Throughout much of the Pleistocene, much of northern North America was covered by two large ice sheets (See the small globe above to determine see these two ice sheets). The western ice sheet, called the Cordilleran, covered nearly all of Western Canada and southern Alaska. During the part of the Pleistocene shown by this map, the Puget Lobe of the Cordilleran Ice extended down into the Puget Sound, covering what is now Seattle to a depth of 1000 meters (3280 feet) and covered Seattle for approximately 1000 years. At Mount Rainier, valley glaciers retreated as the ice sheet occupied the Puget Lowland. On the west side of the Cascades, several large lakes formed as the ice sheet dammed valleys draining the mountain.

The lower division of the time line, the Pleistocene, actually extends from 1.6 million to 10,000 years. It is a period of time during which several global ice ages occurred. You can see how Mount Rainier and its surroundings might have looked during the late Pleistocene by clicking on 15,000 or 20,000 years ago. The upper division, the Holocene, is period of a global warming, during which time the world's glaciers have retreated and sea level has risen. To see how Mount Rainier's glaciers changed during the late Holocene, click on 1896, 1913,1974, or 1994. The red and blue bar next to the Holocene and Pleistocene bars show average global temperature during the past 20,000 years. The dark and medium blue bands show periods of global cooling. The red and pink bands show periods of global warming. Dark blue is coldest and bright red is warmest ^(c).