210 ALASKA GLACIERS 



differential erosion. A strip of lowland between Dutch 

 Harbor and Iliuliuk is so hummocky and so set with lake- 

 lets as to remind one of a terminal moraine of the Lauren- 

 tide ice-sheet, and I half expected to find it a heap of glacial 

 waste; but every discovered break in its turfy mantle 

 revealed volcanic rocks in situ, and I was forced to regard 

 its undulations as products of sculpture. 



PRESSURE AND EROSIVE POWER OF TIDAL GLACIERS 



The discussion, in the last chapter, of the fiords of the 

 Alexander Archipelago assumes (page 163) that a tidal 

 glacier is partly supported by the sea, so that the full 

 weight of the ice does not press on the rock floor, and 

 the glacier's power to erode is correspondingly diminished. 

 This assumption has been often made, and is usually given 

 quantitative form. The sea is said to sustain a portion of 

 the glacier equal in weight to the body of water displaced 

 by the ice, and correspondingly to dimin- 

 ish the pressure of the glacier on its bed. 

 When the subject is approached in a 

 certain way this view seems altogether 

 plausible. In figure 103, a represents in 

 section a sea 2,000 feet deep with a flat 

 bottom. In the sea floats an iceberg of 

 which the sides are vertical and the top 

 and bottom are horizontal planes. Its 

 thickness is 1,600 feet, and the submerged 

 part measures 1,400 feet, the densities of 

 the ice and the water having the ratio of 



FIG. 103. DIAGRAMS . 



ILLUSTRATING FLO- 7 to 8. The water sustains the entire 

 TATION THEORY OF weight of the ice. Now conceive the 

 water of the sea to be drained away. 

 The block of ice sinks to the bottom (3) and is wholly 

 supported by it. Again, conceive the water of the sea to 

 be only partly withdrawn, the depth being reduced from 



