Speight. — Structural and Glacial Features of Hurunui Valley, 97 



backwards as if the beds had been dragged down along a fault-line. Farther 

 north towards the Hurunni Gorge, opposite their junction witli the Mandamus, 

 they appear quite normal, but in the Pab.au again their structure is obscure, 

 though that may be attributed to the disturbance in the immediate neigh- 

 bourhood of a volcanic vent. Farther north-east towards Culverden their 

 arrangement is again normal. The floor of the basin is almost completely 

 masked by the gravels of the Hurunui and Pahau Rivers, the only indication 

 of what is underneath being given in the vicinity of Hurunui Mound. Here 

 the Tertiaries rise like an island in the sea of gravels, and they are evidently 

 folded acutely. In the cliffs on the bank of the river near the railway-bridge 

 the structure is anticlinal, but at the Mound itself, about half a mile to the 

 north-east, the beds are also folded, though not on the same line. There 

 is evidence, therefore, of a more complex structure under the plains — that 

 is, they approach a synclinorium. 



The southern margin of the plain from the road-bridge eastward is 

 determined by a fault-scarp, along the foot of which the river flows. The 

 settlement of the block of country under the plains appears to be more 

 marked on the south-east side (cf. the Waikari and Greta Valleys, also 

 the fault system of the Kaikouras : Cotton, 1914), and the river has therefore 

 occupied it as the lowest level possible on the plains. The outlet, however, 

 is marked by high-level terraces indicating a former higher level of the 

 river. It is almost certain, therefore, that the deformational movements 

 which caused the basin had not terminated w T hen the river had established 

 its course through the gap at the south-east corner of the plains. Of such 

 recent movements the surrounding districts furnish ample evidence (cf. . 

 the fault-scarps near Hanmer, at Glen Wye, and also the recent gorge of 

 the Middle Waipara). The course of the river from the junction of the 

 Mandamus has followed the line of steepest descent to the fault-line, and 

 is therefore approximately at right angles thereto. This explains the 

 necessity of the sudden sharp turn when the line of the fault is reached. 

 Although the high-level terraces at the outlet may be attributed to recent 

 movements in the basin itself, they may be correlated with the uplift which 

 all this region has recently experienced, and therefore are the result of the 

 river accommodating itself to a new and lower base -level. The river-course 

 across the plains is marked by terraces of no great height. It here follows 

 a direction consequent on a surface of its own making, for which the 

 term auto-consequent could be used. Thus the courses of the Rakaia, 

 Rangitata, and other large rivers of Canterbury across the plains are auto- 

 consequent. 



(3.) The Waikari-Kaiwara Basin. 



After leaving the Culverden Plain the river flows through a gorge cut 

 in greywacke for about six miles till it enters on the Waikari-Kaiwara 

 basin. This extends down the river to the immediate vicinity of the 

 Ethelton railway-station, when the river passes through another gorge 

 cut in greywacke. The basin is therefore completely enclosed by pre- 

 Tertiary rocks. Although the area has a basin-shaped form, its origin is 

 somewhat different from the Waiau-Hurunui intermontane area, and owes 

 its formation entirely to the faulting-down of a strip of Tertiary beds and 

 the subsequent enlargement of the tributary valleys through the rapid 

 erosion of relatively weak beds. These consist chiefly of sands with harder 

 concretionary bands, sandy clays, and marls, with occasional irregular 

 layers of shells, mostly in a fragmentary condition : they are, in fact, the 

 equivalents of the Motunau or Greta beds. Mount Brown beds are existent 

 4 — Trans. 



