200 Frank Carney 



Glacial topography . The deposits made by an ice sheet account 

 for much of the resulting relief. Morainic bands, which mark 

 relatively stationary positions of the ice margin, almost invari- 

 ably are irregular in surface. This irregularity is partially genetic, 

 more debris being deposited at one point than another, and is 

 sometimes due to the erosion of waters flowing away from the ice 

 margin. A morainic surface is always slightly irregular, and fre- 

 quently has sharp relief. In valleys, the loops of drift generally 

 form ridges. These ridges vary from a few feet to over one hun- 

 dred feet in height. 



The esker is another ridge which makes glacial topography irreg- 

 ular; as already discussed, eskers do not always follow valleys or in 

 their direction show complete control by preexisting topography. 



Kames are mounds of prevailingly stratified deposits; these 

 mounds frequently have intervening depressions, sometimes con- 

 taining water, called ^'kettle lakes." Kames from 100 to 200 feet 

 in height are not uncommon in connection with morainic bands. 

 The size of kettle lakes and of depressions in drift areas varies 

 much. These depressions are sometimes due to detached blocks 

 of ice being buried beneath accumulating drift. Ice thus covered 

 decayed very slowly. The larger the block, the larger the result- 

 ing depression. Even if no deposits were made about the block, 

 as it was covered, a slight basin would mark its position; as it 

 decayed much of the debris in the ice would slump down the sides 

 of the mass, coming to rest on the plain beneath; the quantity of 

 this debris would decrease as the block grew smaller, hence a 

 depression would be formed. Sometimes large portions of a valley 

 tongue, the glacier being stagnant, have been thus buried. No 

 doubt many centuries passed before such a large mass of ice entirely 

 disappeared. In Alaska are forest trees, growing on a soil 

 horizon that accumulated over ice plains thus protected from 

 decay ; plants took root, and eventually trees spread over the area. 

 No one can tell how long such a buried mass of ice may endure. 



Drainage changes. The preglacial stream pattern is sometimes 

 altered through glaciation. Fewer changes result (1) when the 

 land drains away from the ice, but modifications are possible, 



(2) when the rivers of the area flow towards the ice margin. Drain- 

 age change is also possible, and no doubt often has resulted, 



(3) when the preglacial streams had a course parallel to the ice 

 margin. 



