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the lines of which are at right angles to the direction of impact; or 

 confused pressure areas of humniocky ice may be formed. The longer 

 the pressure lasts, the greater the chaos produced. Pressure ridges 

 may be as high as 50 feet where grounded against a coast, but in deep 

 water away from land the greatest height is from 20 to oO feet, al- 

 though it is more usual to find ridges of 10 to 15 feet. A ridge is at 

 its highest when first formed. A certain amount of settlement soon 

 takes place, owing to the sinking down of the whole mass under the 

 weight of the hummocks until hydrostatic equilibrium is reached. 

 The weight of a ridge is ultimately supported by a downward exten- 

 sion of ice under water, which may be as much as 4 to 5 times the 

 height of the ridge above. During summer, the pressure ridges change 

 in outline and the sharper features soften to the form of rolling hil- 

 locks. Snowdrifts form against the ridges, the balance of the weight 

 alters, cracks form due to differential loading, and the opening and 

 closing process goes on. 



The release of pressure gives rise to lines of weakness in ice fields 

 in the form of cracks or lanes. These are often parallel to pressure 



Figure 1 1 . — Hummocky ice floes in Eureka Sound. 



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