DEPTH OF WAVE ACTION 11 



ments of the force of gravity from a submerged submarine — some- 

 thing that can rarely be done from a surfaced vessel because of its 

 uneasy motions. We might note in passing that deep-sea divers 

 have reported being tossed to and fro when working as deep as 100 

 feet. 



The question of the depth to which wave action may extend is also 

 a matter of interest to the student of submarine geology, because move- 

 ments of the water, so small that they would be of no concern to the 

 seaman, may still be great enough to shift sand, mud, or even small 

 stones about from place to place over the bottom. It is known by ex- 

 perimental measurements that a velocity of 0.3 foot per second is strong 

 enough to move grains of sand or gravel as large as 0.1 inch in diam- 

 eter, and it is at this velocity that the water particles would, theoreti- 

 cally, be moving to and fro 'at a depth of 92 feet, in a wave that was 10 

 feet high at the surface and 200 feet long. In the case of swells no 

 higher but longer, say one 500 feet long, such as are often encountered 

 at sea, the orbital velocities and, consequently, their abrasive power 

 would be as great as this down to a depth of at least 192 feet; the in- 

 fluence would be noticeable to an even greater depth with still longer 

 waves. And the observations that have been made on the depth at 

 which sand and even stones may actually be shifted about on the bot- 

 tom are in line with the foregoing. Thus, swells have been said to 

 wash stones as heavy as one pound into lobster pots off the mouth of 

 the English Channel in depths as great as 180 feet; rocks weighing 

 several hundred pounds have been reported as moved by wave action 

 in depths of 90 to 120 feet off the western coast of Ireland; and coarse 

 sand is sometimes brought up from 150 feet by storm waves, to be 

 dashed against Bishop Rock Lighthouse, England, to quote a few in- 

 cidents only. It has even been stated repeatedly — though not on very 

 strong evidence — that wave action may affect the distribution of sub- 

 marine sediments to a depth as deep as six hundred feet along the 

 slopes that front the continents. But it is generally held that this is 

 about the extreme depth to which wave action affects the water, in any 

 part of the sea or at any time. (For further discussion of this sub- 

 ject, with additional examples and references, we refer the reader to 

 Johnson, 1919, p. 76.) 



We ought perhaps to caution the reader, in this connection, that the 

 presence of ripple marks on the bottom, such as have been regarded 

 sometimes as evidence of wave action in deep water, may equally be 

 the result of currents flowing over the sea floor. For example, sub- 

 marine photographs have proven the presence of ripple marks at a 

 depth of 498 feet in the Gulf of Maine, although the waves that had 

 been running for some days previous had been far too small to have 

 disturbed the sand at so great a depth (fig. 4). 



