18 AUSTRALASIAN ANTARCTIC EXPEDITION. 



5. Shore Ice. Sea ice did not remain in Commonwealth Bay for more than a couple 

 of days during 1912 and 1913 ; in the boat harbour, however, the ice was able to remain 

 and thicken. If sea ice did form at winter quarters, it would be held in by the Mackellar 

 Islands, and results similar to those for lake ice described by Chamberlin and Salisbury* 

 might be possible. When the ice has once formed, a further lowering of temperature 

 will cause contraction ; and the water will then rise along the contracted margins and 

 immediately freeze. If the water is shallow, ice may become attached to rocks and 

 boulders on the bottom. A subsequent rise in temperature causes the ice to expand, 

 and the marginal layers, with their burden of enclosed boulders, may be pushed some 

 distance up the shore ; final melting will then leave a bank of boulders. It is possible 

 that similar action has contributed to the formation of the " lower moraines " on 

 Cape Denison. 



6. Nature of the Rock. Here, as usual, the hard resistant nature of the rock is 

 important. Foliation planes and cross jointing, and the junction planes of the gneiss 

 with the amphibolites have facilitated frost action. The amphibolites are usually 

 softer than the grey granodiorite gneiss, and in consequence there are frequent 

 depressions due to this fact. In one instance (Plate XVI., fig. 1) the reverse is true, 

 and the dyke rock appears as a low thin wall. The detailed structures and compositions 

 are discussed in a subsequent chapter. 



OBIGIN OF THE VALLEYS. 

 The valleys above mentioned may have resulted from 



(a) The travel of the main ice sheet. 



(6) Valley glaciers existing after the partial recession of the ice sheet. 



(c) Thaw water action. 



(d) Wind action. 



(e) Pre-glacial strike valleys. 



(a) The ice sheet has certainly been more extensive. Assuming that the ice sheet 

 does erode its under surface in its forward movement, we naturally expect valleys to 

 be carved approximately parallel to the direction of travel ; but still differential move- 

 ments are known to exist in a large ice sheet, and a small valley depression diverging 

 45 might result from such. In the present case, the four parallel valleys would require 

 four different parallel sets of differential movements. This does not seem possible 

 when we realise that we are dealing with a very small area only three-quarters of a 

 mile broad. 



(6) The glacier rises very steeply to the south of the rocks, approximately 1,000ft. 

 in three miles and 1,500ft. in five and a half miles. The valleys are in the direction 

 of greatest slope, and hence the formation of these valleys by subsidiary valley glaciers 

 is quite possible. It is to be noted that three of the four valleys now contain ice 



* " Earth Processes," p. 389. 



