CONDITIONS OF SEDIMENTARY DEPOSITION. 483 



ries. If, on the other hand, currents be continuous and constant, 

 the zones of sand, clay and silt deposits will occur each beyond 

 the former. But this is a question of distribution as well as of 

 sorting of sediments. 



(<:) Moderate volumes of sediments. — Sediments are also 

 more or less completely sorted by waves or currents according 

 to the relation between the volume of sediment and the force of 

 the sorters. When waves breaking upon a coast have only the 

 product of wave erosion to handle they sort most completely; 

 the material is washed again and again until no trace of clay 

 remains mingled with the sand grains ; and the under-tow, bur- 

 dened only with the clay washed out by the waves and the fine 

 products of abrasion, carries them all away. But where a river 

 pours out a large volume of sediment, and waves or currents are 

 consequently overloaded, both sorting and transportation fail to 

 a greater or less degree. Deposition takes place too rapidly for 

 the separation of fine from coarse and the deposit is of mixed 

 character. The effect of waves is then seen in ripple-marked 

 and ill-assorted beds of tide flats. 



DISTRIBUTION. 



The conditions under which sediments are more or less widely 

 distributed, depend upon movement of the waters and the nature 

 of the sediment; those favorable to distribution are : 



Favorable conditions : 



(#) Efficient wave action prevailing from one direction 

 oblique to the shore. 



(b) Continuous currents. 



(c) Uniform or gradually increasing depths of water. 



(d) Fine or light sediment. 



The reverse of these conditions favor deposition, and will be 

 discussed in that connection. 



[a) Efficient, oblique wave action. — Distribution of shore 

 drift is fully discussed by Gilbert, and has already been referred 

 to in stating the effect of sorting by waves of the Atlantic on 



