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GEOMORPHOLOGY: 



water collects in glacial lakes and/or runs off in 

 glacial streams. In polar areas, glaciers may reach 

 the sea and there be dissipated in the form of large ice 

 chunks or icebergs. Currents move these icebergs 

 toward the equator, but the ice normally is melted in 

 cool temperate waters. 



The features of local glaciation are best seen by 

 considering an area before, during, and after the 

 formation of ice. The alpine type will be used as an 

 example. 



Preglacial Landscape. Most mountain topography 

 is modified primarily by streams and some weather- 

 ing. Typical preglacial land forms are dome moun- 

 tains, smooth slopes, V-shaped river valleys, and fairly 

 deep soil. The V-shaped valleys tend to unite into a 

 winding, irregular pattern; waterfalls are generally 

 not present (Figure 4.14A). 



Glacial Landscape. During glaciation, the valleys 

 are filled with ice, and the summits of the lower peaks 

 may be entirely covered by ice. However, most of the 

 glaciers are confined to the valleys, and there is a 

 glacial field that forks and flows in many directions. 

 When considered as a whole, the field is a piedmont 

 glacier (Figure 4.14B). 



These glaciers display their greatest erosion power 

 at their heads. There, at the tops of exposed peaks, 

 the effect of accumulating snow and ice is a freezing- 

 thawing cycle that loosens the peaks' rocks. The 

 loosened rocks are then carried downward by the 

 moving glacier. At the head of a single glacier, the 

 consequence of this continuous erosion is an amphi- 

 theater-like excavation called a cirque. 



Along the bottom and sides of the path of down- 

 ward movement, the freezing-thawing, filelike, plow- 

 like, and sledlike forces produce somewhat different 

 terrain. In relation to the mass of glacial ice, there is 

 a downward and lateral cutting by the glacier that 

 produces a distinct U-shaped valley profile. These 

 valleys, called glacial troughs, follow the path of previ- 

 ous streams but do not always join at the same alti- 

 tude. There is a tendency for much wider and deeper 

 cutting in the area of the previous stream's main 

 trunk and less U-shaped erosion in the previous tribu- 

 taries, with resultant steep cliffs at old points of junc- 

 tion between tributaries and the main trunk. The 

 U-shaped valleys above such cliffs are caWed hanging 

 valleys. 



The processes of glacial erosion cause rocks and 

 other materials to fall upon the tops of the flowing 

 glacier. Owing to the addition of debris upon moving 



Figure 4.14 Land forms produced by local glaciation. 



A. Before glaciation sets in, the region has smoothly rounded divides 

 and narrow, V-shaped stream valleys. 



B. After glaciation has been in progress for thousands of years, new 

 erosionol forms are developed. 



C. With the disappearance of the ice, o system of glacial troughs is 

 exposed. (Used, by permission, from A. N. Strohler, Physical Geogra- 

 phy, John Wiley & Sons, Inc., New York 1961. After W. M. Davis and 

 A. K. Lobeck.) 



ice, fragments on the ice are organized in lines paral- 

 lel to the direction of glacial flow. These lines, called 

 moraines, accumulate along the sides of a glacier as 

 lateral moraines and at any distance nearer to the 

 middle as medial moraines. A medial moraine forms 

 from two lateral moraines when two forks of a glacier 

 are joined; hence, lateral moraines on the inside of 

 each joining fork unite to produce one or, perhaps. 



