There are active earth hummocks in eastern Hokkaido. Ground freezing and frost heave were observed at an earth hummock site in Bekkai town in 1991-92 winter. The ground was frozen to 47cm deep and heaved about 5cm at an earth hummock, while it was frozen to 19cm deep and heaved about 3cm at an inter-hummock depression due to the difference of snow depth. Differential frost heave is considered to be one of the factors which promote the upheaval of the surface of earth hummocks. Internal structure and soil properties such as dense roots, low bulk density and ratio of solid phase suggest that the surface upheaval of earth hummocks is caused by the repetition of “frost heave and imperfect subsidence”.
