We may get different ideas from the US and Brits. (This is not an unusual phenomenon). Probably most Brits would see Martin Carthy as a pivotal figure - maybe Ewan McColl would be there too. When I think of someone who embodies US folk music, I always think of Pete Seeger or Woody Guthrie. I will admit to bias though. Guthrie was essentially a busker - probably a prodigiously talented one - and that sums up the spirit of folk music to me. It is universal, travels well and can be performed simply in any community. No doubt plenty of Catters can improve on this rash assertion!