Speight. — Structural and Glacial Features of Hurunui Valley. 99 



wacke barrier the limestone also occurs, with reversed dip, and under the 

 limestones are exposed sands and greensands with saurian bones, and thin beds 

 of impure coal. On following the beds across the strike a synclinal arrange- 

 ment is found, and the limestone forming the south-eastern limb appears 

 as a reef at Port Robinson, striking out to sea just as the limestone reefs 

 do at Amuri Bluff. This syncline is well seen in the cliffs at Gore Bay, and 

 it no doubt extends south-west as far as the Hurunui, and appears where 

 the rocks dip up-stream just above the lowest bridge across the river. The 

 upturned beds of the south-easterly wing of this syncline rest at Port Robin- 

 son on greywackes, and at the Hurunui Bridge on the same rocks. In the 

 last-mentioned locality there is evidence that the Tertiaries are bent over 

 this core of grey wacke in mild anticlinal arrangement. The river has cut 

 a gorge through the greywacke, which has been used as a solid basis for 

 the abutments of the bridge. 



Up-stream from this, the traces of the anticlines and synclines which 

 occur between Port Robinson and Cheviot can be seen occasionally where 

 the Motunau beds are exposed in the river-terraces, but no limestone is 

 visible ; the general arrangement is, however, synclinal. 



An important point as regards the history of the river is the compara- 

 tively recent elevation of the coast-line. This has amounted to as much 

 as 600 ft., judging from the shore-platforms extending to that height at 

 Port Robinson, between the mouths of the Hurunui and the Blyth Rivers 

 (three miles to the south), and just south of the Blyth River on the summit 

 of the Napenape Cliffs. This elevation has been noted previously by 

 Haast, Hutton, and McKay. In the last-named locality there are sea- 

 planed limestone surfaces 600 ft. above sea-level covered in places with 

 marine gravels. In the country just south of the Hurunui this plain of 

 marine denudation extends back from the present coast-line for some five 

 miles to the base of the greywacke hills, and exhibits a peculiarity in that 

 the wave-cut surface is higher near the coast than farther inland. This 

 suggests that a slight warping has taken place since the plain was cut ; but 

 the peculiarity may perhaps be explained by the more ready erosion of the 

 softer beds farther inland than the harder limestone exposed near the 

 coast where it forms the floor of the high platforms. The first explanation 

 is, however, the more reasonable, and if it is correct the axis of warping 

 would approximate to that of the line of the greywacke bar near the river- 

 mouth. Apart from the effect on the river in this vicinity, probably apparent 

 in the gorge of the river incised in a somewhat wide flood-plain, an elevation 

 of the land totalling some 600 ft. would exert considerable influence on a 

 river which had reached approximate base-level, as it is reasonably certain 

 that the Hurunui had, before the coastal elevation took place. The power 

 of vertical corrasion would be greatly increased over a considerable part 

 of the course of the river. At the present time a considerable portion of the 

 Culverden Plains are under 600 ft. above the sea, and unless some compensa- 

 tions in level have taken place inside the coastal belt the level of the river- 

 bed should have been greatly affected as far as the junction with the Man- 

 damus at least, where the solid bars of rock would delay adjustment for a 

 long period after it had taken place in the relatively weaker beds farther 

 down-stream. There is, however, evidence of a lowering of the inland 

 portion of the river-basin relative to the coastal portion as a result of the 

 faulting which took place on the Greta line, on the Waikari-Kaiwara line, 

 and again in the deformational movements of the Hurunui-Waiau basin. 

 The effect of this would be to make this portion of the stream an aggrading 

 one if the lowering were in excess of the coastal elevation. This has 

 certainly been the case, for the aggregate throw of the faults must total 

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