LAND FORMS, THEIR DEFORMATION AND FORMATION 



51 



two lines of debris that flow down the interior top ol 

 the ice. 



Moraine material is constantly being added, but 

 large rock piles do not accumulate on top of the ice. 

 Because of debris weight and the characteristics of 

 ice, moraine fragments gradually sink from the top of 

 the glacier to the bottom. This sinking of materials 

 provides constant renewal of the particles at the 

 glacier bottom which are responsible for the filelike 

 action of a glacier. 



Posfglacial Landscape. The retreat and final disap- 

 pearance of a glacier reveals a landscape that has 

 been altered markedly (Figure 4.14C). Valleys are 

 distinctly U-shaped and their bottoms have lateral, 

 medial, and terminal moraines. Terminal moraines 

 are collections of glacial debris that form at points of 

 greatest extent of the glacier and at places where the 

 glacial front was located for some time during the 

 cycle of retreat. In addition to glacial erosion, the 

 U-shaped valleys display the effects of weathering and 

 stream erosion. For example, landslides are common 

 and may act to dam a stream, fashioning a landslide 

 lake. In addition, the site of a hanging valley entrance 

 is usually emphasized by a waterfall that is cutting 

 a notch into the mouth of the hanging valley. 



The old mountain summits may be completely 

 ringed by cirques that are now lakes. Also, the ring 

 of cirques may leave a sharp, almost needle-like, cen- 

 tral matterhorn peak. In addition, adjacent cirques 

 and valleys often are separated by knifelike ridges 

 (aretes) that have occasional passes (cots). Finally, 

 down the mountain from the cirques, may be rock 

 steps. These steps are the final cirque-producing ac- 

 tivities of a contracting glacier. The steps are pro- 

 duced at each place where the glacial head remains 

 for a while as it moves downhill during the general 

 contraction cycle. 



CONTINENTAL GLACIATION 



Piedmont glaciers are not considered separately 

 because their features are those of a miniature con- 

 tinental glacier or of interrelated local glaciers. 



At the present time, only one-eleventh of the land 

 is glaciated and the only true continental glaciers are 

 found on the continent of Antarctica and on the 

 island of Greenland. Now there are about 6,000,000 

 cubic miles of ice, enough to raise the sea level about 

 200 feet. However, during the last one million years, 

 the Pleistocene Epoc, glaciers covered nearly a third 



of the land. The ice involved was perhaps four times 

 the amount present today. In North America, most 

 of Canada, the higher western mountains, and the 

 eastern United States south to Louisville, Kentucky, 

 were covered during one or more of the four advances 

 of continental glaciers (See Figure 19.18, p. 381). This 

 ice rounded the New York and New England moun- 

 tains, expanded and deepened the Great Lakes, cre- 

 ated many other lakes and rivers, diverted the Ohio 

 and Missouri Rivers, shaped the Mississippi and 

 Missouri valleys, and stripped eastern Canadian soil 

 to form the rich Midwest farmlands. The tremendous 

 forces involved were due to the action of ice that was 

 as much as 2 miles thick. 



Preglacial Landscape. Consideration of the land- 

 scape prior to continental glaciation is not as im- 

 portant as is the topography preceding local glacia- 

 tion. Before continental glaciation, almost any type 

 of land form can exist. In general, the postglacial 

 consequences are lowering and leveling of the previ- 

 ous topography. 



Glacial Landscape. If there were now a mean an- 

 nual temperature decrease of less than 10 degrees, 

 perhaps as few as 5, there would be great accumula- 

 tions of snow and, as a result, continental glaciation. 

 Any continental glacier is so large that there is little, 

 if any, downhill slope to provide gravity flow. How- 

 ever, the ice creates its own kind of movement by 

 causing a "High" pressure area to form upon the 

 glacier. The movement of winds out of the "High" 

 carriers snow to an ever-widening margin. This 

 growing margin, and perhaps pressure of the thick ice 

 mass, cause much of the movement and serve to pro- 

 vide and maintain the filling, plowing, and sled ero- 

 sion of glaciation. 



Continental glacial movement flattens the country 

 by removing soil and rock masses. These erosion 

 products are deposited at the front and under the 

 glacier. If the glacier continues forward, much of 

 the front debris is pushed ahead of the ice, but even- 

 tually the ice no longer advances and a cycle of re- 

 treat starts. The point of maximum advance is 

 marked by a large collection of debris, a terminal 

 moraine, which after glacial retreat is recognized as a 

 pile of debris that has a glacially eroded landscape 

 to one side and no such landscape on the other side. 

 In addition, as any continental glacier shrinks, many 

 other terminal (more accurately, "recessional") 

 moraines may be deposited at each site where the 

 contracting front stays for some time. 



