220 Transactions. 



The remaining stages are marked by changes in the courses of the Ohau 

 and other streams over the Ohau fan, for the explanation of which Ferrel's 

 law is invoked, though it is much more probable that such changes as have 

 taken place in stream-courses have resulted from spilling over as a normal 

 accompaniment of aggradation. At the same time the shore-line advanced 

 steadily seaward. It is not clear whether this is regarded as entirely the 

 result of an inferred movement of uplift, but this is probably what the 

 author had in mind, as he speaks of the whole lowland as a coastal plain. 

 The new land formed thus was progressively covered with sand-dunes, which 

 impounded lakes. 



The account is somewhat difficult to follow, but the foregoing is a 

 fair summary. Adkin regards the gravel plains, such as the Ohau fan, as 

 the oldest, instead of placing them among the youngest elements of the 

 lowland physiography, as is done in the explanation now offered ; and his 

 conclusion that any portions of the existing fan-surfaces or gravel plains 

 were in existence prior to the deposition of the sands of the older lowland 

 is an extremely doubtful one, whatever the correct explanation of the mode 

 of accumulation of those sands may be. 



It must be added that Adkin's work is obviouslv based on a large 

 amount of careful field-work ; and his mapping in the Ohau River district 

 (Horowhenua) is extremely useful. 



Subdivisions of the Lowland. 

 The Otahi Series. 



The oldest physiographic element in the lowland is, then, the dissected 

 peneplain of soft sandstone corresponding to the " older dune-sand areas " 

 of the theoretical discussion previously given, and comprising the " raised- 

 beach formation " of Adkin. To the lithological formation of sandstone 

 thus indicated the name Otaki series may be applied as a local formation- 

 name, as it is well developed just north of the Otaki River and town. 



Topography on the Otaki Series. — The topography is that described in 

 the theoretical section as developed on the " older dune-sand areas," and 

 shown diagrammatically in fig. 2, F (see also Plate XV, fig. 2). I agree with 

 Adkin* that the gently undulating tops of the broad benches of this forma- 

 tion are parts of a surface of erosion and not of deposition. In some parts 

 of the district a considerable area of the surface of the Otaki series consists 

 of broad terrace-remnants of valley-floors at intermediate levels developed 

 generally by very small streams which now make their way along the 

 flat, swampy floors of inner valleys. A striking characteristic of the small 

 dissecting streams arising within this formation is the steepness of their 

 valley-sides, which remains practically constant as the width of the floor 

 increases, and the same slope continues around the valley-heads. To their 

 very heads the valleys are box-shaped rather than V-shaped. The slopes 

 separating the broad terraces at intermediate levels were evidently once 

 exactly similar to those of the inner trenches, though they have become 

 somewhat dissected and broken down since the streams were revived 

 and undercutting of these slopes ceased. Where the terraces have been 

 developed by gravel-bearing streams heading in the old land they are 

 gravel-covered. 



Distribution of the Otaki Series. — Between the southern end of the 

 coastal lowland and the Manawatu River the Otaki series, with its charac- 



* Loc. cit., p. 509. 



