Valley, the Columbia Plateau, and the Central Great Plains), China, and Europe. In areas where loess is abundant,
it is sometimes used as a building material for dwellings.
3. Ice. The recrystallization of perennial snow under overlying pressure results in the formation of ice. As more
and more ice layers accumulate, the thickness of the entire mass eventually becomes sufficient to allow for the
plastic behavior of the ice layers near the base of the mass. Under gravitational influence, these plastic layers may
begin to flow, causing the entire ice mass to move downslope. This mobile accumulation of ice is called a glacier.
Because glaciers are yet another transportation agent, their action results in the formation of both erosional and
depositional features. As glaciers travel across an area, they tend to pluck rock fragments from the underlying
bedrock and incorporate them into the ice mass. Once the fragments have been incorporated, they may serve as
abrasion agents through the gouging, scraping, or scouring of any subsequent bedrock with which they come into
contact. Both plucking and abrasion are erosional processes associated with glaciation. Depositional processes
occur when the glacier begins to melt and can no longer carry its load. There are two basic categories of
glaciation-alpine, or valley, glaciation and continental glaciation.
a. Alpine or Valley Glaciers. Large ice masses often accumulate in bowl-shaped hollows called cirques,
which are located near the peaks of mountainous regions in areas of heavy precipitation and low temperatures. As
the ice in the cirque becomes plastic, it begins to flow down the mountainside, forming long, narrow rivers that
comprise alpine, or valley, glaciers. This type of glaciation is usually associated with erosional features, although
the formation of depositional features is also possible.
(1) Erosional Features Created by Alpine Glaciation. In addition to the plucked and abraded cirques
previously mentioned, there are several other distinctive erosional features associated with alpine glaciation.
Figure 2-15, page 2-24, depicts some of the more important erosional features associated with alpine glaciation.
An arete is a rough, irregular ridge that serves as a single wall separating two adjacent cirques. In areas where the
headward erosion of these cirques has been extensive, a notch or pass, called a col, may be carved into the arete.
A horn is a sharp, isolated peak formed by the intersection of the walls of three or more cirques. As glaciers flow
down a mountainside, they continually deepen and widen their channels, eventually forming steep-sided, U-
shaped glacial troughs. As the ice flows through these troughs, it cuts through numerous subordinate ridges that
extend from the crest of the mountain. In this way, faceted structures known as truncated spurs are formed.
Tributaries, like the main glacier, are also characterized by U-shaped valleys. However, the floors of these
valleys are topographically higher than those of the main trough, so they are referred to as hanging troughs. The
floors of erosional features, such as cirques and troughs, are highly irregular and uneven. This allows for the
sporadic pooling of water, which results in the formation of small lakes called tarns.
(2) Depositional Features Created by Alpine Glaciation. Masses of sediment contained within a
glacier tend to become streamlined as a result of ice flow; therefore, when the glacier melts, the unconsolidated
particles are frequently deposited as elongate ridges known as moraines. These ridges commonly exist where
sediments have been dragged along between the glacier and the valley wall (lateral moraine) or along the margin
between two adjacent glaciers (medial moraine)(see figure 2-15, page 2-24). In addition, curvilinear deposits of
unconsolidated material may accumulate at the terminus of a glacier. These
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