The Phenomenon of Frost Heaving (P4)

PASSAGE 4

This vertical movement of the fieldstones is not simply an artifact of soil erosion, it is the result of frost heaving. In the fall the soil freezes first beneath stones, because stones are a better conductor of heat than soil. Or, put another way, soil is a better insulator than rock in a sea of insulation, stones are chilly islands. Because most glacial till has a fairly high water content, ice forms beneath fieldstones when they freeze, and the expansion of this ice forces them upward. Even when the ice thaws, the stones do not return to their original positions because during thawing particles of soil seep into the cavity beneath, partially preventing the stones from dropping. Like a ratchet on a car jack each freeze-thaw cycle gradually lifts the fieldstones toward the surface. Ina very cold winter there may actually be two thrusts per freeze. Ice expands when it initially forms, but as the temperature plummets, the ice contracts. In the reverse process, when this very cold ice finally melts, it must expand a second time, pushing the stone once more.

In theory, the upward movement of fieldstones should result in pure soil, all the stones above the frost line having been pushed to the surface and carried away. What a vision! Acres of pure, deep soil and crowbars rusting away unused. Alas, the fastest stones move only an inch or so a year, and most are orders of magnitude slower.