WATER MASSES AND CURRENTS OF THE OCEANS 219 



the edges of which are broken and hummocked. The pack ice is kept 

 in motion by winds and currents. The effect of the wind is conspicuous 

 when observations are made from a vessel drifting with the ice, because 

 every change in wind direction and velocity brings about a corresponding 

 change in the drift of the ice. Careful observations of the ice drift were 

 conducted by Brennecke during the drift of the Deutschland in the 

 Weddell Sea in 1911-1912. He found that the direction of the ice drift 

 deviated on an average 34° from the wind direction, and not 45° as 

 required by the Ekman theory of wind currents (p. 125). The dis- 

 crepancy was explained by Rossby and Montgomery as due to a layer 

 of shearing motion in the water directly below the ice, but it may be due 

 in part to the resistance offered by the ice because the wind does not blow 

 uniformly over large areas, wherefore the ice is packed together in some 

 areas and torn asunder in others. The ice resistance appears, however, 

 to be small in the Antarctic compared to that in the Arctic, because in 

 the Antarctic the drift of the ice is not impeded by land barriers on all 

 sides. The antarctic pack ice consists therefore of larger ice floes than 

 the arctic pack ice and is less broken and piled up. 



The antarctic pack ice has in all months of the year a well-defined 

 northern boundary beyond which no great amounts of scattered ice are 

 encountered. The northern limits of the ice at the end of the winter 

 (September) and the end of the summer (March) are shown in fig. 62. 

 Parts of the antarctic coast are always ice-free in summer, such as the 

 Pacific side of Graham Land, and other parts are often ice-free, such as 

 the coasts to the south of Australia and Africa. The eastern areas of the 

 Weddell Sea and the Ross Sea are always ice-free. The Weddell Sea can 

 often be entered from the east without encountering pack ice, but in 

 order to enter the Ross Sea it is always necessary to pass through a broad 

 belt of pack ice. 



The great icebergs of the Antarctic originate mainly from the shelf 

 ice, which represents the direct continuation of the antarctic ice cap 

 where it extends into the shallow waters surrounding the continent. 

 The shelf ice is partly afloat, and when it is pushed far out it breaks off 

 in enormous pieces that may be tens of kilometers wide and up to 100 km 

 long and rise 90 m out of the water, corresponding to a thickness of about 

 800 m. These giant bergs, which have occasionally been mistaken for 

 islands, drift through the pack ice and often move in a direction opposite 

 to the ice, because they are carried by currents and are less influenced 

 by wind. Since the melting of these bergs takes a long time, they drift 

 to greater distances from the continent than the sea ice. Fig. 62 also 

 shows the average northern boundary of icebergs according to British 

 Admiralty chart No. 1241, but icebergs or remains of icebergs have been 

 reported much further north than is indicated by this average limit. On 

 April 30, 1894, a small piece of floating ice was sighted in lat. 26°30'S, 



