194 SEA AND LAND 



the southern coast have, by loner education, become well 

 adapted to the peculiar task of holding- wind-blown detritus, 

 much of the finer arenaceous material is blown across the 

 sand hills next the sea and distributed in a more sheet-like 

 form over the lai^oon which lies between the g-rowing barrier 

 and the mainland. This accumulation is often much greater 

 in amount than the coarser sands lying in the distinct dune 

 ridees on the seaward side of the island. It is likely to shal- 

 low the lagoon waters until they are completely closed, and 

 become converted into a strip of lowland which slowly widens 

 toward the main shore. In many cases these blown sands 

 would occlude the shallow waters of the lagoon in a complete 

 way, were it not for a peculiar effect of the tide which we 

 shall now consider. On examining the map of any lagoon- 

 bordered shore it will be perceived that these basins, though 

 shallow, are often very extensive in a horizontal direction, 

 and they are always receiving a considerable share of water 

 from the land. Thus it comes about that where the sand 

 reef encloses a great extent of shore, the lagoons become 

 overfilled with the land waters and break away through to 

 the sea. This opening is apt to be scoured out to a consider- 

 able depth by the energy with which the imprisoned waters 

 discharge through the easily eroded material. 



As soon as a channel is cut through the sand barriers it 

 naturall)- becomes a way through which the tidal waters ebb 

 and llow. Where, as is often the case, the lagoon area acces- 

 sible to the tide is of great extent, the volume of water which 

 four times a day passes through the inlet may be so large as 

 to develop a channel having a width of several miles. To a 

 considerable extent this tidal ebb and flow serves to scour out 

 not only the passage of the inlet but a number of channels. 



