Cotton. — Geomorphology of South-western Wellington. 221 



teristic topography, covers an area of perhaps sixty square miles, of which 

 about half consists of nearly flat summits. One considerable area occurs 

 between the Otaki and Ohau Kivers ; an island " of it, surrounded by 

 gravel plains evidently parts of the Ohau fan spread by distributaries of 

 that river, occurs at Weraroa, where the Central Development Farm of 

 the Department of Agriculture is situated partly on this formation and 

 partly on the fan ; and there is then a nearly continuous bench, broken 

 only by some gravel-covered valley-floors, extending north-eastward for 

 twenty miles. 



Lithology and Structure of the Otaki Series. — The prevailing material in 

 the Otaki series is grey sand similar in mineral composition to that of the 

 present beach and the associated dunes. In addition to quartz, the sand 

 contains a considerable proportion of feldspathic, ferromagnesian, and iron- 

 oxide grains. The mineral grains, including those of quartz, though not 

 completely rounded, have their angles smoothed off, and they thus contrast 

 very strongly with the sharp, angular grains of the present beach. This 

 suggests aeolian accumulation. All the samples examined are somewhat 

 weathered, however, and the rounding of grains may be ascribed in part to 

 weathering. 



The more or less coherent sandstone formed of this material weathers 

 at the surface to a residual sandy clay, usually containing scattered 

 spheroidal masses of the sandstone. The permeability seems not to be 

 great, for the water-table is generally close to the surface. Much water 

 seeps out along the bases of even low scarps, and necessitates draining. 



In the few sections where bedding has been noted the beds are inclined 

 at about 35°. It is quite clearly cross-bedding on a large scale, and again 

 suggests subaerial accumulation. Cross-bedding and also ripple-mark are 

 noted by Adkin.* A horizontal pseudo-stratification, due, apparently, to 

 deposition of iron, is also generally present. Though less prominent, it 

 seems to resemble that noted by Berkeyj* in the aeolian San Juan 

 formation of Porto Rico. 



The non-discovery of fossils, J though a negative character, points also 

 to the possibility of subaerial accumulation of the sand of the Otaki series. 



Clay lenses and bleached soil-beds, which are interbedded with the sand- 

 stone, accumulated, no doubt, in lakes and swampy areas impounded among 

 dunes. The clay-bed between upper and lower sands noted by Adkin, § 

 which he ascribes to marine deposition at a period of maximum depression, 

 is perhaps one of these. 



In one section near Shannon layers of small pebbles occur interbedded 

 with the sand. Here is probably the course of one of the smaller streams 

 from the old land, or perhaps the margin of one of the larger fans. If 

 gravel fans and dune sands accumulated side by side, as is assumed in the 

 theoretical section, there must be a considerable amount of intermixture of 

 material along the transition lines where gravel passes laterally into sand. 

 Along these lines, indeed, a complex interfingering of gravel and sand beds 

 may be expected. 



The Fans or Gravel Plains. 



Throughout the length of the lowland there are numerous gravel fans, 

 both great and small. The largest are those of the Otaki and Ohau Rivers, 



* Loc. cit., p. 507. 



t C. P. Berkey, Geological Reconnoissance of Porto Rico, Ann. N.Y. Acad. Sci., 

 vol. 26, pp. 1-70 (see p. 50), 1915. 



J G. L. Adkin, loc. cit., pp. 497, 507. 

 § Loc. cit., p. 507. 



