Glacial Landscapes
The interaction of the glaciers with the landscape is a complex process. Glaciers alter landscapes by eroding, transporting, and depositing rock and sediment. They 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 6.4).
Figure 6.4: Rock and sediment derived by plucking and abrasion. These loose materials are subsequently transported to a glacier’s ablation zone where they are deposited by melting ice.
The nature of the glacier causing the erosion is also crucial. Because continental glaciers spread from a central accumulation zone, they can’t 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 places, making them lower still. While this gouging increases the overall relief of an area, anything directly in the path of the ice is flattened.
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 6.5). Dramatic results include marine reefs lifted high above sea level and marine sediments found as coastal bluffs.
Figure 6.5: 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 landscapes are most visible in areas where glaciers have retreated (Figure 6.6).
Figure 6.6: Glacial features in Cascade Pass—North Cascades National Park, Washington.
Valley glaciers carve long, U-shaped, steep-walled valleys (Figure 6.7). A river, in contrast, will erode a sharp notch, creating V-shaped valleys. As the glaciers retreat from these valleys, they leave behind thick layers of till, a chaotic mix of rock and sediment that covers the valley floor. If the valley is flooded with seawater as a glacier recedes it becomes a fjord (Figure 6.8).
Figure 6.7: A glacial valley on Steens Mountain, Oregon.
Figure 6.8: Misty Fjords National Monument in the Tongass National Forest, Alaska.
Drift-covered plains with lakes and low ridges and hills appear near the end, or 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-shaped hills called drumlins (Figure 6.9) are composed largely of till and reflect the final flow direction before the glacier receded. Small kettle lakes are formed by blocks of ice that calved from the glacier as it melted onto the outwash plain (Figure 6.10).
Figure 6.9: A drumlin field.
Figure 6.10: Steps in the formation of a kettle lake.
