LAND FORMS, THEIR DEFORMATION AND FORMATION 



49 



little more than a remnant of the closed-ring drainage 

 may indicate the past existence of a dome mountain. 



HOGBACKS 



In addition to those produced by dome mountain 

 erosion, hogbacks can form from the erosion of other 

 land forms. Hogbacks can develop whenever strata 

 that slope at high angles have rocks of unlike hard- 

 ness, and, therefore, differential erosion occurs. They 

 can form on the slopes of folded mountains, on a fault 

 line where strata edges are upturned as a result of 

 adjacent uplift, on a batholith, or on overturned 

 strata. 



The steep slope of most hogbacks faces toward, or 

 rests upon, an elevation (see Figure 4.25). The gen- 

 eral exception is found when overturned beds occur. 

 Overturned beds are formed when layers are first 

 turned upward and then arc backward in relation to a 

 rise in land. Therefore, the steep hogback face is 

 turned away from the elevation. 



Hogbacks are actually overgrown cuestas. As a 

 rule-of-thumb differentiation, cuestas have their 

 strata angled up to 12° from the horizontal and hog- 

 backs have theirs at an angle of more than 12°. 



GLACIATION 



During the last million years at least parts of North 

 America, Europe, and Asia experienced four advances 

 and four retreats of glaciers. Due to these ice move- 

 ments, much of the northern and northeastern parts 

 of our continent contain land forms molded by past 

 glaciation. This is the case because no other erosion 

 force is as powerful as glaciation. 



Glacier Formation. Glacier formation can be sum- 

 marized in a few steps. First, more snow must fall 

 than melts each year, so obviously some snow is 

 present throughout the year and accumulates from 

 year to year. Such a mass of snow is called a snow 

 field and the lowest elevation at which it occurs is 

 called a snow line. Snow falling upon a snow field is 

 usually dry and flaky, but as more snow falls on top 

 the lower parts of the snow field are compressed. 

 Compression causes the snow to melt, but the cold re- 

 freezes it into granular snow. Further precipitation 

 upon the snow field surface causes greater compres- 

 sion and the granular snow is transformed to ice. 

 While these processes are taking place, there is a 

 transition from snow field to glacier. A snow field 



becomes a glacier when either granular snow or ice 

 starts flowing. 



Glaciers are not solid masses of ice. In addition 

 to snow and granular snow in the upper layers, 

 debris, especially rocks, is scattered throughout the 

 mass. Also, there are cracks — small fissures and 

 large crevasses. Associated features include ice caves, 

 ice bridges, and once dislocated but refrozen blocks 

 of ice. All of these structures, like those already men- 

 tioned, are irregularly distributed, but their locations 

 generally reveal areas of stress upon the ice mass. The 

 cracks develop when ice is stretched, as when it moves 

 over some surface elevation. 



Erosion Acfion. In moving rocks and other debris 

 from a landscape, a glacier acts in a manner that can 

 be likened to a plow, a file, and a sled. Its plowlike 

 action is provided by the tremendous force exerted at 

 the moving front. The filelike action, which produces 

 grooves, or glacial scratch, on rocks beneath the ice, 

 is due to rock fragments. These fragments originate 

 either from material that falls upon the glacier and 

 then "falls" through the ice, or from debris that is 

 overridden and accumulated. The sled action comes 

 from the mass of ice flattening the land and rounding 

 elevations. 



Glacier Types. Glaciers are classified into three 

 types: local, piedmont, and continental. The local 

 type consists of any single, tongue-shaped ice mass 

 that is confined to a valley and represents the site of 

 a former stream. Actual naming of local glaciers is 

 based upon the surrounding environs. There are 

 alpine, mountain, and valley glaciers. Piedmont 

 glaciers are the product of several local glaciers that 

 have come together. Continental glaciers cover vast 

 expanses and are the only glacier type that moves 

 forward without regard for terrain. 



LOCAL GLACIATION 



Most high mountains of the world have some snow 

 the year round. Owing to climatic zonation, the vari- 

 ous snow lines are lowest at the poles and become 

 progressively higher as they approach the equator. 

 At the poles the snow line approaches sea level; near 

 the equator only the truly high mountains have 

 permanent snow. 



Many high mountains with snow fields obtain suf- 

 ficient snowfall for glaciers to form. Even now some 

 of these glaciers become over 1000 feet thick and ex- 

 tend some distance before the ice melts and the melt 



