Geoff, I'm curious about the 1961 reference. What started that year? Most historians have it starting in the 54-56 era, and then being reborn with the Beatlemania/British Invasion of 64-65.
There's no real start date; it's spread out over a period of time. When it reached critical mass is up to interpretation. Looking at the iTunes 1961 charts, it seems to me a good year for the handoff from the old 'corny' Top 40, which still charts to this day, but has a lot of clearly cool shit too. It just happens to be the year I got really hooked on radio, so it's also personal.
I Fall to Pieces (Single) Patsy Cline
Crying Roy Orbison
My True Story The Jive Five
Running Scared Roy Orbison
Exodus Ferrante and Teicher
Where the Boys Are Connie Francis
Travelin' Man Rick Nelson
Shop Around The Miracles
The Boll Weevil Song Brook Benton
On the Rebound Floyd Cramer
Calendar Girl Neil Sedaka
Wings of a Dove Dolly Parton
Little Sister Elvis Presley
Blue Moon The Marcels
Daddy's Home Shep & The Limelites
(I Don't Know Why ) But I Do Clarence "Frogman" Henry
Runaround Sue Dion
Hurt Timi Yuro
Hello Mary Lou, Goodbye Heart (Live) Rick Nelson
Surrender Elvis Presley
Walk Right Back The Everly Brothers
The Way You Look Tonight The Lettermen
Moody River (Single) Pat Boone
One Mint Julep Poncho Sanchez
Gee Whiz (Look at His Eyes) The Emotions
Stand by Me Ben E. King
Spanish Harlem Ben E. King
It's Gonna Work Out Fine Tina Turner
(Marie's the Name Of) His Latest Flame Elvis Presley
Bless You Tony Orlando
Breakin' In a Brand New Broken Heart Connie Francis
My Kind of Girl Matt Monro
Tonight My Love, Tonight Paul Anka
San Antonio Rose Floyd Cramer
Big Bad John Jimmy Dean
Dum Dum (Single) Brenda Lee
You Can Depend on Me (Single) Brenda Lee
Take Five Dave Brubeck
Are You Lonesome Tonight? Elvis Presley
Sea of Heartbreak Don Gibson
Please Stay The Drifters