First, the climate was cold. The revolutionary period coincides with the end of the “Little Ice Age”, an unusually cold period during the Holocene (think Valley Forge). Second, and because of the cold, New England was mostly deforested to build houses and barns and to keep them warm. As a result, water penetrated deeply and the heavy freezes and thaws caused ice heaving, pushing the old Laurentide cobbles and boulders upward. Farmers began plowing up rocks in their fields and piling them at the edge of their pastures. Hence came the stone walls. Now you know why you see them commonly in northern Virginia and points north, but you never see them (except the ones that were recently built) much further south. The effects of the Laurentide ice sheet were smaller, so not as many boulders and cobbles left behind and the effects of the Little Ice Age were less, so not as much ice heave.