? Eh, stone walls are a byproduct of removing stones from fields. The humus part sets, new stones disappear, the game goes on and on. Fields do only become stonefree, if you add earth from some external source on top. Source: My youth on my dads farm.

Additional annoying: https://en.wikipedia.org/wiki/Land_consolidation Because not all those fields were properly consolidated (the internal walls moved to the outside, you get stripes in the new larger field, were there are stones again.