19.9: Glacial Depositional Landforms

Glacial Depositional Landforms

A moraine Links to an external site. is an accumulation of glacial till derived by the grinding and erosive effects of a glacier. In valley glaciers, moraines can be produced by material falling on the sides of the glacier via landslides from the valley walls. Overall, glaciers act like conveyor belts, always flowing downslope, carrying sediments inside and on the ice and depositing them at the end of their flow and along their sides. There are few varieties of moraine that are categorized by deposition location and glacial type.

A terminal moraine Links to an external site. is a ridge of unsorted till at the terminus, or farthest extent, of a glacier (Figure 19.18). As glaciers retreat, they typically leave behind deposits called recessional moraines Links to an external site., which are similar to terminal moraines but instead form when glacier retreat pauses. Behind the terminal and recessional moraines is a thin sheet of till on top of bedrock called the ground moraine Links to an external site.. (Figure 19.18).

In valley glaciers, moraines located along the side of a glacier are called lateral moraines Links to an external site. and mostly represent material that fell via landslides onto the sides of the glacier from the valley walls. When two tributary glaciers join, the two lateral moraines merge and combine to form a medial moraine Links to an external site.. (Figure 19.19).

---

Other depositional landforms can be produced by meltwater streams as they carry sediment from the melting glacier. This sediment is deposited at the terminus of the glacier in an area called the outwash plain Links to an external site.. These areas, particularly those at the terminus of continental glaciers, contain an abundance of additional landforms (Figure 19.20). For example, large blocks of ice can be left behind on the plain to melt creating a depression called a kettle Links to an external site. that can be later filled with surface water like a kettle lake Links to an external site..

As glaciers melt, the meltwater flows over the ice surface until it descends into crevasses, finding channels within the ice or continuing to the base of the glacier into channels along the bottom. Such streams located under continental glaciers carry sediment in a sinuous channel within or under the ice, like a river. When the ice recedes, the sediment remains as a long sinuous ridge known as an esker Links to an external site. (Figure 19.20).

Meltwater descending through the ice or along the margins of the ice may deposit mounds of sediment that remain as hills called kames Links to an external site. (Figure 19.20). The meltwater that forms eskers and kames results in these landforms being composed of sediment (stratified drift), which is layered, sorted, and more rounded than till.

 

Other landforms typical of the outwash plain include drumlins Links to an external site., elongated teardrop-shaped hills. Drumlins form with their steepest side pointing upstream to the flow of ice and streamlined side (low angle side) pointing in the direction the ice is flowing (Figure 19.21). Vast quantities of drumlins occur in drumlin fields Links to an external site., common in the US in places like New York, Wisconsin and Minnesota. The origin of drumlins is still unknown; however, leading ideas suggest incremental accumulation of till under the glacier, large catastrophic meltwater floods beneath the glacier, or surface deformation by the weight of the overlying glacial ice.