eboy, you've gotten good answers so far. I would add that if you are in a given site, that has un-disturbed soil, then you might start to develope a strata verses years criteria, on a case-by-case basis. But it would change from site to site. And can even change from one side of a site to another. For example: coins at the base of the hardpan that surround large shade trees, can be shallow (as there's no lush turf surrounding them, and instead, hardpan). But walk 15 ft. further out into the moist turf *next* to that tree, and old coins will be deeper.
Within a given undisturbed site though, you can develope a time-line, by dating the items. For example: if you're in a turfed park, and ....... after 100 targets, have noticed a fairly reliable date strata placement: Clad is in the top 4", then '50s loss-wheaties tend to start at 5" or more. '20s loss green wheaties and earlier mercs (as can be discerned by the tell-tale circulation amount on the coins) tend to start at 7" or more, and so forth. You can see by that example, that if you found an item of jewelry at a certain depth, you *might* conclude that it would fit in the date-strata of whatever the coin-age loss-era stratas are.
I have found seated coins and reales, and even gold coins, that were so shallow, I could hear them on my pinpointer even before I started to dig (thus were only an inch or two deep). And then I have found zinc at a foot deep. Beaches will have absolutely no strata rules, because storms and erosion are constantly changing the face of the beach over the decades. Furroughed fields, of course, will have lost all context. And so forth.
