Speight. — The Intermontane Basins of Canterbury. 347 



Waiau-Hurtjnui Basin. 



This basin lies across the middle courses of the Waiau and Hurunui 

 Rivers, and is perhaps the most typical of all those within the Canterbury 

 area. The rivers have cut deep gorges through the barrier which bounds 

 it on the east, and furnish a most interesting example of anomalous drainage. 

 Cotton has suggested that the rivers were antecedent to the present land- 

 surface, and that the gorges were cut as the surface was warped upwards. 

 The other explanation, which was originally advanced by Hutton, is that 

 it is a case of superimposed drainage. 



The basin through which these rivers run extends for nearly thirty 

 miles in a south-west to north-east direction, and has a maximum breadth 

 of about eight miles, its elevation above the sea lying between 500 ft. and 

 800 ft. The greater part of its surface is formed by the combined aggraded 

 flood-plains of the Waiau, Pahau, and Hurunui Rivers, but at one or two 

 places the Tertiary beds rise through this covering. An extension of this 

 basin lies on the south side of the Hurunui River, and reaches the Waipara 

 River in the neighbourhood of Heathstock, with outlying portions in the 

 Upper Okuku and Ashley Rivers : and another connection with it lies 

 toward the upper head of the Waikari Creek, the dividing ridge being quite 

 low. and constructed entirely of Tertiary rocks. 



The general sequence of beds exposed in the area is as follows :- — 



(1.) Sands and clays, with beds of greywacke, gravel, and very occa- 

 sionally impure coal. These are well exposed in the banks of the Pahau, 

 on the western side of the basin, in the valley of the Mason River to the 

 north-east, and in the deep gorges cut by the various tributaries of the 

 Waipara River near Heathstock. 



(2.) Limestones, frequently interstratified with volcanic tuffs. These 

 are typically developed on the north-west side of the area between the 

 Pahau and Waiau Rivers in the valley of the Mason, and near Heathstock. 

 In the central portion, on the south-east flanks of Mount Culverden, they 

 dip steadily to the south-east at angles of about 15° : farther west they 

 have been folded into an anticline, and in the lower part of the gorge of 

 the Pahau River the dip is to the north-west, but the directions are much 

 disturbed by volcanic action. The limestone outcrop can be traced round 

 the north side of the basin, across the Hurunui. and on the flanks of Mount 

 Mason, but the outcrop is not continuously visible. In this last-mentioned 

 locality the limestone, is folded back sharply where it abuts against the 

 Trias-Jura rocks of Mount Mason, although the general dip appears to be 

 towards the south-east. On the south side of the basin, along the Hurunui 

 River, the limestone has evidently been faulted down and covered up by 

 gravels, but farther west it reappears, the fault grading into a fold, and 

 the outcrop follows round the western end of the Trias-Jura mass forming 

 the Mount Alexander Range and joins on to the Weka Pass stone near 

 Waikari. which, when followed north-east down the Waikari Creek, forms 

 one of the strips of limestone in the floor and on the north-west flanks of 

 fault valleys which are so characteristic of this region of Canterbury. This 

 fault also grades into a fold in the upper part of the Waikari Creek basin. 

 There is a marked difference, however, in the fossil-content of the lime- 

 stones in the Culverden area from those of the Weka Pass stone, for the 

 former seem to accord more closely with those of the Mount Brown beds, 

 which lie above the Weka Pass stone, a fact which is probably explained 

 by the gradual and slow transgression of the sea over the Culverden area 



