100 Transactions. 



considerably over this. The effect of this has no doubt been to make the 

 river an aggrading stream in that part of its course which lies in the Cul- 

 verden Plain, and to neutralize the effect of the elevation perhaps as far 

 down as Ethel ton, but to rejuvenate the part between the Greta and the 

 sea. Even this part is near a temporary base-level, judging by the great 

 amount of shingle in its bed and the very low terraces of some parts of its 

 course. This rejuvenation enabled the river to maintain its course in its 

 lower portion across the grain of the country, to cut deep gorges through 

 greywacke rocks, and to do this in spite of movements which would tend 

 to turn it from its original course. As the present valley of the river is 

 situated, there are several easier routes than that which it actually follows, 

 such as that past Hawarden down the Waikari Valley, or past Hawarden 

 into the valley of the Waipara and thus into the sea near Amberley. But 

 it appears that under certain conditions, when the course of a river is once 

 definitely established it will maintain that position in spite of influences 

 which should divert it from its original path. 



Development of the Course of the Hurunui Eiver. 

 The geological features of this region which have primarily determined 

 the course of the river are briefly stated as follows : On a greywacke sur- 

 face, incompletely base-levelled, a series of beds was deposited chiefly in 

 middle and late Tertiary times. These consist of sands sometimes with 

 coal, greensands, limestones, marls, sandy shell-beds, and sandy marls 

 passing up into conglomerates, the higher members being of Pliocene age. 

 The general character of the strata indicates deposition on a sinking sea- 

 bottom in the early part of the period, followed by deposition on a rising 

 bottom at the end, the whole sequence being laid down without a physical 

 break. It is probable that there was some differential elevation towards 

 the close of the time, so that some of the earlier beds were eroded in places 

 while continuous deposition was going on elsewhere. The sea in which 

 deposition took place gradually extended over a wider area with the sinking 

 of the land, since the higher members overlap the lower and cover a more 

 extensive area. Thus it is that limestone is very thin or entirely absent in 

 certain localities — for example, the Greta and Waikari Valleys — the land 

 occupied by those localities being the last to be invaded by the sea during 

 submergence, and having an entirely different form from that which it now 

 has. No doubt a slightly elevated area occupied the site of those valleys 

 in early Tertiary times. 



The covering beds extended far to the eastward, but have been cut 

 back bv marine erosion, which is at present making marked inroads on 

 the sea-cliffs composed of loose sands and marls ; while to the westward 

 the Tertiaries extended beyond the Mandamus Eiver, probably to the 

 vicinity of Maori Gully, but fragments of the greywackes rose like islands 

 in the cover of more recent beds, though not in the position of the high 

 lands existing at present. 



On this surface of covering beds as it emerged from the sea a consequent 

 drainage was established, consisting of subparallel streams running seaward 

 in an easterly direction. Although it cannot be stated with certainty, it is 

 probable that the first elevation of the land took place with comparatively 

 little deformation, and the river-courses were well established before the 

 dislocations became pronounced. After the rivers had been completely 

 established, folding and faulting took place on lines cutting the direction of 

 the main streams at an angle of approximately 45°, and these lines have 

 determined the courses of the principal tributaries, most of which enter the 

 main valley along fold and fault lines. The recency of the movements is 



