Adkin. — PoKf-tertiari/ Qeological nialory of Ohau Hirer, 507 



valley of the stream would have formed an iusurinoiintable barrier to the 

 Ohau River when the course of the latter was chanj2;ed in the manner to 

 be described later. 



The structure and lithological character of the material of the inclined 

 plain of marine deposit (raised beaches) may now be examined. I am 

 indebted to Mr. John Young for a hint as to the origin of this formation. 

 From original observations made while travelling up and down the railway 

 he concluded that it was nothing more or less than an extensive series of 

 raised beaches, and I have since proved — to my own satisfaction at least 

 — that such is indeed the case. At the foot of the ranges the raised- 

 beach formation is composed of sandstone, but further out from them — 

 in Levin Borough, for example — the formation consists of an U})per bed 

 of sandstone, a middle zone of yellow clay, and a basal layer of sand- 

 stone, the first and last being identical in all respects. Along the lower 

 slopes of the Tararua foothills especially, the thickness of the raised-beach 

 formation varies according to its remoteness from or j^roximity to the 

 smmiiits of the Ohau, Otaki, and other fans. Two miles south of Shannon 

 the sandstone lies on the lower northern slope of the Ohau fan, and there 

 has a thickness exceeding 500 ft. Opposite Levin its thickness was orighi- 

 ally 200 ft. to 240 ft., but has since been much denuded. 



The raised-beach formation is evidently a double one. When the 

 country sank (terminating the early Pleistocene elevation), and the sea 

 advanced over the land in this locality, sandy beaches were formed, only 

 to be submerged and superseded by others as the subsidence continued. 

 When the subsequent uplift took place, the sea while receding repeated 

 the process of deposition, and formed a second series of sandy beaches 

 above the first. The intercalated masses of clay represent, of course, the 

 finer sediment laid down while the sea was advancing and receding, and 

 also when this portion of the country was at its recent minimum elevation 

 —i.e., when the sea washed the Tararua foothills. 



The sandstone of the formation is usually rudely stratified, and gene- 

 rally finely laminated, the laminae sometimes exhil)iting false beddmg.. 

 [ts colour varies from light and dark grey to various shades of red aiid 

 vellow. Quartz grains, which constitute the greater part of the sandstone^ 

 are coated with iron-oxide in the red and yellow varieties, and associated 

 with black particles in the grey. Ripple-marks are of frequent occurrence 

 in the sandstone, usually at some little distance from the upppermost sea- 

 margin at the foot of the hills, and are without doubt due to the friction 

 of fairly deep water, and not to the action of wind. 



In this district both the clay and the sandstone are quite destitute of 

 fossil remains. This deficiency is probably due to the seas which the)i 

 washed these shores being too turbid to be favourable to the life of shell- 

 fish and similar creatures which were likely to be preserved in the fossil 

 state ; or it maj- be that shells which were originally preserved have since 

 been removed by dissolution. In some places, especially at the foot of the 

 Arapaepae Range, the sandstone outcrops are ,seen to be riddled, or even 

 honeycombed, with tubular holes, usually from | in. to 1 in. in diameter. 

 The absence of fossil remains within these holes renders it difficult to deter- 

 mine whether they were the homes of marine worms or borers, or have 

 merely been made by land-insects since the uplift. The size of the perfo- 

 rations and certain other characteristics seem, however, to disfavoui- the- 

 latter suggestion. 



