I sorta think of them the same way when I find shards of glass bottles and crockery. Family groups and people live there until life changes move them to other areas or maybe death or marriage events. People leave trash and even non trash laying about and move on because they don't consider it important to keep anymore. Example being maybe like how I would foresee in my future. I've lived here for years and raised my family and when I cross over the things I consider important will probably not be important to my boys. If they stay in my home stuff might get tossed outside to rot away or just sit there until someone comes along and re-purposes it. So it's likely that very old things got left behind because it wasn't useful anymore in the way it used to be, or maybe it was buried in burial mounds or trash pits like the glass and metal used to be and now with all the time and erosion it's washing out broken up or maybe even whole. Likely too that modern families were taught disrespect for Native American items and tossed them around or used them for target practice and broke them into pieces. Just like a young friend of mine told me they used to throw the old bottles that were left behind in piles behind their home place just to hear them break and she winced when she told me and now regrets it, but they were kids and the parents didn't care. Seems I read a story about how the kids were given treats for picking up the "arrowheads" in the fields and gardens and tossing them out like they were rocks, which in those days there was no respect for any race but white so they didn't mean anything to them and were considered rocks. And that is sort of one of my theories about the shards.