730 j. a. udden composition of clastic sediments 



General Discussion of Data 

 modes of sedimentary sorting 



On the basis of these analyses as well as on general principles, it 

 seems correct to distinguish between two or three main modes of sedi- 

 mentation^ in the atmosphere as well as in water. These modes may be 

 called drifting, silting, and washing, when occurring in water; and re- 

 spectively as blowing, dusting, and winnowing, when effected by the 

 atmosphere. 



By drifting I mean the sedimentation which is effected on the bot- 

 tom under water by the lowest part of a current always running in the 

 same direction, when this current is strong enough to move materials 

 by its own impact. Drifting is going on in all streams. It is without 

 doubt also taking place in many parts of the sea, as on the continental 

 shelves, and even at considerable depths farther out in the sea, as in the 

 northeast part of the Gulf of Mexico. Our knowledge of submarine 

 currents is very limited. 



The analogous transportation and sedimentation effected by the wind 

 on the surface of the ground we may distinguish as blowing of coarser 

 sediments, «uch as sand and gravel. Such work is general and effective 

 in dry regions and also on many shores of the sea. It produces the sand 

 dunes and the lag gravels. In the atmosphere the direction of the cur- 

 rent is more variable, but the mode of work is identical with drifting in 

 water. Sand grains are picked up short distances and fall down. There 

 is usually also a drifting, progressive motion of the deposit in the direc- 

 tion of a prevailing wind. 



Washing is a mode of sedimentation in water which likewise takes 

 place as a result of an interaction between bottom current and drift on 

 the bed of some river, lake, or sea. It differs from drifting in that it is 

 effected not by a continuous current in a more or less constant direction, 

 but by currents that are intermittent and alternating, going back and 

 forth usually at a somewhat divergent angle and following the rhythm 

 of waves produced by the wind or by the tide."^ The directions of the 

 alternating currents are more or less constant for each locality. This is 

 the prevailing mode of work on all shores, but it extends out to any 

 depth in the sea that may be affected by rhythmic bottom currents caused 

 by the tide. Concerning these we have very little knowledge. As tidal 



•5 It is understood that the term sedimentation is here used in its widest sense, im- 

 plying the three processes of transportation, sorting, and deposition. The first two of 

 these processes always accompany each other and always also accompany deposition. 



■^ G. K. Gilbert : Lake Bonneville. U. S. Geological Survey, Monograph I, p. 37 et 

 seq. 



