The evidence of separate Labradorian and Keewatin centers of radial glacial flow is confined almost wholly to very late Wisconsin time, and indicates that these centers shifted their positions widely even during that short time. There is no evidence that these centers ever were the sites of independent glaciers; their importance has been overemphasized. The North American ice sheet, during the Wisconsin maximum, was recognized as a single mass and named the Laurentide ice sheet before the names Labradorian and Keewatin were applied.
The Laurentide ice sheet is believed to have originated as mountain glaciers chiefly in the conspicuous highlands of eastern Quebec, Labrador, and Baffin Island. Nourished by moist maritime air masses derived mainly from the south and southeast and moving northward and eastward, these glaciers coalesced into, piedmont glaciers. By continued growth southward and westward toward the principal sources of their nourishment, the piedmonts thickened and spread, burying the highlands in which they had originated. Ultimately they formed a vast ice sheet that extended from the east coast to the Cordillera. Further eastward expansion was prevented by the deep water of the Atlantic, in which the ice broke up and floated away.
Glacial-anticyclonic winds are assumed to have been subordinate to cyclonic storms in nourishing the ice sheet. The Labradorian, Keewatin, and other centers of outflow recorded by striae were broad low domes on the surface of the ice and were caused by exceptional concentrations of snowfall. While the ice sheet was shrinking, these domes shifted position.