also suggests a large amount of sedimentation. During the 

 summer floods, many large rivers, such as the Yukon and 

 Kuskokwim, carry great volumes of sediment, which is pre- 

 sumably deposited in large part on the shelf. However, the 

 possibility that the shoal depth of the Bering Sea is due to 

 post-glacial upwarping, such as has taken place in the arctic, 

 must not be overlooked, although such upwarping has been 

 largely confined to areas which were glaciated and plastically 

 deformed by the weight of a thick ice sheet. 



In the Bering Strait (fig. 4 J) the bottom is slightly deeper, 

 presumably because of strong north-setting currents which 

 scour the bottom and prevent deposition of fine-grained 

 sediment. The bottom is also slightly irregular or hummocky. 

 Bottom samples and bottom photographs obtained there showed 

 that a stony bottom is associated with this irregularity. 

 Examination of fathograms from other shelves of the world 

 has likewise shown that a rocky, hard, stony, or other coarse- 

 grained bottom invariably shows an irregular bottom trace, 

 while the trace of a sand or mud bottom is characteristically 

 smooth. 



SEA FLOOR TOPOGRAPHY OF THE CHUKCHI SEA (fig. 5) 



The floor of that section of the Chukchi Sea traversed by 

 the USS NEREUS (figs. 4K to 40, 6, and 7 A to 7G) is in most 

 respects similar to the Bering Sea shelf but is even flatter 

 and more featureless. Whereas the Bering Sea tended to 

 shoal toward the north with a minimum depth abeam St. Law- 

 rence Island and had an average depth of about 30 fathoms, 

 the depth of the Chukchi Sea is between 15 and 30 fathoms 

 and averages about 25 fathoms in the area traversed. 



The floor of the Chukchi Sea is a portion of a broad and 

 shallow nonglaciated continental shelf which extends out into 

 the Arctic Ocean from Siberia to Alaska. The findings of the 

 MAUD expedition 12 show that, as to both topography and sedi- 

 ments, the shelf off Siberia is probably similar in character 

 to the Chukchi shelf. The arctic shelf east of Alaska (off 

 northern Canada) has been severely glaciated and is strik- 

 ingly different in nature. The hummocky bottom trace from the 



