Glacial Landscapes

The interaction of glaciers with the landscape is a complex process. Glaciers alter landscapes by eroding, transporting, and depositing rock and sediment. Scouring abrades bedrock and removes sediment, while melting causes the ice to deposit sediment.

Continental glaciers also affect the landscape by depressing the Earth’s crust with their enormous mass, just as a person standing on a trampoline will cause the center to bulge downwards. The effect is quite substantial, with surfaces being lowered by hundreds of meters. Of course, this means that when the glacier retreats and the mass is removed, the crust will rise to its former height in a process known as isostasy (Figure 6.3). Dramatic results include marine reefs lifted high above sea level and marine sediments composing coastal bluffs.

Figure 6.3: Isostatic rebound resulting from glacial retreat.

Glacial erosion can produce rugged mountainous areas with knife-edge ridges (arêtes), pointed rocky peaks (horns), and bowl-shaped depressions (cirques). These landscape features are most visible in areas where glaciers have retreated (Figure 6.4).

Figure 6.4: Common alpine glacial landscape features.

Erosion

Figure 6.4: Common alpine glacial landscape features. Thousands of years of scraping by ice can have dramatic, and sometimes dramatically varied, effects on a landscape. Glaciers erode the land they flow over via abrasion and plucking. Harder bedrock will be scratched and polished by sediment stuck in the ice, while frost wedging, when water freezes and expands in cracks, can eventually break chunks of rock away. Softer bedrock is much more easily carved and crushed. Abrasion, or scouring, occurs when rock fragments in the ice erode bedrock as the glacier moves over it. Plucking involves glaciers literally pulling rock from underlying bedrock. The flowing ice cracks and breaks rock as it passes over, pieces of which become incorporated in the sheet or bulldozed forward, in front of the glacier’s margin. The less resistant rock over which glaciers move is often eroded and ground-up into very fine sand and clay (called rock flour). Once eroded, this material is carried away by the ice and deposited wherever it melts out (Figure 6.5).

Figure 6.5: Common erosional glacial landscape features.

More resistant igneous and metamorphic rock is often polished and scratched by the grinding action of sediments trapped in the glacial ice. Streams of meltwater from the glacier, frequently gushing and full of sediment, cause significant amounts of scour as well. The abrasive sediments in the flowing water create potholes in the bedrock and plunge pools at the base of waterfalls. At the edge of the sheet, where the ice at last succumbs to melting, the rock is finally deposited. Piles of this rock form some of the distinctive landforms found in the Dakotas and Montana today.

The nature of the glacier causing the erosion is also crucial. Because continental glaciers spread from a central accumulation zone, they cannot go around peaks in their path, so they instead slowly crush and scrape them away. For the most part, this results in flatter landscapes. Conversely, alpine glaciers tend to follow the existing topography, flowing downhill. This frequently causes them to scour existing low points, making them lower still. While this gouging increases the overall relief of an area, anything directly in the path of the ice is flattened. For example, a glacier might deepen a valley while surrounding peaks remain high, yet the valley itself, initially cut by a narrow stream into a sharp V-shape, is smoothed into a distinctive U-shape by the wider glacier (Figure 6.6).

Figure 6.6: A glacially carved valley in Glacier National Park, Montana.

Deposition

As glaciers scrape over the earth, sediment is incorporated into or shoved ahead of the advancing ice. The unsorted mixture of boulders, gravel, sand, silt, and clay that is picked up and later deposited by glaciers is called till. It is important to note that whether a glacier is advancing, in equilibrium, or retreating, its ice is still flowing forward, like a conveyor belt that is constantly depositing till at its margin. In places where a glacier stopped its advance and then melted back, a ridge of till that had been pushed in front of it is left behind, marking the farthest extent of the glacier’s margin, or terminus. A ridge of till formed this way is called a terminal moraine, and it may range in length from hundreds to thousands of meters. Moraines can also form when till is pushed to the sides of an advancing glacier (Figure 6.7).

Figure 6.7: The snakelike ridge in the foreground is a now-forested lateral moraine deposited by a valley glacier. Today, it curves along the left side of Mission Reservoir, located about 48 kilometers (30 miles) north of Missoula, Montana.

Drift-covered plains with lakes and low ridges and hills appear near the terminus of a glacier as dwindling ice leaves behind glacial till. Beyond the terminus, meltwater streams leave more orderly deposits of sediment, creating an outwash plain where the finest sediments are farthest from the terminus, while cobbles and boulders are found much closer. Spoon- or teardrop-shaped hills called drumlins (Figure 6.8) are composed largely of till that was trapped beneath a glacier and streamlined in the direction of the flow of ice moving over it. The elongation of a drumlin provides an excellent clue to the direction of flow during an ice sheet’s most recent advance and reflects the final flow direction before the glacier receded.

Figure 6.8: The Eureka Drumlin Field, near Eureka, Montana. These drumlins are arranged in a north-south orientation, indicating the direction of ice flow.

Meltwater flowing off a glacier also leaves behind deposits. Unlike till deposits, meltwater deposits are well sorted: large rocks can only be moved by high-energy water, while finer sand and mud are washed downstream until enough energy is lost so that even they are dropped. In other words, the faster the water is moving, the coarser the sediment deposited (Figure 6.9). As a glacier melts, streams of sediment-laden meltwater often create networks of braided streams in front of the glacier. Streams of meltwater flowing under a glacier can deposit sand and gravel, and when an ice sheet retreats, these snaking ridges of stream deposits, known as eskers, are left behind (Figure 6.10).

Figure 6.9: Moving water deposits sediment in what is known as a horizontally sorted pattern. As water slows (i.e., loses energy) with decreased gradient, it deposits the large particles first. The sizes in the figure represent the boundaries between categories of sediment type.

Figure 6.10: Eskers are sinuous deposits composed of sand and gravel deposited by streams that once flowed under the ice.

Other glacial features include kettles, kames, and erratics. Kettles are depressions left behind by the melting glacier. Blocks of ice may be broken off from the glacier and buried or surrounded by meltwater sediments (Figure 6.11). When the ice eventually melts, the overlying sediments have no support, so they frequently collapse and form a depression that often fills with water to become a lake. Kames are formed in nearly the opposite way: layers of sediment fill in depressions in the ice, leaving mound-like deposits of sorted sediment after the glacier retreats (Figure 6.12). Often, kettles and kames occur near one another.

Figure 6.11: Kettle lakes form where large, isolated blocks of ice become separated from the retreating ice sheet. The weight of the ice leaves a shallow depression in the landscape that persists as a small lake.

Figure 6.12: Glacial sediment deposits and the resulting hills called kames.

Erratics are rocks that the ice sheet picked up and transported farther south, sometimes hundreds of kilometers (miles) from their origin. They are often distinctive because they are a different type of rock than that making up the bedrock in the area to which they have been transported. For example, boulders and pebbles of igneous and metamorphic rocks are often found in areas where the bedrock is sedimentary. It is sometimes possible to locate the origin of an erratic if its composition and textures are highly distinctive. The pink-colored Sioux quartzite, which originates in southeastern South Dakota as well as northeastern Nebraska and several of the Midwestern states, is one such example. Erratics from this Proterozoic outcrop are found across much of northwestern Kansas and north-central Iowa, carried there by ice during the Quaternary.

Periglacial Environments

Though a large portion of the Northwest Central was covered by ice, even unglaciated areas felt its effects. The land covered by the ice sheet was scoured and covered with glacial deposits, while the area south of the ice sheet developed its own distinctive landscape and features due to its proximity to the ice margin. This unglaciated but still affected area is called a periglacial zone.

There are a variety of features associated with a periglacial zone that also provide clues to the extent of the most recent ice sheet. In the tundra-like environment of a periglacial zone, aeolian, or windblown deposits, are common. Sand dunes and wind-transported sediments, such as those found in the Sandhills of Nebraska and Wyoming’s Red Desert, are found in former periglacial areas of the Northwest Central. The permafrost associated with the periglacial area, in which the ground is frozen much of the year, can cause mass movement of sediment. When the surface layer of the permafrost ground thaws, it is full of moisture. This water-heavy layer of soil may move rapidly downhill in a process called solifluction.

Physical weathering of the bedrock is magnified in the periglacial environment because of the freeze-thaw cycles associated with permafrost. When water enters cracks and fissures in the ground and subsequently freezes, the ice wedges the cracks farther and farther apart (Figure 6.13). Freeze-thaw is important in any climate that cycles above and below the freezing point of water. Because ice takes up more space than water, the pre-existing cracks and fractures are widened when the water freezes. Along ridges, rocks are eventually broken off as ice wedges continue to expand in joints and fractures. The boulders and blocks of bedrock roll downhill and are deposited along the slope or as talus. Frost action also brings cobbles and pebbles to the surface to form nets, circles, polygons, and garlands of rocks. These unusual patterns of sorted rock are known as patterned ground. Solifluction and ice wedging are found exclusively where the ground remains perennially frozen yet is not insulated by an ice sheet. Such conditions only occur in areas adjacent to ice sheets, and evidence for them can be seen all along the glacial margin of the Laurentide Ice Sheet, from Nebraska to Idaho.

Figure 6.13: Physical weathering from a freeze-thaw cycle.