VERY low flying was legal in the 1950's in many cases. I was in a F2H-3 (Banshee) Squadron, VF-152. We had just become Special Weapons squadron, and we were madse to do long trips at very low level, TO and return to NAS Moffett.
I recall flying just above the tops of wheat stalks in one of those huge farms in The Valley. Looking in my rear view mirrors, I could see a real "wake" in the wheat. RA was reading 25'.
I recall another time flying wing on a lead who returned with a flying fish impaled on his pitot tube. I flew stepped up on him after that.
At the loft bombing range in NV used by us at NAAS Fallon, we flew RA 15' over the desert floor to the IP or the target itself.