Grateful Dead legend Bob Weir joined one of today's most popular groups, the Avett Brothers, on stage this weekend at the 10th annual Mountain Jam festival.

At the top of this page you can watch them all play a cover of country legend George Jones' 1964 hit 'The Race is On,' which Weir and the Dead covered on their 1981 live album 'Reckoning.' Weir also joined the Avetts for a run-through of 'Go to Sleep,' a track from their 2007 breakthrough album 'Emotionalism.'

The Avett Brothers, arguably one of the biggest folk-rock bands in the world right now, held the crowd in the palm of their hands through their 18-song set. You can watch the final number, 'Head Full of Doubt / Road Full of Promise,' below, and check out Diffuser.fm's exclusive interview with the group as well.

Later in the evening, Weir closed down the second night of the festival with an extended set by his group Ratdog, giving everybody the chance to stare and stare and stare at that absolutely amazingly bushy mustache one more time.

Watch the Avett Brothers Perform 'Head Full of Doubt / Road Full of Promise'

Watch An Exclusive Interview with the Avett Brothers