THE BUILDING OF THE LAND 23 



In Hawaii the natives with surf boards swim 

 out from just such shores and upon the wave 

 crests are swept in with great rapidity. At the 

 beach the wave-formed current turns back sea- 

 ward, retreating underneath into deeper water, 

 rapidly at first and more slowly as the depth in- 

 creases. This backward movement is called the 

 undertow, and swimmers are sometimes carried by 

 it out to sea. This outgoing undertow rapidly 

 slackens because of its friction against the bottom 

 and also against the incoming water above, and at a 

 certain distance from the shore, by reason of this 

 friction and of the increasing depth, it ceases to 

 advance and mingles with the comparatively slow 

 moving, incoming tide. All this water contains 

 silt and often the coarser sand, but only a little 

 of it is deposited between the beach and the line 

 of slack tide well off shore because of its too rapid 

 motion between these two points. Naturally a 

 considerable quantity of sand and mud must be 

 released and deposited where the undertow slack- 

 ens and ceases to flow. 



Thus a ridge of silt begins to form along the line 

 of these mixed currents and slack water, parallel 

 to the shore and at some distance from it. Once 



