The answer to that very question, has troubled me too, for a long, long time!!
I've said before, If I was dropped inside the perimeter of King Tut's tomb, I would *STILL* find clad!!!!
When I'm out there I'm always saying, "Hmmm...quarter hit. I wonder if its from farmer Johnny come lately or from farmer Aalouwishes P. Coltrane! "
But, with this current place there is something even more odd about the clad I'm finding. Not all, but the majority of all of it, from 1970 to 1979!! Sure, some newer but almost *nothing* from earlier! So strange!
I might could understand if there was a lot of fill dirt but, there's not.
Anyhoo, thanks for the inquiry!
Merry Christmas!