in fairness Cyclocross goes way back to the late 18th Century..as a race between towns..
I remember seeing it in Yorkshire in the late 70's and thinking thats nuts !
and I'm pretty sure one of the first regular offroad cycles was undertaken by a DUblin Road Club, I can't remember which one, they used to cycle annually over Table Mountain to Donard back in the 70's too..I remembr seeing them twice when I was trying to open up Glenmalure as a climbing area !! In fact one of the earliest offroad spins I did was push a Triumph 20 to the top of Table Mountain and ride back down the fireroad with my mates !
I'd say that as long as theres been bikes, lunatics have always gotten them to the top of mountains and tried to ride them down them..its just a natural thing to do !!
I remember riding a triump twenty with two rear tyres one inside the other and a big pair of cowhorn handle bars up and down 3 rock in about 81,82 even before riding my BMX down it. Lot easier on a MTB. Until I got on the Epic pain train.... But even before that I remember Hego riding his Sun Squadra 5 speed up there with flat bars... and I 've noticed recently that The Dutch claim to have started BMX and off road cycling in the 50's. Anyway once it's off road it's good.