The Grateful Dead & The Band - original Americana groups?
Whenever people talk about the beginnings of the Americana movement, there are two groups that I don't ever hear mentioned that I think deserve recognition. If we take "Americana" to mean music that has its roots in American myth and folklore (and of course folkloric music, including the blues), both the Dead and the Band qualify in every respect.
The Band started off as a rockabilly combo backing Ronnie Hawkins, and moved on to back Dylan before stepping out on their own with "Music From Big Pink". This was a record created in the pastoral setting of Woodstock, NY and the surrounding Catskill mountains, with vividly painted characters and an almost archaic yet timeless sound. Echoes of back-porch jams in the hollers blended with hints of old brass bands and high gospel harmony. Their subsequent recordings delved deeper, reaching a pinnacle (for me) on the following record simply titled "The Band" and known to fans as the Brown Album. Characters like Civil War veteran Virgil Cain in "The Night They Drove Old Dixie Down" and the struggling farmer in "King Harvest (Has Surely Come)" are as quintessentially American as can be....ironic that four-fifths of the group were Canadians.
"Music From Big Pink" turned out to be a tremendously influential album. George Harrison came to Woodstock to seek out the mountain men making this new music. Eric Clapton credits Big Pink with his shift from the psychedelia of Cream and Blind Faith to the more relaxed, country-flavored sounds of his first solo records. And on the West Coast, psychedelic voyagers the Grateful Dead heard Big Pink and also shifted gears into a more rootsy, acoustic sound. The companion 1970 albums "American Beauty" and "Workingman's Dead" are a radical shift from the previous experimental acid-fueled sounds of "Anthem of the Sun" or "Aoxomoxoa", and began lyricist Robert Hunter's exploration of American mythology. (The fact that he and Garcia appropriated a large amount of material from old folk and blues songs and took credit is a matter for some debate, though most of that material is public domain and of untraceable origin).
Back in England, Elton John and Bernie Taupin began their own exploration of American myth with albums like "Tumbleweed Connection" (a meditation on 19th century America, with several Civil War songs and opening with a song about a gunslinger on the run).
In Southern California, of course, the country-rock sound that began when Gram Parsons joined the Byrds flourished and became a major part of the sound of the 1970's. This seems to be where most people focus when talking about Americana music on the rock side, but I do believe that the contributions of the Band and the Dead merit recognition as well.