19.10: Glacial Erosional Landforms

Glacial Erosional Landforms

The sediment carried glacial ice and subglacial runoff is responsible for the majority of the erosion of glacial environments. As we saw above, the sediment grinds, carves, and polishes the bedrock surface.

Continental glaciers act as bulldozers, and as a result produce uniformly subdued topography. Valley glaciers, on the other hand, are confined by their topography and typically produce distinct erosional features, visible once the glacier recedes (Figure 19.22): for example, glacial troughs or U-shaped valleys Links to an external site., and, in coastally glaciated areas, fjords Links to an external site. (submerged glacial valleys). Between two adjacent U-shaped valleys you will typically find a ridge called an arête Links to an external site.. At the head of a valley glacier there is often a cirque Links to an external site., a bowl-shaped feature where the head of the glacier is eroding against the mountain by plucking rock away.  

After the glacier is gone, the bowl at the bottom of the cirque can be occupied by a tarn lake Links to an external site.. Headward cirque erosion by three or more mountain glaciers produces glacial Links to an external site. Links to an external site.horns Links to an external site., steep-sided, pyramid-shaped mountains. Low points along arêtes or between horns are termed cols Links to an external site., or mountain passes. Smaller tributary glaciers can intersect larger glaciers, and, after the ice recedes, the tributary valley becomes a  hanging valley Links to an external site., sometimes with a waterfall, like Bridalveil Falls in Yosemite National Park (Figure 19.23). 

Lastly, roche moutonnée Links to an external site. (or sheepback) are bedrock formations that are produced when the glacial ice abrades and plucks the underlying bedrock into an asymmetric form (Figure 19.24).

Lakes are common features in glacial environments. In addition to the tarn lakes of valley glaciers, there may be a series of lakes, called paternoster lakes Links to an external site., which can develop when recessional moraines isolate multiple basins within a glaciated valley. Common in both valley and continental glaciated areas, elongated lakes may occupy glacially carved depressions, known as finger lakes, the most famous being the aptly named Finger Lakes of central New York Links to an external site. (Figure 19.25). In areas of continental glaciation, the crust is depressed isostatically Links to an external site. by crustal loading from the weight of thick glacial ice. Basins are formed along the edges these continental glaciers will fill with meltwater and form proglacial lakes Links to an external site. (Figure 19.20). These glacial lakes often have unique sediment deposition, including glacial varves Links to an external site., which represent seasonal sediment accumulation.