The best way to visualize this is to think of what's happening around our coast line (seen the programme "Coast").
In some places, particually on the east coast coastal erosion is taking big chunks out of our coast line, in other places it is actually adding to it. The reasons are not the same what you describe, but the point is that local variations happen all over the place, there can be very many reasons why what happens in one locality does not happen in an other. For example farming methods over the centuries - has the land often been ploughed or always
left for grazing?