unless of course "skid sheets" in those days were hermetically sealed in non biodegradable plastic. Oh, and BTW Tim, "cellophane" is 100% biodegradable.
As you must know, Bob, cellophane is both waterproof and
impermeable to microorganisms. Biodegradation, of course, requires the presence of microorganisms (see definition below), and probably some amount of oxygen. Both of these may be insufficient at 985 feet to do such rapid damage to cellophane. BTW, it appears to me that many of the "cellophane" packs have been heat-sealed. You may not be able to see such fine details unless you are able to take advantage of the 960x540 high definition version of the video.
Of course, maybe these bags were made of something even more durable that cellophane. We really don't know, do we?