2i8 PRINCIPLES OF STRATIGRAPHY 



becomes steeper than its back, its crest curls forward and at last 

 plunges into the trough ahead of it, splashing and surging up the 

 beach. Just at the time of breaking, the water may be seen ascend- 

 ing in the concave front of the wave and curling forward at the 

 crest. Breaking is therefore the result of normal orbital move- 

 ment at a place where the water is so shallow that there is not 

 enough of it to build up the front of the wave." Davis thinks that 

 this is a more effective cause than the "drag" of the waves on the 

 bottom in shallow water, and their consequent retardation. 



In general the critical point at which breaking of waves occurs 

 is found where the depth is equal to the height of the wave, or the 

 diameter of the orbit in which the water particles move. This is, 

 however, complicated by the ground swell over submerged banks, 

 such as the Dogger bank, the Newfoundland banks, the Agulhas 

 banks, etc., where the shoaling of the water results in the breaking 

 of waves over water several times deeper than the height of the 

 visible waves. Thus waves have been known to break in water 

 25 to 30 meters deep, on the north coast ©f Spain, in 48 meters 

 near Terceira, in the Azores ; 46-57 meters off Punta Robanal, 

 North Spain, and at 84 meters on the Syrian coast. During heavy 

 tidal runs at Faira Island, north of the. Orkneys, heavy surf was 

 found, according to Stevenson, in water 70 meters deep, and Airy 

 mentions surf on. the outer margin of the "Grounds" at the mouth 

 of the English Channel where the water was 180 to 200 meters deep 

 ( 100 fathoms). Again heavier and shorter seas have been observed 

 over the Wyville Thomson ridge between Faroe and Scotland 

 than on either side of it, although the ridge culminates at a depth of 

 300 to 500 meters below the surface of the ocean. 



On the Banks of Newfoundland the water is often stirred to 

 the bottom, although the depth is 50 meters and over. Heavy waves 

 breaking on the decks of the vessels in water 20 to 25 meters deep 

 often leave there sand stirred up from the bottom. The stirring up 

 of the bottom is also shown by the fact that the remains of Mya 

 truncata, which lives buried at a depth of 20 to 25 cm. in the sandy 

 bottom, are found in the stomachs of bottom-feeding fish, which 

 could have obtained them only after they were dug up by the 

 waves. 



The sand and mud of the bottom of the North Sea is kept in 

 constant motion by the waves and the tides, so that no seaweeds 

 can become attached, except on the rocky cliffs of the coast. Meas- 

 urements made in Lake Ontario showed that stirring of the sand 

 at the bottom by storm waves does not extend down to 20 feet. 

 Four empty boxes were anchored in the sloping sand bed of the 



