It wasn't one of the Clarkes. It was John Selva's teenage son on an '88 YZ250. He actually crashed the bike in a later race and was run into by another rider. The bike cart wheeled all the way down the stair case. The subframe was folded down onto the rear wheel and you could clearly see the shape in one of the down tubes from the wheel of another bike. Totally destroyed the frame. The young bloke wasn't hurt and took it all in his stride.
Cheers,
K