Glaciation describes time periods in which extensive ice sheets developed over large continental areas and extended even into temperate mid-latitude zones. Glacial periods appear to be related to times of global cooling, whereas interglacial periods describe periods of global warming, when the large ice sheets shrink and exist only in polar areas or areas of high elevation. At present, the Earth is in an interglacial period, but it is likely that glaciation has occurred throughout the entire history of the Earth. Over the last 2 million years (Quaternary Period), the global climate has fluctuated repeatedly: this has resulted in a series of continental-sized glaciers that advanced and retreated over most of the northern hemisphere. During the Quaternary, four major periods of glaciation have been identified in North America, although there were several advances and retreats of the ice during these periods. From oldest to youngest, these glacial periods are the Nebraskan, Kansan, Illinoian, and Wisconsinan - separated by interglacials called, from oldest to youngest, the Aftonian, Yarmouth, and Sangamon. In southern Saskatchewan, there is evidence for at least eight glacial advances during the Quaternary Period.
The last glacier that advanced over the province was called the Laurentide ice sheet and occurred during the Wisconsinan glacial period. This ice sheet reached its maximum size some 18,000 years before present (BP) when it covered most of Canada. At that time, a thick layer of glacial ice a kilometre in depth covered most of Saskatchewan; it is now believed that a few areas called nunataks escaped ice cover, for example sites of high elevation in the Cypress Hills and around Grasslands National Park. After 18,000 years BP the Laurentide Ice sheet gradually retreated northeastward, present only in the far northeast corner of Saskatchewan around 8,000 years BP, with the last vestige of ice likely gone by 6,000 years BP.
Today, much of the present landscape of Saskatchewan owes its origin to glacial erosion and glacial deposition of the Laurentide Ice Sheet. The effects of this glacier can be conveniently separated between the north and south of the province using the edge of the Precambrian Shield as a boundary. The hard exposed bedrock of the Precambrian Shield and proximity to the ice centre of the Laurentide ice sheet in Hudson Bay resulted in a landscape dominated by glacial erosional features in the north. As the ice sheet advanced it eroded and transported rocky materials southward, leaving an erosional surface behind: areal scouring, roches moutonées, whalebacks, and striae are common erosional landforms. The most common depositional landforms in the north are the eskers that extend hundreds of kilometres and illustrate the southward movement of materials during glacial retreat in the many meltwater channels that developed in the glacier.
In southern Saskatchewan glacial deposition is more evident, although several erosional features are apparent. It is believed that the softer Phanerozoic bedrock of the south was initially eroded and deposited by the earlier continental glaciers; but it appears that subsequent glacial advances were unable to remove all of these deposits and erode down to the bedrock each time. Instead, sizeable deposition of materials called drift and defined as deposits of glacial origin appears to have occurred in the southern part of the province: after each glacial advance, another layer of drift was laid down over the underlying deposit. Thus in most areas south of the Precambrian Shield, the bedrock is covered by a thick deposit of glacial drift ranging from 50 to 300 m in thickness. It was into this drift that several spectacular glacial meltwater spillway valleys have been carved to form such features as the Frenchman River Valley in the southwest corner and the Qu'Appelle Valley in central Saskatchewan. The Qu'Appelle Valley runs hundreds of kilometres and is some 130 m in depth; it has a U-shaped cross-section with wide flat floors and steep sidewalls, now subject to mass wasting processes. The valley floors typically contain glaciofluvial and fluvial deposits of fine clays and silts, conducive to intensive cropping. Elsewhere, evidence of glacial deposition includes proglacial lake deposits, eskers, deltas, and moraines of various types.