The Impact of Glaciation in the Northwest Central

During the Pleistocene, continental glaciers covered much of Canada, Alaska, and the northern edge of the continental United States (Figure 6.17). Continental ice sheets blanketed the Central Lowland and the northern Great Plains, scraping away rock and overlying sediment. When the glaciers retreated, glacial drift and till were deposited. Today, large swaths of the Dakotas and Nebraska are covered in glacial debris. Besides carving vast sections of the northern landscape and depositing huge quantities of sediment in low-lying areas, the glaciers’ impact was felt throughout the landscape as glacial outburst floods carved into Idaho and winds laden with glacial loess reached deep into Nebraska and Wyoming.

Figure 6.17: Extent of glaciation over North America during the last glacial maximum.

Glacial Erosion and Deposition in the Northwest Central

The Drift Prairie, a relatively flat area consisting of glacial drift, is located in North and South Dakota. In some areas, there are small hills or ridges underlain by glacial debris. The Glaciated Missouri Plateau, also called the Missouri Coteau, is another hilly area underlain by glacial moraines and containing many small, closed kettle lakes. The eastern edge of the plateau, extending north and south across both Dakotas, is marked by a gentle slope that exhibits topographical relief from moraines and pre-existing river valleys. It marks the western extent of glacial ice coverage in the Dakotas during the Wisconsinian glaciation, and is also the boundary between the Central Lowland to the east and the Great Basin to the west.

Eastern Nebraska is covered by glacial till, indicating that glaciers covered that portion of the state. Deposits of loess—silt-sized windblown material that is commonly associated with continental ice sheets—are also common throughout Nebraska (Figure 6.18). Strong winds blowing off of the ice sheet deposited sand, silt, and other glacial debris over the landscape to create relatively flat areas as well as forming hills and dunes (such as the Sandhills). In northwest Idaho, the Columbia River Basalts are also covered by loess. Because the sediment was not deposited evenly over the basalts, the area is characterized by hummocky terrain.

Figure 6.18: Loess deposits in Nebraska and surrounding states.

The drainage of rivers and streams was also changed by the Pleistocene glaciation (Figure 6.19). Prior to the ice age, North Dakota’s water—including the Missouri River—drained to the north, into Hudson Bay. During the ice age, glaciers created dams that diverted rivers to the south and formed lakes such as Glacial Lake Agassiz (Figure 6.20). This enormous lake, stretching from Saskatchewan down into Minnesota and eastern North Dakota, was formed by water that accumulated in front of the Laurentide Ice Sheet. Most of the area that is now eastern North Dakota would have been close to the shoreline of Lake Agassiz, and waves along its coastline modified the area. Today, the Red River Valley in North Dakota marks the extent to which Lake Agassiz covered the state. The James River Valley, extending from central North Dakota across South Dakota to the Mississippi River, was also carved by a lobe that extended from the ice sheet.

Figure 6.19: Drainage valleys of North Dakota. A) Pre-glacial river valleys drained into the Hudson Bay. B) Ice coverage and drainage during the Pleistocene. Water flowed along the margins of the ice to the south. C) After the Pleistocene, the Missouri river flowed south through the channel created during glaciation. Note that a few of the pre-glacial valleys became river valleys once more, after the glaciers retreated.

Figure 6.20: The maximum extent of Glacial Lake Agassiz.

Glacial Lake Missoula, another massive glacial lake located in Montana, was created when the Clark Fork River in Idaho was dammed by ice over 610 meters (2000 feet) high. As the lake grew deeper and higher, waves eroded the ground along the shoreline (Figure 6.21), and water pressure against the ice dam increased, eventually causing catastrophic failure of the dam. Water flowed out of the dam at a calculated speed of 105 kilometers (65 miles) per hour, allowing the lake to drain in a few days. Along with water, ice and glacial debris were carried to the west as the lake drained. This event carved the Channeled Scablands, a barren, scoured landscape that extends from Idaho through Washington and Oregon (Figure 6.22).

Figure 6.21: Wave-cut terraces along Mt. Jumbo near Missoula, Montana mark the ancient lakeshore of Lake Missoula.

Figure 6.22: Glacial Lake Missoula and the extent of the Channeled Scablands.

Alpine Glaciers in the Northwest Central States

One of the hallmarks of alpine glaciers in the Northwest Central States is the rugged mountain terrain they carve. The stunning characteristics of the Rocky Mountains—from jagged peaks and bowls to glacial valleys and high meadows— are largely a result of glacial erosion and deposition during the Pleistocene. In several cases, these glaciers coalesced into ice caps covering entire mountain ranges. In other instances, they merged with advancing continental ice sheets, eventually becoming indistinguishable as separate glaciers, only to regain their distinctiveness as the ice sheets retreated. As these glaciers retreated, they not only exposed characteristic U-shaped valleys, but they also revealed a diverse collection of peaks, bowls, ridges, and lakes scraped into the bedrock (Figures 6.23, 6.24).

For instance, in the Wind River Mountains of Wyoming, the Beartooth-Absaroka Range in Montana, and the Sawtooth Mountains of Idaho, glaciers have carved a series of horns, arêtes, and cirques. Below these prominent features, we often find chains of lakes that form when meltwater pools behind lateral and terminal moraines. Likewise, in and around Yellowstone National Park and the Teton Mountains of Wyoming, a network of small Pleistocene glaciers merged like streams flowing into a large river. As the glaciers retreated, they left behind a collection of smaller U-shaped valleys (known as hanging valleys) that drop abruptly into a much larger valley. This phenomenon is responsible for the formation of spectacular waterfalls like Tower Fall (Figure 6.25).

Figure 6.23: Glacially sculpted mountain ranges with horns, arêtes, and cirques are common in Glacier National Park, Montana.

Figure 6.24: Glacial meltwater lakes near Yellowstone National Park in Wyoming.

Figure 6.25: Tower Fall, Yellowstone National Park.