Glacial Landforms

The most striking glacial erosional features are associated with alpine glaciation. In fact, rugged mountainous areas can be made even more spectacular by glacial action. Alpine glaciers transform V‐shaped valleys made by streams into deeper U‐shaped valleys called glacial troughs—the ice is too massive to follow the stream bed and pushes right through, scouring out a U shape. The ice also erodes away the ends of any ridges along the valley walls. These eroded ridges are called truncated spurs. The valleys of tributaries can also be truncated, forming hanging valleys that are higher than the main valley and often marked by waterfalls.

The mass of ice at the top of a glacial valley ultimately forms a steep‐sided, circular hollow called a cirque. Mass wasting and frost wedging also contribute to the formation of a cirque. A bergschrund is a crevasse that forms where the glacier separates from the cirque wall and is commonly filled with rock fall debris. A horn is a sharply defined peak that has formed from erosional processes along the rim of the cirque. A steep ridge called an arete commonly extends downward from a horn to separate two adjacent glacial valleys.

An advancing glacier scours out a series of depressions in the underlying bedrock, which later fill with water and become rock‐basin lakes, or tarns. Tarns are best developed in softer or highly fractured bedrock. Tarns are less common on smooth, hard bedrock surfaces because it is more difficult for the glacier to “grab hold” and break off pieces of rock.

Ice sheets, with their slower rates of movement and greater weight, tend to grind down and smooth out the irregular, or sometimes mountainous, underlying surface. Exposed bedrock is polished and striated. The rounded geologic landforms and extensive, flat, bedrock surfaces in Ontario, Canada, are good examples of how an ice sheet affects the surface.