. Shingle on the East Coasts of Neiv Zealand. 73 



line suddenly receding, the wave-action must have been 

 insufficient to drive the shingle along the shore of the bay 

 as fast as it accumulated at the commencement of the 

 indentation in the coast-line. The shingle would then be 

 deposited in a line between the points of the shore termin- 

 ating the bay. No such action takes place, ordinarily, across 

 the mouth of a bay when the waves can break fairly 

 on the shores of the bay, as in Caroline Bay at Timaru, 

 and at the present time north of Napier. Other instances 

 are to be found where the shore-line, aiono- which shingle is 

 moving, suddenly recedes and, being protected in some 

 way, the wave-action is insufficient to drive the shingle 

 along the receding shore-line as fast as it accumulates at the 

 point. The point will then advance steadily further and 

 further out. This was probably the action going on south 

 of Napier before Scinde Island was connected with the 

 mainland. Similar action is going on at Hurst Castle and 

 Dungeness, on the south coast of England, and at other places. 



49. As the action at the mouth of the River Ngaruroro, six 

 miles south of Napier, is due to a similar cause, the case may 

 here be briefly described. The mouth of this river has been 

 for some time past shifting steadily northwards, the shingle 

 travelling from the southwards. The shore behind the shingle 

 is formed of earth, and would be easily cut away but for the 

 shingle beach in front. The line of the beach is fixed, and 

 could be altered but slowly. The stream from the river 

 having first worked as far northwards as the bank would 

 allow, began to cut away the bank and to form a channel 

 behind the shingle. 



50. This has gone on, the river near the mouth having 

 changed its course, extending further and further north, 

 parallel with and behind the beach, and cutting away more 

 and more of the original shore, and would go on indefinitely 

 if there were only a steady fresh- water stream to form the 

 current. When heavy floods come down the river at inter- 

 vals, a fresh mouth is opened through the shingle, when 

 the water can more easily escape in this way than along 

 the channel. The old mouth of this channel is then 

 quickly closed up by the shingle, and that opened by the 

 flood continues in use ; but, as before, it gradually shifts 

 more and more to the north, and the same action as 

 before goes on. If the channel were kept open by the 

 tidal flow, this flow would gradually become less and less 

 as the channel lengthened, and the mouth of the channel 



