14 GENESIS AND DEVELOPMENT 



then the deposition of successive layers over the front slope of the 

 elevation. If the currents are transient, alternating with conditions of 

 still water, the obliquely laminated beds will alternate with others 

 horizontally laminated. Such laminations may be due to changes of 

 wind or tide, or to the periodical or occasional fluctuations in the 

 volume of rivers. 



The flow- and plunge structure has been caused by plunging waves 

 accompanying the rapid flow of a current, through which action the 

 oblique laminae have been broken up into short, wavelike parts. This 

 lamination bears evidence of being the result of an agent less variable, 

 and moving slower than that which has formed the irregular structure 

 so characteristic of most eolian deposits. 



THE DEVELOPMENT OF COASTAL SAND FORMA- 

 TIONS. 



The sand formations, which will come under discussion in connection 

 with our present subject, all have their origin ultimately due to the 

 action of the sea. We can conveniently divide these marine formations 

 into following groups: 



1. Submarine sand banks. 



2. Sandy islands. 



3. Sandy spits. 



4. Sandy beaches. 



1. The first class of formations or submarine sand banks are formed 

 by the combined action of streams and the waves of the sea, or by the 

 latter alone. Most of these accumulations contain more or less of river 

 detritus, which is brought down to the sea during floods. The ocean's 

 waves and currents meet it as the tide sets in, with a counter action, 

 or one from the sea landward; between the two the waters, as they 

 lose their velocity, drop the detritus over the bottom. Where the river 

 is very large and the tides feeble, the banks and reefs extend far out 

 to sea. Where the tide is strong, sand bars are formed, and the stronger 

 the tide, the closer are the sand bars to the coast. Where the stream 

 is small, the ocean may throw a sand bank quite across its mouth, so 

 that there may be no egress to the river waters except by percolation 

 through the sand; or, if a channel is left open, it may be only a 

 shallow one. 



In other cases the material constituting the sand banks is derived 



