52 



GEOMORPHOLOGY: 



Posiglacial Landscape. When a glacier disappears, 

 the past is disclosed by the landscape (Figure 4.15). 

 The ground shows that much of the surface soil and 

 rock was removed. The terrain is much flattened, 

 with topographic features similar to those of a pene- 

 plain produced by other erosion mechanisms. Such a 

 terrain often has an abundance of mature to old 

 streams and lakes. 



Perhaps the most distinctive things in postglacial 

 landscapes are the various kinds of glacial deposits. 

 These deposits are called erratics, till, or drift and con- 

 sist of assorted clay, sand, gravel, and boulders. When 

 a glacier advances, some till is dropped as a fairly 

 even layer of ground moraine. In certain areas, larger 

 till masses are relased (probably under the glacier) 

 and formed into inverted spoon-shaped or canoe- 

 shaped hills (dnimlins) that "point" toward the direc- 

 tion of past glacial movement. 



Glacial streams produce other deposits. Streams 

 are created where melt-water flows through the ter- 

 minal moraine that dams a temporary glacial lake. 

 Each of the melt-water streams, upon cutting through 

 the moraine, leaves a low alluvial fan. Where the 

 alluvial fans join, an outwash plain that gently slopes 

 away from the moraine is accumulated. A second 



stream feature is a ridge (esker) that once was the 

 rock- and debris-covered bottom of a stream flowing 

 within the ice mass. Such ridges can extend for miles 

 in a sinuous pattern that also contains the forks of 

 former tributaries to the stream. Another stream 

 product is a rounded, conical heap of gravel and 

 boulders (called a kame), deposited by a stream enter- 

 ing a moraine lake. The conical heaps are composed 

 of larger materials because finer particles are not im- 

 mediately dropped by the stream. 



A final glaciation feature is fairly common, because 

 blocks of ice often are buried under glacial deposits. 

 After the buried ice melts, the covering of till slumps, 

 leaving a somewhat rounded depression, called a 

 kettle hole, in the landscape. 



In summary, the more conspicuous land forms after 

 glaciation are erosional cirques, rock steps, and 

 glacial troughs; residual matterhorns, aretes, cols, 

 and ridges; and depositional moraines, outwash 

 plains, eskers, kames, and kettle holes. 



WAVES 



We have already considered the creation of certain 

 features of a coastal landscape by wave action. In 



crevasse 



,jf^ ; termina I moraine::::::::::::::::::::::::::::::x:::::::::::::::::::x^^ 



erratic 



v:::-:::-:::::^ ^vr^ivi^^jsivi:: kettlehole in kame :::::>::::::::::::j:j:j:J:|xj:j:J:|:j:ixii::::;:::::j:J:;S 



Jjlljilj-r-^v^ii^^:::-:::-::::!^:::::::::;!^^ 









older moraine ! 



•••:v:S\SSS-:SSv:-:-:S-:S ground moraine (till) v:::::::::::-:::::::::::::-:-:^ 



< J= 



outwash 



Figure 4.15 Continental glaciation, showing o retreating glacier, its terminal moraine (a reces- 

 sional moraine) and features on both sides of tfie true terminal moraine of maximum glacial extent. 



