CORAL REEFS 171 



known as *' talus," and this in the course of time forms a 

 foundation for the further outward extension of the reef. 

 The action of the sea, whose breakers unceasingly beat 

 against the edge of the reef, causes portions of coral, shells 

 and the like, to be thrown on to the " reef flat " as the 

 flattened surface of the reef is called. As a result, the 

 outer edge is marked by the presence of a long mound, 

 known as the " boulder zone," which projects above the 

 surface of the sea. On the seaward side of the boulder 

 zone, in the region where the surf breaks upon the reef, 

 the surface becomes cemented forming a smooth " reef 

 crest." 



The region immediately below water level on the outer 

 edge of the reef is the area of most active coral growth. On 

 the other hand, the reef flat between the boulder zone and 

 the land becomes hollowed out to form a shallow, lagoon-like 

 channel, sometimes deep enough for the passage of native 

 boats. It is not known definitely exactly how these 

 channels are produced, but probably the principal agencies 

 at work are the scouring action of the sea as the waves 

 rush over the surface of the reef flat, the action of the 

 multitudinous boring bivalves, worms, sponges and so 

 forth and, much more doubtfully, the direct dissolution of 

 the limestone by the sea water. Occasionally, when an 

 especially deep channel has been formed in this manner, 

 living corals may be able to establish themselves once more 

 in this region and, by their subsequent growth, fill up the 

 channel. Finally, nearest the shore will be a second flat ; 

 very often formed of coral on its outer side only, the inner 

 part being of land origin, having been formed by the sea 

 which, at the end of its rush across the reef flat, has cut 

 its way into the land. 



This explanation of the formation of fringing reefs clearly 

 cannot be applied to barrier reefs or atolls, the outer slopes 



