Adkin. — The Horowhtnua Coastal Plain. 113 



Lateral corrasion was a negligible factor during the building of the 

 fan and valley-plain ; but with the cessation of deposition it became pre- 

 dominant, and the hill-spurs and ridges bounding the built part of the 

 valley-plain were cut back on a level with its surface. The large amount 

 of material derived from this lateral corrasion was transported across and 

 deposited beyond, and not upon, the surface of the fan, the spoil being 

 delivered to the trunk Cook Strait River, of which at that time the Ohau 

 was but a branch. 



In several places the old rock floors still exist up to their original level, 

 and form small flat-topped rocky hills. Elsewhere they have been incised 

 by the Ohau and its tributaries rejuvenated in the present cycle of erosion, 

 and their former extent is marked by rock terraces capped by gravel veneers 

 of varying thicknesses. Little, if any, gravel or sliingle was deposited on 

 the original surface of the rock floors, showing how complete was the 

 transportation of coarse detritus by the river during the time of lateral 

 corrasion. Similar rock floors will doubtless be found in the intermont 

 portions of the valleys of the other Horowhenua rivers. 



Dr. Cotton's Interpretation of the Physiography of the Coastal 



Lowland.* 



Cotton's views regarding the history and sequence of deposits of the 

 coastal lowland which extends from Paekakariki to Palmerston North and 

 beyond appear to be the result of a misconception arising from observa- 

 tions based for the most part on the extreme southern part of the coastal 

 belt. As shown above, that part of the coastal belt is not typical of 

 the whole, the important incidents of deformation and sea erosion being 

 peculiar to that locality. I hasten to say that Cotton's observations on 

 the area a few miles north-east of Paekakarikif are undoubtedly substan- 

 tially correct : it is the application of inferences drawn from that locality 

 to the coastal lowland in general to which exception must be taken. 



The Sequence of the River-fans and the Sandstone Formation. 



In the paper referred to, Cotton dissents from my view that the coalescing 

 fans of the rivers along this coast form the basal member of the Quaternary 

 formations, and that it (the basal member) is directly overlain by the sand- 

 stone formation (my Horowhenua coastal plain). Cotton puts the sandstone 

 formation at the base of the series, and regards the river-fans, with which 

 he classes all the gravel deposits of the lowland, as being " among the 

 youngest elements of the lowland physiography." 



One of the chief causes of Cotton's divergent opinion as to the sequence 

 of these formations would seem to be the greater complexity of the lowland 

 than he at present recognizes. In addition to the coalescing fans (piedmont 

 alluvial plain) of the larger rivers, and the sandstone formation (Horowhenua 

 coastal plain), the deposits and activities of the minor streams, large and 

 small, have been the cause of innumerable complications. As these minor 

 streams drain the same terrains as the rivers, the gravels and other 

 deposits of both are in many respects similar, and frequently indistinguish- 

 able ; hence great confusion and false deductions may result from any but 

 the most careful study. The classing-together of the great river-fans and 



* Loc. cit. (1918.). ^ Ibid., ]^. 21S. 



