EPICONTINENTAL SEDIMENTATION 479 



2. Transportation by ocean currents, to be subdivided into 



(a) tidal currents, especially in shallow water and in narrows; 



(b) seiches due to swinging water masses (nontidal) ; (c) wind- 

 driven currents, either shallow and variable in force and direction 

 or deep and more stable; (d) jet currents due to differences in level; 

 (e) density currents due to differences in temperature or salinity, 

 and turbidity currents due to the potential energy of suspended 

 sediment seeking a lower level, under the influence of gravity. 



3. Wave action: (a) undertow and rip currents with a dis- 

 tinction made between waves perpendicular to the coast, or ob- 

 lique to the coast, or those combined with marine currents; (b) 

 tsunamis in shallow water; and (c) internal waves at breaks in 

 slope. 



All data on the movement of water should prove useful to the 

 sedimentologist, but the information geologists seek is mainly 

 from the boundaries of the ocean, and it is my impression, en- 

 tirely from the geological point of view, that physical oceanogra- 

 phers have concentrated on the wrong aspects. Waves have been 

 studied at the surface, but where they change to surf and swash, 

 the interest of the physicist apparently slackens. This is just 

 where the destructive and transporting actions begin to become 

 really exciting to geologists. The surface spread of freshwater has 

 not received the attention it deserves from physical and chemical 

 workers. They would greatly assist geologists by taking samples 

 off the mouths of rivers in flood to find out how much and in what 

 form sand, clay, and organic matter are carried, how these drop to 

 the salt water, and how far they are transported out to sea. 



Below the surface the action of waves on the bottom, for in- 

 stance, the maximum velocities and movement of particles, is 

 left almost entirely to the geologist to determine. Can sand be 

 moved offshore near the depth limits of its disturbance or only 

 inshore? How deep is this limit under various conditions of ex- 

 posure? Repeatedly one hears of sand grains being caught in nets 

 or bottles below the surface. This has been ascribed to attached 

 bubbles, but is that not contrary to the principles of submarine 

 navigation? An explanation and unquestionable quantitative data 



