It really was in the 1970s that rock music on FM took off in the US. Before then, it was almost exclusively the domain of classical music and local broadcasting

It really was in the 1970s that rock music on FM took off in the US. Before then, it was almost exclusively the domain of classical music and local broadcasting. The typical top 40 stations were on AM, and the phrase "clear channel" meant a 50 kW AM transmitter that was allowed to run at max power 24x7 (so it could be received over a vast region after dark). Rock on FM was local, and very experimental in those days. Then, in the 1980s, consolidation started happening, and the 1996 removal of all ownership limits turned radio in general (both AM and FM) into the cesspool it is today. There are precious few rock stations from those days that have managed to stay around, not get borged into the cesspool, and keep their format without going stale. In my neck of the woods, WXRT 93.1 in Chicago is the best example.