classic rock, if you're old enough, is late 60s, early 70s.
led zepplin. rolling stones. the guess who and american woman. who can miss the intro of that song? we even went through a disco phase and rock held on tight. face it - ever see a "classic disco" station? please.
even the late 70s and into the 80s rock just went, for lack of a better word, corporate. CORP ROCK SUCKS!!! but looking back it all was but that rock is considered "classic" today. van halen, night ranger, bon jovi, ac/dc, KISS and many more who while some may have roots in the 70s to be sure, their "rock" bled into the 80s and certainly still today for a few.