From formula (15), it is easy to see that the decrease in the length of the ice field in the 

 direction of the wind, and consequently also the dimensions of the ice hummocking with the elapse 

 of time, approach 0. 



LITEEATUEE: 77. 



Section 98. The Aspects of Ice Jamming 



It is possible to divide ice jamming into three aspects, according to origin: 



1. Thermal - the weakest, which takes place under exceptional conditions. 



2. Tidal - which is caused by the non-simultaneous change of the velocity and direction of 

 the tidal currents at a close interval, 



3. Wind - the most powerful ice jamming, which exerts the greatest forces during pressure 

 winds on the shore or on fast ice . * 



According to external form, three aspects of ice jamming are distinguished: 



1. Marginal, which consists of formation of an inclination of comparatively large broken, 

 open fields into a vertical state, with relatively small piling up of fragments on one another. 



2. The complete breakup of the collided edges of ice beams with a subsequent piling up of 

 ridges and rollers which consist of fine fragments. 



3. The pushing up of flat ice beams on one another, which is observed often, particularly 

 with pancake or young ice. 



Usually, all these aspects accompany one another, but with the predominance of the first 

 aspect a hummock ice jam of marginal crushing type is obtained (figure 91); with the predominance 

 of the second, the hummock from complete breakup (figure 92) and with the predominance of the 

 third, packed ice (rafting). It is clear, that the dimensions in both these and other ice jams de- 

 pend also on the relative speed of movement of the mass of the collided ice beams. 



According to the observations of Zarya, the ice hummock forms from the complete breakup 

 of the ice as characterized more by the limited area than by the great height with an angle of 20 to 

 30°. Among floating ice, the ice hummock formed from marginal crushing seldom reaches a 

 height more than 5 to 6 m, while the ice hummock from a complete breakup is 6 to 8 m high. The 

 height of a packed ice hummock is usually less . 



As Nansen indicates, at the time of his journey along the ice of the central part of the Arctic 

 Basin, he had a chance to see an ice hummock 7 m high. "The highest hummocks, which were 

 measured by me, and there was enough of that type, " adds Nansen "were from 5.5 to 7 m high, 

 and I can positively affirm that the piling up of sea ice to heights greater than 8 m is a rare 

 exception." 



*Ice jamming sometimes results with flowing of the current under fast ice, but these cases 

 are rare. 



260 



