.... as the shallower coins are almost always cleaned out(not counting the clad). ....
This is a phenomenon that occurred d/t the "hay-day" of md'ing in the 1970s/80s/90s : There were some geographic locales parks that got hit hard for their obvious deep high conductor-cherry-pickens, during that time. But the machines of that era were doing-good to just work out the top 7" . Eh ?
And that generation of md'rs eventually gave up on those worked-out parks.
But in the early 2000's, as machines like the Explorer opened up the 8 to 9" window, an odd phenomenon happened: Certain parks had more silver to harvest. But oddly, the vast majority of dates were 1920s and earlier. D/t all the 1940s to 1960s stuff had already been had. Hmmmm.
We saw the same phenomenon in the very early 1980s: There were a few parks were the TR disc. era had stripped out all the 4 to 5" layer. But the advent of the first motion discriminators (6000d) to hit those same parks, opened up another inch or two. And yes, not many silver washingtons or roosies to come up. D/t the "prior generation" of shallow seekers had already had that layer.
Strange how the generations keep going on !
But now, all the generations "skipped the shallowest layer of clad" (ignoring the 1 to 2" deep stuff, sadly). So today: There is a layer of clad, that everyone refuses to harvest, that will forever mask the deeper silver
The only "frontier" now, is if tractors blade off the top 6" of turf. Doh !