MD-dog, I got the chance a few years ago, to see this first-hand, in a perfectly stratified park-scrape in a late-1800s San Francisco park. They were scraping to make ready for astro-turf, and it was a rare occasion to be in on various levels of the scrape. The park was essentially un-hunted, because it was in a blighted neighborhood that local md'rs avoid (tending to go to the cleaner upscale parks, unless they want to punish themselves with clad and wino-caps).
The top 6" was very stratified: Ie.: clad within the top several inches, '50s/60s coins starting at 4 to 5", '40s losses at 6" or so, '20s/'30s losses at 7", etc... What was interesting, is that ........ if a person were merely to look at the age verses depth speed, of the top few decades, he could assume that ........ if that speed of burial continued, then by logical conclusion, a coin that was 100 yrs. old, would be a foot or more, right? But we noticed that once the scrape got down to 10" to 12" deep, where we could dig un-disturbed coins (which were now very shallow d/t the scrape), we could see that barbers lost in the teens, and seateds lost in the 1870s, were nearly identical in depth (with maybe a slight 1" difference). That meant that once coins got to a certain depth, the sinkage rate slowed and then, I guess, stops. Below a certain point (when the scrapes got to 1.5 ft. or whatever), the soil was completely sterile. So yes, there does seem to be a point where, in un-disturbed turf, coins tend to slow and stop their depth drop.