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 
7 to 8. The water sustains the entire 
weight of the ice. Now conceive the 
water of the sea to be drained away. 
The block of ice sinks to the bottom (£) and is wholly 
supported by it. Again, conceive the water of the sea to 
be only partly withdrawn, the depth being reduced from 
a 
FIG. IO 3 . DIAGRAMS 
ILLUSTRATING FLO¬ 
TATION THEORY OF 
TIDAL GLACIERS. 
