GEOLOGICAL OBSERVATIONS 



also its methods of freezing. The maximum thickness 

 measured by us amounted to about 7 ft., in the ease of 

 ice formed, in a sheltered position at Backdoor Bay near 

 our winter quarters, between the middle of March and 

 middle of September 1908. Ice had formed over the 

 same area, a little earlier in March, to the depth of a few 

 inches, but this was cracked up, and drifted away by the 

 blizzards. This thickness of 7 ft. of ice was no doubt 

 increased between the middle of September and early in 

 December. 



Ferrar states that the maximum thickness of sea ice 

 which formed during the year 1903 at Hut Point, 

 McMurdo Sound, was 8 ft. 5% in. We observed that 

 in places the sea ice was fractured, through pressure of 

 wind and tidal currents, and the broken slabs were forced 

 over one another forming pressure ridges, from 10 to 20 

 ft. in height. 



It was interesting to note the elFect on the sea ice 

 of a sudden fall of temperature. The contraction fol- 

 lowing on such a fall would put the sea ice, especially at 

 its surface, into a high state of tension, and from time to 

 time the surface would crack open with a loud report. 

 These contraction cracks gaped to a width of 3 to 6 ft., 

 and the sea water between the walls of the crack, of 

 course, began to freeze over. Frequently after ice had 

 formed to the thickness of a few inches a rise of tem- 

 perature would expand the ice. This expansion would 

 tend to expend itself on all weak spots, especially on the 

 planks of thin ice formed between the walls of the con- 

 traction cracks. These would be buckled into small over- 

 folds, until at last they became cracked through excess 

 of thrusting, and overthrust faults resulted. In many of 

 these cracks this process was frequently repeated. 



Another feature worth noting in the sea ice is, that 

 owing to the great difference between the temperature of 



Vol. II.-19 289 



