The soil on sides of hills slowly moves down the hill, covering old houses etc. also vibration from minor earth tremors causes things to sink and settle. This is moreso on soft soils, but on chalk, such as the south downs, there is only a few inches of soil, then solid chalk, so stuff 100's of years old stay near the surface. I found ( with a metal detector) a bronze age axe head, 3000 years old, about 3" down on beachy head. and, amazingly, there was a chalk/rockfall some years ago and the skeleton of a wooly mammoth came down with the chalk. that was a million years old old more, but I think that got buried because of the ice age, not just standing there slowly sinking.
percy.