62 



THE BUILDING OF AN ISLAND. 



The diagram (Fig. 23) represents two rocky points with a sandv bay 

 between them. Outside the points the general east to west current runs. If 

 the sandy shore were in a straight Hne with the points, the current would soon 

 remove a part of the sand and cut the shore back into something like the 

 curve indicated in the diagram, and we find that it has actually done so. But 

 whv does it not continue to cut it back ? Why does the outline of the bay 

 not recede still farther? It is because the current cannot penetrate any 

 farther, but creates a cou^iter-curroit in the bay, a current which, on its inner 

 side, is so sluggish that it would quickly drop any sand or mud which it might 

 have seized on. 



An interesting proof of the existence and effect of tiie counter-current 

 may sometimes be seen when a small stream runs into the water of the bav 

 and brings down pebbles from the surface debris, in which case the pebbles 

 are deposited, not on the leeward side of the stream's mouth, as we might 

 expect, but on the windward side. They move against the direction of the 

 general current under the influence of the counter-current. This is shown in 

 the diagram, which fairlv represents an actually existing bav on the north 

 shore of the island. Storms may make small temporary changes in the curve 

 of such a bay, it is true, but when the usual daily conditions set in again, the 

 shore quickly regains its former outline. 



Fig. 23. 



^ 



>. 



fi 



c ^ 



v^ 



*>f) 





.0 



^' 



(1. c. Direct current, 

 c. c. Counter current, 

 r. p. Rocky points. 



sa. b. Snndy lieacli. 

 sli. b. Shingly beach, 

 s. s. V. Seaside vesetation. 



Onlv the wearing back of the rockv points which protect it can cause the 

 sandy shore of such a bay to recede inland. But this wearing back of the 

 rocks is undoubtedly always in slow progress, the rocky protectors are gradu- 

 ally driven back and the sandy shore must slowly recede with them. This 

 result is occasionally recorded by the beach limestone remainiag as an evidence 

 of the distance to which the shore has been recently pushed back ; a good 

 instance of which is seen in Christiansted harbour, between the Fort and the 

 mouth of the Lagoon, where a strip of shallow water, twelve feet or more in 



