 During the ice age, much of North America was covered by the Laurentide Ice Sheet.As the ice sheet began to melt and recede, the meltwaters became trapped between the higher-elevated land to the west, and the remaining portion of the ice sheet to the east, which steepened the slopes of the escarpment to give it the form that it has today.The Pasquia and Porcupine Hills, and the Duck and Riding Mountains, did already exist before the ice age, but it was after the ice age that the eastern slopes were steepened into an escarpment.