The Quaternary: Mountains of Ice

At the start of the Quaternary period, about 2.5 million years ago, continental ice sheets began to form in northernmost Canada. Throughout this period, the northern half of North America has been periodically covered by continental glaciers that originated in northern Canada (Figure 1.15). The Quaternary period is divided into two epochs: the Pleistocene and Holocene. During the Pleistocene, ice sheets advanced south and retreated north several dozen times, reaching their maximum extent most recently 25,000 - 18,000 years ago. The Holocene epoch is the most recent (and current) period of retreat, and is referred to as an interglacial interval. The beginning of the Holocene is considered to be 11,700 years ago, or about 9700 BCE.

Figure 1.14: The path of the Yellowstone hot spot over the past 16 million years, including the Snake River Plain (part of the Columbia Plateau region) and Yellowstone National Park.

The entire United States was affected by the cooling climate during the most recent ice age. A cooling climate contributes to the growth of continental glaciers: as more snow falls in the winter than melts in the summer, the snow packs into dense glacial ice. In this case, as snow and ice continued to accumulate on the glacier, the ice began to move under its own weight and pressure. The older ice on the bottom was pushed out horizontally by the weight of the overlying younger ice and snow. Glacial ice then radiated out from a central point, flowing laterally in every direction away from the origin (Figure 1.16). As a result, the continental glacier that originated in Canada migrated southwards toward the United States. During this time, the Laurentide Ice Sheet reached into Montana, the Dakotas, Nebraska, Kansas, and east into the Great Lakes. The Cordilleran Ice Sheet reached into Washington, Idaho, and western Montana. Alpine glaciers covered the mountain heights in Idaho, Montana, Wyoming, Utah, Colorado, and New Mexico, as well as the Cascades and Sierra Nevada in the western states.

Glacial lakes formed in low areas between or in front of glaciers, and also during times between glacial advances. These lakes included Lake Missoula in Montana and Lake Agassiz in south-central Canada, Minnesota, and North Dakota. The catastrophic release of an ice dam on Lake Missoula carved the Channeled Scablands in northern Idaho and eastern Washington. (Figure 1.17)

Figure 1.15: Extent of glaciation over North America during the Quaternary.

Figure 1.16: Continental glaciers originating in Canada spread across North America, including the northern portion of the Northwest Central, during the Quaternary period.

Figure 1.17: The extent of ancient Lake Missoula between 15,000 and 13,000 years ago, and the modern Channeled Scablands, carved by the lake’s outburst flood.

Effects of glaciation on the Northwest Central’s landscape include carved glacial cirques and valleys, and deposits of glacial till in moraines and outwash plains. Glacier National Park in Montana contains many good examples of these features. Fine silt from glacier-ground rock was picked up from the glacial outwash by wind and deposited in thick layers of loess across large areas of the midcontinental US. Sand dunes, formed where a supply of outwash sand was picked up and blown by the wind, include the Sandhills of Nebraska and Killpecker Sand Dunes in Wyoming.

The ice age continues today, but the Earth is in an interglacial stage, since the ice sheets have retreated for now. The current interglacial period has slowed both erosional and depositional processes in the South Central—this and a higher, more stable sea level allowed coastal features such as barrier islands and lagoons to form, resulting in the landscape we know today. The glacial-interglacial cycling of ice ages indicates that the world will return to a glacial stage in the future, that is unless the impacts of human-induced climate change radically shift these natural cycles.