The balance between accumulation and ablation, which is largely dependent on climate. On a valley glacier or ice sheet, mass is added at the top in the accumulation zone and taken away through melting and calving at the terminus in the ablation zone. Consequently the surface profile of the glacier will steepen with increasing accumulation (M. Bennet and N. F. Glasser 2009). Thus, the mass balance gradient is the glacial driving mechanism. See J. Holden, ed. (2012), p. 471 on measuring the mass balance of major ice sheets from space.