As the title suggests, what years do you describe (https://pulsemusic.proboards.com/thread/219178/transitional-years-different-popular-music) as being “transitional (https://carleton.ca/lifelong/wp-content/uploads/McCuaig-The-Story-of-Rock-and-Roll-1951-1966-2.-1958-1964.pdf) periods” between different eras in popular music?

Interesting question: Since I write much about the history of rock and music in general, I can answer this, I believe:

I would say that the first "transitional" period of rock and roll came between
1959 and 1964 in a five-year span. In February 1959, the deaths of Buddy Holly, Richie Valens, and the Big Bopper indicated the end of the "Big Bang" of rock and roll with Elvis, Chuck Berry, Jerry Lee Lewis, and others. Elvis had left music for the army, Berry and Lewis fell from favor due to scandals and legal woes, but with the death of rising star Buddy Holly, this tragedy took the fire out of the momentum of rock and roll. One young star, Eddie Cochran, died at the age of 21 in 1960 in an auto collision in Britain.

So during this time music and radio was dominated by teen pop singers such as Bobby Rydell and Fabian, as well as girl groups and "doo-wop" acts. R & B groups were on the rise. There were no real guitar-oriented rock and roll acts as of yet, though instrumental surf-rock was becoming popular with groundbreaking guitarists such as Dick Dale with "Miserlou" and songs such as "Pipeline" by The Chantays, "Wipeout" by the Surfaris, "Surfin' Bird" by The Trashmen, and "Walk Don't Run" by The Ventures. Between 1958 and 1964, surf-rock was all the rage. In 1962, "Green Onions" by r &b act Booker T & the MGs was also high on the charts. This set the stage for the next big phase in rock and roll in 1964.

Dick Dale was one of the first guitarists to play at high volume and create amplification to accommodate for his unique sound. This would prove to be pivotal in the new decade.

One music scene which rivaled the surf guitar sound in the early/mid 60's was the American folk revival which began in the mid-late 50's and continued through the mid-60's. The scene was initially tied in with the Beatnik counter-culture where many fans and hipsters flocked to coffee houses to hear musicians sing songs of social dissent and other similar themes. Some artists such as the Kingston trio avoided such themes and became popular with a clean-cut, collegiate look and sound to rise to the top of the charts in 1958. Folk songstress Joan Baez arrived with her debut album in 1960 and was overtly political. Such socially conscious themes were taken from folk pioneers as Pete Seeger and Woody Guthrie, and worked well in the new decade which was seen as the era of social progress and awareness. Singers Harry Belafonte and Odetta were prominent during this time. Many of these artists were based in the scene in Greenwich Village in New York City.

Of course the scene would not be complete without folk legend Bob Dylan and Peter, Paul, and Mary, who both performed at the 1963 Civil Rights march in Washington D.C. with Martin Luther King. Much of the music by these folk revivalists, called Freedom songs, were heard at freedom rides, lunch counter sit-ins, and voter registration drives during the early mid-60's. Songs such as Dylan's "Blowin' In the Wind" and "If I Had a Hammer" by Peter, Paul, and Mary were performed. Many of the artists had obvious left-of-center leanings, such as Phil Ochs, which brought criticism and scrutiny from more conventional persons in society as the country dealt with the Cold War "Red Scare". It appears that the conventional music status quo felt more comfortable with standard pop and rock tunes than with the radical messages of the folk troubadours rising on the scene. The folk movement was introduced to American television with the comedy duo of The Smothers Brothers (RIP Tom Smothers 2024), who not only introduced a raucous performance by The Who in a 1965 episode, but faced censorship by the network when their brand of social commentary was deemed "too controversial" for prime time.

The clean-cut collegians of The Kingston Trio;
Joan Baez;
Bob Dylan and Joan Baez;
Peter, Paul and Mary at the March On Washington 1963;
Phil Ochs, who tragically died early due to mental instability;
The Smothers Brothers. with Tom Smothers displaying the acoustic guitar Pete Townsend of The Who smashed onstage during one TV episode;
Odetta, whom MLK called "The Queen of Folk Music":