Actually it was not called "clunking," it was just mountain biking. This image is a lot newer than most old images of mountain biking; you can clearly see Cunningham roller-cam brakes on the fork and other mountain-specific parts. Clunkers were mainly Schwinn cruisers with big tires, drum brakes, and whatever other heavy-duty parts the riders could find on various other bikes. Also, it was not done on singletrack, it was mostly fire roads, including the famous "Repack" trail, so named because riders would have to repack their hubs after every descent since the grease within them would burn up during the run.
ya, seraph knows where it's at. this looks like one of the last corners on repack. thats probably a fisher-kelly mountain bike. More info on the old stuff is in the klunkerz movie.