The Pleistocene epoch extends from 0.1 to 1.6 million years ago and is characterized by cycles of major glacier advance and retreat on a global scale. In the Upper Skagit Basin, significant glacier advances occurred at least five times between 10,000 and 350,000 years ago. (c) During these episodes, alpine glaciers grew and coallesced, filling all the valleys with streams of ice. During the largest of these advances, the Fraser glaciation, most of the Upper Skagit Basin was probably covered by a single ice sheet. Only peaks rising above 1800 meters would have been visible above this sea of ice. (c) Furthermore, it is likely that this ice sheet was an extension of the Cordilleran ice sheet .
The impact of these glacial advances on the landscape was significant. Ice age glaciers deepened and steepened the walls of the valleys they flowed through. They also deposited thick layers of glacial till on all the valley floors. Along their upper reaches the glaciers carved many of the Horns, Aretes, hanging valleys, and Cirques seen high in the basin. Pleistocene glaciers even redirected the course of the Skagit River. Prior to the Fraser glaciation, the Skagit River probably flowed north into Canada. Late in the Fraser, as the Cordilleran ice sheet retreated northword, meltwater from the glacier was trapped in the Skagit Valley by a low ridge at the headwaters of the ancestral river. (c) As the valley filled, water from the lake cascaded over the ridge, eroding down into it. Eventually, a narrow gorge was formed above what is now the town of Newhalem, Washington. When the glaciers had retreated, it was easier for the Skagit to flow westward through this gorge, than flow out along its previous course.