With the lower air temperatures of approaching winter, the freezing plane slowly penetrates the soil. In a fine-grained moist soil a peculiar phenomenon occurs. At the freezing plane, the water in the soil turns to ice. This is, in effect, a drying action and water in the unfrozen soil beneath moves toward the freezing plane in the same way that water will move from moist soil to dry soil. This water, on reaching the freezing plane, is able to flow through and around the soil particles there and to join the ice crystals above, thus adding to the growth of a lens or layer of pure ice.
Pressure is developed so that the ice and soil above it are lifted.
When there is an adequate supply of water to the freezing plane in soil of the proper type the ice lens can grow almost indefinitely. At the same time the freezing plane is prevented from penetrating further into the unfrozen soil because of the heat made available from the water as it freezes. The balance between the heat from freezing of the water and the heat loss to the surface is then disturbed, and the freezing plane advances until the conditions for growth of a new ice lens are restored. This results in the formation of a series of ice lenses separated by layers of frozen soil, and is the most common situation in nature. Ice lenses frequently develop in the soil under road surfaces and cause them to heave.