3 . Increase the quantity of deep ice and bottom ice . 



4. Increase the total quantity of ice in the given region by comparison with the quantity of ice 

 which might form in the same region under the same meteorological conditions but in the absence 

 of tides. In the melting period strong tides, as a rule, assist in the destruction of the ice. In 

 relation to this, not only are the variations in level a contributing factor, assisting in the break-up 

 of fast ice, but also the gyrations of the ice which come about as a result of the tidal currents 

 which have diverse directions even in comparatively nearby regions. The ice is thus crumbled into 

 smaller and smaller parts as a result of which the total ice area subject to the action of radiation 

 from the warm air and water is increased. In addition, the areas of clear water which are continu- 

 ally opening up due to the tidal disruptive pressures strongly absorb the solar radiation. This heat, 

 stored up by the water, is subsequently expended in melting of the ice, 



LITERATURE: 62, 77. 



Section 122. Tidal Cracks (Treshchiny) 



Tidal variations in sea level in the winter season make themselves felt in the freezing of the 

 littoral shallow water right to the bottom and in formation of tidal cracks in the fast ice. 



Figure 116. Diagram showing formation of tidal cracks in fast ice. 



Let us suppose (figure 116) that MN is the level of high tide and nn the level of low tide and 

 that a layer of ice of a certain thickness is formed at high tide (with sufficiently cold temperature). 

 With the falling of the water level, the ice cover will also begin to drop, but at the very shore line 

 there will remain a layer of ice broken off from the main ice mass . Irregular ice formations will 

 thus become lodged along the whole slope of the shore, as the level drops down. This slope remains 

 uncovered during low tide and the ice formations frozen to it become larger due to the water flowing 

 along the slope. With the next rising of the water, new ice is added to the ice pieces frozen to the 

 ground and in this manner the whole area ANn gradually becomes frozen through to the bottom. 



Obviously, as long as the ice remains thin it will bend due to its plasticity, but as it becomes 

 thicker and its strength increases, it will crack up with the crack parallel to the shore line. At 

 full development this crack will trace an isobath equal to the amplitude of the tide in the given 

 region of the sea. On this account, in case of great amplitude of tide, huge expanses of the shallow 

 bank may be piled up with ice to the very bottom . 



Between the isobaths n and fe , the difference in depth of which is equal to the thickness of 

 ice, the ice will partly lodge on the bottom during ebb tide and partly break off. 



Thus, we will find in the coastal tidal belt: (a) a coastal strip of ice always lying on the 

 ground, (b) a strip of ice closer to the sea lying on the bottom only during ebb tide, and finally (c) 

 the basic ice mass of thickness AC , which is on the surface. These strips are separated from each 



331 



