sediments is largely explainable when the following factors 

 are considered: (l) the depth; (2) the topography of the bot- 

 tom; (3) the distance from a source of sediments such as the 

 shore or a river mouth; (4) the exposure of the bottom to 

 currents related to internal waves, the tide, semipermanent 

 currents, or surface waves. Tidal currents are an especially 

 important cause of bottom erosion because both theory and 

 actual measurement show that they extend to the sea floor 

 with little loss of velocity except that caused by frictional 

 drag against the bottom. Currents in general reach maximum 

 velocities wherever the flow is constricted, either horizon- 

 tally or vertically, such as in narrow bay entrances, over 

 submarine hills, in straits, over sills, or at breaks -in-slope. 



On the shelves of the Bering and Chukchi Seas, rocky, 

 stony, and coarse sandy areas appear to be largely confined 

 to topographic highs on the bottom, to the vicinity of the 

 break-in-slope between the shelf and the continental slope 

 of the Bering Sea, and to bottoms swept by strong semi- 

 permanent currents. A zone of coarse sand appears to lie 

 along the margin of the shelf of the Bering Sea near the break- 

 in-slope. According to Trask, 14 coarse sediment is also 

 found down to a depth of 1,080 fathoms (2,000 meters) on this 

 continental slope. The presence of coarse sediment near the 

 break-in-slope appears to be a characteristic of most conti- 

 nental shelves. Stetsonll recorded coarse sediment at the 

 edge of the shelf off the eastern United States. He ascribed 

 this finding to vigorous wave action during lower Pleistocene 

 sea levels which washed out the mud, and suggested that since 

 that time fine material from shore has been deposited before 

 reaching the break-in-slope. The authors of this report, 

 however, are inclined to ascribe the coarse sediment along 

 the margin of the shelf to the stronger currents, especially 

 tidal currents, which winnow out the mud. Fleming and 

 Revelle 5 have shown theoretically that such a concentration 

 of currents must take place. Currents moving onto the shelf 

 from the open ocean are greatly speeded up because the verti- 

 cal cross-sectional area of the ocean is reduced. 



The stony and rocky bottom present in the Bering Strait 

 is undoubtedly related to the strong scouring action of the 

 north-setting current which funnels through this strait. The 

 USS NEREUS measured a surface velocity of 2 knots at the 

 time of her passage (see Dynamic Topography and Currents, 

 below). This strong current continues northward along the 

 Alaskan coast toward Point Barrow and probably accounts 

 for the coarse sediments found at stations 22 and 26 outside 

 of Kotzebue Sound (fig. 5). 



