sleep' and George Jones' golden country hit 'The Race Is On' (which was often covered live in the Dead's heyday).

Before the show, we had the chance to sit down with the Avett Brothers, and bassist Bob Crawford had a heady Dead experience or two in his day. "Back in the late '80s and early '90s, I may have been seen wandering Giants Stadium, and numerous coliseums of the northeast and the Midwest, outta my mind, getting tickets and going in," he told us.

Elsewhere in their set, the Avetts rolled out a set list packed with fan favorites, including 'Pretty Girl From Chile' and a version of 'I and Love and You' that had Scott Avett standing at the head of the stage, arms outstretched, as the band's excited and many female fans shouted along.

And the Avetts managed to slip in a reference to R&B legend Sam Cooke, sneaking in a few lines of 'You Send Me.'

Check out some of our Instagrams from the show: