The relationship between ice-face ablation and headwall retreat is described for three ground-ice slumps in the Sand Hills moraine, southwest Banks Island. Retreat exceeds ablation by a geometric factor that depends on the slope of the ice and the gradient of the surrounding terrain. Amounts of retreat predicted from ice-ablation calculations compare favourably with field measurements, except at the start of thermokarst activity in the spring or when the ground ice remains covered by debris for long periods.Long-term headwall retreat for slumps in southern Banks Island with different orientations and ice contents can be estimated using a model based on meteorological information. The model predicts headwall recession of 11 m/a for a ground-ice slump facing south and 8.8–9.3 m/a for one facing north, with inputs of a 35 °ice face, a ground slope of 5°, and a volumetric latent heat of 270 MJ/m3. These predictions are close to the maximum rates of retreat over a 10 year period as measured from air photographs.