Philadelphia Folk Festival
This video is an historical document. It shows a very important point, the beginnings of jamgrass.
It show the nexus of traditional music and what John Hartford called "that New York sensibility" (weed fueled organic improvisational  music).
Hartford & Bromberg met at the 1970 Philadelphia Folk Festival. They made an immediate connection and were inseparable, a spark was made, the fuse was lit.
Bromberg was scheduled to play solo he instead formed an hoc band and invited John. (The first time Bromberg played in public with a band)
John said he was greatly inspired by Bromberg's creative attitude and shortly after this fest, he asked Bromberg to produce his seminal jamgrass album, Steam Powered Aereo Plain.
And the rest is history, as they say.