Figure 117. Crater- shaped hummocks of ice over shoals. 



Fluctuations in sea level during winter are caused not only by tides but also by wind driving 

 water towards and away from the shore. So, besides tidal cracks, wind-driven cracks are charac- 

 teristic of the ice cover. The latter are particularly well developed and most typical when the bays 

 are already covered with a rather firm ice cover and the adjacent sea is still free of ice. With a 

 strong wind blowing towards shore the water covers the fast ice. When the wind blows off-shore, 

 due to the lowering of the level, cracks are formed in the ice. These are typical in that they 

 intersect perpendicularly entries into the gulfs, separate the gulf from its shallower bays, and 

 stretch from one island to another across the straits. Such cracks appear in any month of the 

 winter. 



In most cases, however, the wind-driven cracks coincide vidth the tidal cracks. This may be 

 readily followed, for example, on the Severnaya Dvina where the wind-driving effect considerably 

 exceeds the tides. For example, in the winter of 1941-42 after the fast ice cover had formed, the 

 amplitude of tidal fluctuations in level did not exceed 60 cm even in spring tides. The amplitude of 

 wind-driven fluctuations of level, however, reached 101 cm in November, 79 cm in December, 

 62 cm in January, 76 cm in February, 76 cm in March, and 69 cm in April. As one can see, the 

 wind-driven fluctuations of level on the Severnaya Dvina did not cease throughout the winter despite 

 the fact that the entire sea was solidly covered with ice. 



The fast ice cover was formed on the Severnaya Dvina on 3 November 1941 and the river ice 

 broke up on 11 May 1942. E>uring this time the maximum level (tidal plus wind-driving effects on 

 shore) was observed on 30 November and was equal to 210 cm while the minimum level was ob- 

 served on 8 February and was equal to -2 cm (relative to the "zero of 1881"). Thus, the absolute 

 amplitude of variation in the ice period was 212 cm which made for sharply defined cracks, winter 

 water-lanes along the coast, freezing of the ice to the shore and to mooring lines, etc. 



Figure 118 shows, according to Voeikov and Stolyarov, the location of tidal cracks in Tiksi 

 Bay in the winter of 1933-34. These cracks ran between all the prominent parts of the shore and 

 along the coastline and remained open all through the winter. With severe freezing young ice was 

 often pressed out of them forming ridges from 10 to 15 cm high and 2 to 2.5 cm thick. Due to the 

 lack of connection with the shore, the ice in the central part of the bay remained afloat. 



333 



