Arguably the genre began with Gram Parsons, Chris Hillman, Chris Ethridge and Sneaky Pete Kleinow when they formed The Flying Burrito Brothers in 1969. The style then grew like Topsy: here’s a pretty good family tree chart of California Country Rock from pre-Byrds to the still-extant editions of Crosby Stills and Nash and Eagles.
I just bought this CD, which contains the first two albums by the Brothers. Parsons either left or was kicked out of the band following the second album’s release; opinions differ.
Here’s one of the better-known songs from the first album.