Fig. 2: A model for ridge formation by diurnal working of a pre-existing crack. (a) After the crack opens and fills with liquid water, ice quickly forms at the fresh surface. (b) The crack then closes, forming a narrowing, but vertically thickening, raft of crushed ice. (c) Some of the slurry is squeezed onto the surface as the crack closes. (d) The crack reopens, leaving a double ridge of surface deposit, and liquid water enters renewing the cycle.