WAVES 219 



lake bottom "at equal distances over a length of 650 yards, in 

 depths of 6 feet, 12 feet, 18 feet and 20 feet. After storms it was 

 found that the first box in the shallow water became filled with 

 sand; the box in 12 feet of water half-full; in the one at 18 feet 

 there was very little sand ; and at 20 feet there was no sand in the 

 box." (Wheeler-73 : 70.) 



In the open Atlantic ripple marks have been found at a depth 

 of 200 meters, but in the English Channel they occur only down to 

 depths of 40 meters, and to depths of 50 meters in the Roman 

 mediterranean. Ofif the Florida coast, too, Agassiz has noted dis- 

 turbances to a depth of 200 meters. 



Destructive wave work, however, does not extend to such depths, 

 especially in the more protected water bodies. Thus submarine con- 

 structions in the Mediterranean scarcely suffer damage from storms 

 at a depth of 5 meters, and the coarse sand at the bottom of the 

 Bay of Biscay in less than 10 fathoms of water is scarcely dis- 

 turbed. (Delesse-2i.) A great oceanic swell may, on reaching 

 a coast, break more than once. The first surf line will occur at 

 about the point where the shoaling water becomes equal in depth to 

 the wave height. If this is far from shore the water mass, after 

 breaking, will roll forward as a wave of diminished height, and 

 this will break again where the further shoaling demands it, and 

 this may be repeated several times. The resultant piling up of 

 the waters on the coast will induce a strong seaward-tending bot- 

 tom current or undertow, which will carry the less heavy substances 

 out along the bottom. 



The height to which a breaking wave may be thrown, or the 

 height of the surf, is sometimes surprising. Lighthouses and even 

 rock cliffs have been destroyed at a height of 100 feet above sea- 

 level. The lighthouse of Unst, the northernmost of the Shetland 

 Islands, which stands nearly 60 meters above sea-level, repeatedly 

 has had its windows broken by the high-dashing surf. "During 

 northeasterly gales the windows of the Dunnet Head lighthouse [on 

 the north coast of Caithness in Scotland], at a height of upwards 

 of 300 feet above high-water mark, are said to be sometimes broken 

 by stones swept up the cliffs by sheets of sea water, which then 

 deluge the building." (Geikie-26: 5<5/.) 



The surf from the ground swell alone will, "even when no wind 

 is blowing, often cover the cliffs of North Scotland with sheets of 

 water and foam up to heights of 100 or even nearly 200 feet." 

 (Geikie-26: 5^7.) 



As the waves advance on the shore, they tend to become more 

 and more nearly parallel to the shore line, mainly as a result of the 



