On the basis of the areal distribution of glacial erosional and depositional features, western Gaspé is divided into four zones. From north to south, glacial erosion increases in depth and the deposits become thicker across Zones I and II; they are absent in Zone III, and reappear in Zone IV. Zone II also includes high-altitude occurrences of periglacial features which formed prior to the last glacial maximum. Glaciation in Gaspé began in the highlands with the development of an ice cap which had a frozen base over the periglacial deposits protecting them from erosion, while elsewhere in the highlands it actively eroded the rock surface with a melting base. Radial flow of this ice cap dispersed local lithologies. The Laurentide Ice Sheet entered the peninsula from the west and northwest, flowed southeastward, and merged with the ice cap to form a single ice sheet over Gaspé. The zonal distribution of glacial features is due to changes in the conditions at the base of the glacier: it was melting in the St. Lawrence channel, freezing through Zones I and II, frozen in Zone III and melting in Zone IV. Breakup of the ice sheet occurred through the calving-bay-surge type of mass wastage in the St. Lawrence Gulf-Estuary system, owing to eustatically rising sea level. The resulting reversed ice flow transported rocks from the south to north, including a few Shield rocks (a few erratic rocks of Precambrian age formerly brought into the area from the Canadian Shield in the north). Further retreat of the ice front caused the development of a Gaspé ice dome which subsequently broke up into several individual ice caps.