Jason Isbell and the 400 Unit perform during the Newport Folk Festival 2018 at Fort Adams State Park on July 27, 2018 in Newport, Rhode Island.

The Newport Folk Festival's opening crowd were treated to one classic shock of a set on Friday evening (July 27) when David Crosby strolled out and joined Jason Isbell on the main stage for the final set of the day.

After blasting through "Cumberland Gap" and other tracks off his 2017 album, The Nashville Sound, Isbell invited a grinning Crosby to come out and plug in before the whole lot launched into Crosby, Stills & Nash's weighty 1969 cut, "Wooden Ships."

Isbell then took a minute to reflect on the threads connecting him and his fellow performers at this year's festival and artists of older generations, and how artists like Crosby effect positive, social change with their music.

"The songwriters, the guitar players and bass players and banjo players and singers -- they're all connected to the people that they were when they were trying to make things change," he said. "We need to get together and try to make things change."

The first chords to "Ohio," Neil Young's protest anthem written in reaction to the 1970 Kent State Shootings, then rang out, and Crosby passionately screamed for the crowd to sing the chorus back to him louder before they wrapped the jam.

Watch Isbell and Crosby perform "Wooden Ships" and "Ohio" at the 2018 Newport Folk Festival below.