Progressive folk

Progressive folk or prog folk was originally a type of American folk music that pursued a progressive political agenda, but in the United Kingdom the term became attached to a sub-genre that rejects or de-emphasizes the conventions of traditional folk music and encourages stylistic or thematic innovation. It gave rise to the genre of psychedelic or psych folk and had a major impact on the development of progressive rock.

Source : http://en.wikipedia.org/wiki/Progressive_folk_music