GLACIERS 3 I I 



question how far a glacier is able to excavate solid rock. The 

 fjords of Norway are glaciated valleys which have been invaded 

 by the sea. 



An equally characteristic kind of topography is due to the 

 constructive work of glaciers. The terminal moraine of a valley 

 glacier or of the lobes and tongues given off by an ice-sheet, 

 surrounds the end of the ice, with its concave slope directed up 

 the valley, thus forming a more or less crescentic dam. A glacier 

 retreating, but with stationary pauses, builds up one of these 

 moraines at each halt, or if the retreat be rapid and continuous, 

 the material is spread over the abandoned ground and is fre- 

 quently worked over by the waters derived from the melting ice 

 into stratified deposits. The morainic dams often pond back 

 waters into lakes. Kettle moraines, which have deep conical 

 depressions in them, are believed to be due to the isolation of 

 masses of debris-covered ice, left behind by the retreating glacier, 

 and the subsequent melting of these isolated masses has formed 

 the depressions. To what extent material can gather beneath a 

 moving mass of ice is still an open question, but the vanished ice- 

 sheets have left over much of their former courses great masses of 

 drift, spread like a mantle over the ground and filling up the 

 valleys. In the northern United States countless stream valleys 

 have thus been obliterated. 



In considering glacial topography, then, we have to deal with 

 the work of erosion and of deposition by the ice, each of which 

 produces effects peculiar to itself. In the central zo?ie of the ice- 

 sheet, where the ice remained longest, it had its maximum thick- 

 ness and destructive efficiency. Here the principal work is that 

 of erosion, and when the ice has retreated, we find great areas of 

 naked, striated, and polished rocks, abounding in roches moutonnees 

 and in lake-filled rock basins. In the peripheral zone of the ice- 

 sheet, the ice was thinner, more sluggish in movement, was sub- 

 ject to episodes of advance and retreat, and did not remain for so 

 long a time. Here the work was prevailingly that of deposition, 

 and the resulting topography has little relief, and that relief is 

 very irregular. Sheets of drift, morainic mounds and dams, which 



