[ChuckinNH]

It is the freeze / thaw cycles in the northern climates that causes rocks to work their way to the top. There is moisture in the soil, and the soil "heaves" when it freezes, and eventually moves the rocks to the surface, and sometimes makes a mess of the secondary paved roads. "Frost Heaves" are a way of life up here, and can be pretty brutal on vehicle suspensions, and steering components.