■i£a^yfe<t|S'^ 



Figure 9. — Sea ice being subjected to great pressure. 



shore, or by elevation of the ice above the normal level under the pres- 

 sure of the wind or the current. They are higher near the shore and 

 lower at the sea end. 



The pack ice in the Antarctic consists of larger cakes and is less 

 broken and piled up than in the Arctic. The pack is heaviest and most 

 closely packed around the coast in summer, and is more open and scat- 

 tered during the winter months. Antarctic pack ice will generally 

 have a great number of icebergs interspersed throughout its entire 

 area, whereas iceberg distribution throughout the pack ice in the Arctic 

 is confined to areas draining active glaciers. 



Sea ice when newly formed is highly plastic and readily conforms 

 to stresses. It acquires brittleness with age, and reaches a state of 

 strain where it may require but a slight impulse to break it. This im- 

 pulse is usually provided by the wind. Strain cracks may be produced 

 by a swell from the open sea moving under a sheet of ice. The sheet 

 suspended between the crests of the swell will be unsupported over the 

 trough, and a crack parallel to the wave front may result. If a family 

 of cracks is produced, the cracks will lie parallel to each other. If a 

 crack assumes the shape of a fan, it indicates the presence of torsion. 



This tendency to crack is always present in an ice field, whether 

 composed of young ice or hummocky floes. All cracks are due to the 

 relief of strain produced by stresses set up by sudden diiferences of 



RESTRICTED 



17 



