becomes frozen fast to the bottom of ice. By surface melting 

 in summer and bottom freezing in winter, sediment tends to 

 work its way toward the top of the floe in a few years. Such 

 grounded floes may be turned over by the pressure of large 

 floes carried against them by currents. The charts of the 

 shoal shelves of northern Siberia and Canada show that there 

 are extensive areas where the water is less than 11 fathoms 

 deep (20 meters), shoal enough for the grounding of ice. 



It would seem that the catastrophic outflow of water with 

 the advent of the summer thaw must also carry great quantities 

 of detritus by rafting in river ice. However, the finding of shell 

 fragments in a sample (NEL 537) collected from the ice by the 

 USS NEREUS shows that this ice picked up its load from the sea 

 floor. Two samples were collected from the ice at the periph- 

 ery of the polar pack; one sample (NEL 538) consisted of silt 

 and clay, and the other (NEL 537) consisted entirely of rounded 

 to angular gravels 2 to 10 mm. in diameter. 



Observation of drifting ice has shown that when the large 

 blocks of landfast ice break loose from the Canadian Archi- 

 pelago, they move west toward the Chukchi Sea and then north 

 toward the pole. Although the circulation in the central part 

 of the arctic basin is clockwise (from east to west), currents 

 along the shore are largely affected by the wind. Since these 

 wind-induced currents are commonly east-setting, the debris - 

 laden ice from the shoal and extensive shelf off North Siberia 

 can also be transported into the Chukchi Sea. 



An undoubtedly significant relationship exists between 

 the shelves of the Bering and Chukchi Seas and the position 

 of seasonal ice cover. According to the U. S. Navy Hydro- 

 graphic Office Ice Atlas of the Northern Hemisphere, 1 " the 

 deep arctic basin is almost everywhere covered by a perma- 

 nent ice pack. The shelves of the Bering and Chukchi Seas 

 are almost entirely covered by seasonal ice, and the position 

 of the Bering Sea continental slope agrees fairly well with 

 the maximum extent of the ice. Although the seasonal ice 

 cover on the shelves is an important factor controlling the 

 type and amount of deposition, the position of these shelves 

 is a cause of the ice cover rather than a result of it because 

 of the fact that shoal shelf waters undergo marked seasonal 

 changes. In this connection, associated physical oceano- 

 graphic factors are important; these are: (l) low salinity, 



(2) the partial restriction imposed by the continental slope 

 of the intrusion of warmer southern water, and especially, 



(3) the rapid cooling owing to the shoal depth. In contrast 

 to fresh water, where a positive temperature gradient be- 

 comes stable near the freezing point, the entire volume of 



