Active layer slope movement in a continuous permafrost environment, Garry Island, Northwest Territories, Canada

Field investigations have been carried out at Garry Island, N.W.T. for the 1964–1980 period in order to study downslope active layer movement at sites with two-sided (downward and upward) freezing and active ice-wedge growth. Movements have been determined with reference to semi-flexible plastic tubes inserted vertically into the ground and by deformation of lines of stakes. The results show that the vertical velocity profile on the hillslopes with clayey hummocks is convex downslope; the movement is plug-like and occurs in late summer; the plug-like movement progressively buries the interhummock peat to form a buried organic layer; and most of the plug-like movement can be attributed to frost creep by thaw of an ice-rich layer at the bottom of the active layer. The ice-rich layer forms by upfreezing in winter and the ice content may be augmented by ice lensing in the summer thaw period. In a sedgy drainage swale, the vertical velocity profile is concave downslope. The active layer of ice-wedge polygons shows a net movement outwards from the centres to the troughs. These studies show that active layer movement at sites with two-sided freezing and active ice-wedge polygons may differ substantially from sites with only one-sided freezing and without active ice-wedge polygons.