One of my first "real" jobs in forestry in the mid-1990's was in southern New England. Although I had seen some stone walls here and there growing up in NY, I had never seen stone walls like those of coastal New England. I remember one property on the Connecticut River that had stone walls at least chest-high and at least as wide (apparently, designed for horseback riding on top of the wall instead of the precious tillable land they protected). That wall was inside a forest that even ~ 25 years ago appeared to be "old growth".
One local expert told me that the early settlers built stone walls primarily to "give value to otherwise value-less land" - a way of staking their claim on the landscape. I guess that makes sense in the context of early colonial farms. Must have worked though as that area has some of the highest land prices in the country today.
I don't even like to pick up a tractor bucket's worth of stones today - much less stack them somewhere - so I can only imagine the work that went in to the walls mentioned in this article: