506 NEW ZEALAND SOUND (AND LAKE) BASINS, 



glaciation, which bhea, in turn, being admissible into the discus- 

 sion, casts a strong light upon the theory of the peneplain. To 

 this we will return shortly. 



(2) The storm wave. 



The wave of translation may be considered as a stream whose 

 efficiency, as a rock-corrader, depends on its velocity and volume. 

 The difference between the wave and the ordinary stream is that 

 the one corrades by descending to base-level, the other acts from 

 base-level to a depth below that plane determined by its gravita- 

 tive thrust. In this way the depths below sea-level of the wave- 

 cut platforms are the counterparts of the depths of flood-holes in 

 ordinary rivers, and, as will be shown directly, also of fiord-basins 

 excavated by ice-floods. Naturally these various gravitative 

 thrust types differ quantitatively, since the ice-floods were 

 frequently 100 times as high as the stream- or wave-flood. 



Ordinary streams generally work normally to the coastal curve, 

 while wave grading progresses in a direction parallel to that line. 

 Each attains a linear development at the outset, but the one by 

 advancing, the other by meandering, form wide areas of corra- 

 sion. But, in all, it is the mightiest floods only which determine 

 the channel grades 



Therefore the wave-cut slope which marks the encroachment of 

 the sea upon the land is a flood grade. Here the flood wave is 

 determined by the heaviest storms. The function of the normal 

 current, tide and wave is simply to equalise the shore slope by 

 aggradation of irregularities of contour (made during heavy 

 floods). The shingle masses are the weapons of the storm wave. 

 In periods of calm the bathers know the apparent stability of 

 position for the boulders where the sand only moves gently 

 amongst their bases. In heavy storms, however, their shrieks 

 may be heard as they attack the cliff or retreat along the shore. 

 That is, these grades have been cut by storm waves, and as such 

 they are too flat to allow of further corrasion by normal activities. 

 It will be well now to see how applicable all this is to ice flood 

 grades, and from these in turn to learn somewhat of the earlier 

 stages of peneplain formation. 



