This is a powerful agent in producing a powdery soil. When water in the soil or in the crevices of hard rocks becomes frozen, it swells up and occupies more space. In cultivated soils the particles are easily pushed asunder, and are often raised up a good deal. In the case of rocks the force exerted by the swelling ice is so tremendous that it is irresistible. The rocks are therefore forced apart, splitting along the line of least resistance, and when a thaw sets in great pieces are broken off. Fresh surfaces are thus exposed to the weather, and the process of disintegration goes steadily on.