THICKNESS OF SEA-ICE. 
59 
from the inner ice which is fixed, and then impinging on it again and again. In 
the belt of pack-ice, hummocks are rare and usually less than 10 feet in height. 
The Ross Ice-sheet presses against the sea-ice on the south-east side of the 
Winter Quarters peninsula, and produces a series of hummocks some two miles long. 
Some of the hummocks rise as buddings of the sea-ice, here 8 feet thick ; in others 
the sea-ice breaks into pieces about 20 feet long and these are forced up on end. Four 
parallel rows stretched out south-west from Pram Point and grew very gradually 
before the movement of the ice-sheet, only becoming conspicuous at the end of the 
first winter. 
The Thickness . — The sea-surface freezes during the winter, but its ice rarely 
exceeds a thickness of 8 feet. Our observations on the thickness of sea-ice are 
rather exceptional for McMurdo Sound as a whole ; for in one case the ice-gauge 
was placed where a strong 
current was known to 
exist, and in the other 
case the ice - gauge was 
placed in a sheltered bay, 
in ice which was always 
wind-swept and free from 
snow. At the former spot 
on March 1st, 1903, a 
water-hole was open : on 
the 24th of April the ice 
in it was 3 feet thick ; 
and by about mid-winter 
(June 27th) it had grown 
to 5 feet. On August 
23rd the thickness was 
6 feet 6 inches, and water 
continued to freeze until December 5th, by which time it attained its maximum 
(8 feet 5| inches). After this date the ice began to disappear from below, and by 
January 28th was all gone. A water-hole off' Cape Armitage (Fig. 32) was observed 
to open each year, which shows this melting action of the sea on sea-ice to be 
important. In 1904 the ice which surrounded the ‘Discovery’ broke up naturally 
and rapidly floated away, and the rate at which the break-up took place seemed to 
be independent of the thickness (Figs. 33, 34). 
Transpoi't. — During the winter-months cracks in sea-ice, radiating from both 
Hut Point and from Cape Armitage, were formed. These cracks are produced by the 
ice in the middle of the strait being pushed forward faster than the ice at the sides. 
The pressure is always in one direction, and is caused partly by the movement of the 
Ross Ice-sheet and partly by accumulations of snow along the shore. The cracks 
Pig. 33. — Sea-ice breaking away from the Winter Quarters in 1902. 
