OF SAND FORMATIONS ON MARINE COASTS. 21 



notice that the sand grains follow the movement of the waves, that 

 is roll up and down. The deposit of sand takes place only when the 

 returning current does not carry back all material brought forward by 

 the wave. It follows that the velocity of the forward movement must 

 be greater than that of the returning current which is possible only on 

 very low strands, the sloping angle of which is not greater than five 

 degrees. If the size of the grains is large, the angle naturally also 

 changes. At the limit to which the wave reaches, an instantaneous ab- 

 sorption of the very thin strata of water takes place in the sand, so 

 that the returning current does not begin at this limit, but at a place 

 lower down. It is easy to determine the width of this belt in which the 

 absorption takes place, as the sand surface first is shining by the water 

 and then quickly turns dull. The width is always varying, and is in 

 direct relation to the strength of the waves, and also to the sloping 

 angle of the beach. During a strong gale and on a very low strand, 

 this belt is from 2 m. upwards on the Baltic coast, and on the western 

 coasts of Australia and the Pacific coast of America, where the mighty 

 waves of the ocean strike the shore with all their force, this belt is still 

 much broader. Secondly, deposits take place only on coasts, the sloping 

 angle of which is not more than 5 to 10 degrees. This angle is about 5 

 degrees with a grain size of 0.5 to 1 mm. in diameter. With finer sand, 

 under 0.5 mm., as is the case on many places on sheltered shores on 

 the Baltic coasts, it sinks to between 1 and 2 degrees, while with larger 

 grains, from 1 to 3 mm., an angle of 7 or even 8.5 degrees is formed. 

 With a steeper slope, deposition does not take place, but a denudation 

 is "commenced. 



Sandy beaches afford a certain protection of the coast line against 

 the erosive action of waves and surf. During the constant landward 

 urging of the sediments the coarser ingredients of the arenaceous 

 material soon cease to roll, and come to rest, and as the deposits are 

 augmented they will offer sufficient resistance to reduce the energy 

 of the wave, and consequently the erosion is diminished. 



That beach sands remain unworn depends to a great extent on the 

 fact that the particles do not touch each other, as each one is surrounded 

 by a film of water. The beating of the waves also compresses the inter- 

 stitial water, and the solitary grains are thus not tossed about and 

 therefore do not grind and wear. 



The presence of a considerable amount of interstitial water in the 

 beach sand washed by the waves is demonstrated when through the 

 pressure of the foot on the sand this whitens because of the expulsion 



