[saalweachter]

My understanding is that the Northeast was primarily deforested for sheep farming. The stone walls delineate old paddocks for rotated grazing.

Most of the ground is really quite marginal; thin soil, rocky as sin, etc etc. It's possible it'd would be economically viable as pasture-land; it wouldn't surprise me if the only The West won as pasture land was that it was all essentially free out there. Maybe if the gov't charged competitive rates for grazing on public land (and was actually able to collect) you'd see a resurgence of Eastern pasture-land.