432 



Transactions. 



This fossil plain is strikingly similar to that of the Shag Valley fault- 

 angle. Obviously it had a similar origin, and it is practically certain that 

 the two were continuous prior to the Kaikoura orogenic movements (though 

 at that time deeply buried under covering strata). 



The surface is somewhat undulating, being " folded " to some extent 

 like the adjacent covering strata. This has introduced slight irregulari- 

 ties in the pattern of the consequent drainage, but the majority of the 

 streams run north-eastward in the direction of the general slope. 



While the dissection is in general shallow, some master streams— e.g., 

 the Waianakarua (Plate XXXI) — are deeply incised in steep-walled gorges, 

 which necessarily become deeper as they are followed back from the 

 debouchures, the gradients of the streams being decidedly less steep than 

 the slope of the fossil plain. 



The Waitaki Graben. 



The Waitaki River is consequent on the Kaikoura deformation. The 

 middle and lower Waitaki follow a straight course in an east-south-east 

 direction through a somewhat complex graben between the highland blocks 

 of North Otago and those of South Canterbury. Its tributary the Haka- 

 taramea enters from a great northerly-trending fault-angle depression. 



In the Waitaki graben there are several small blocks of great interest 

 as examples of tectonic forms. One small tilted block just behind Kurow 

 exhibits an almost perfectly preserved back-slope and even crest-line (see 

 fig. 3). 



^>L ^i 



CO. C.C-ton 1916 



Fig. 3. — Small tilted block in the Waitaki Valley graben. View looking south-west- 

 ward across the bed of the Waitaki River. The height of the scarp at the eastern 

 end is about 1,000 ft. At the western end a glimpse is caught of the crest-line 

 of the northern highland of Otago. 



Another striking feature is a splinter from the northern fault-scarp of 

 the graben just opposite Duntroon (Plate XXX, fig. 2). The fossil plain or 

 stripped surface of the greywacke undermass, which is similar to that of 

 the back-slope of the Hunter's Hills,* forms a plateau considerably over 

 1,000 ft. above the level of the valley-floor, and a strip of it descends in a 

 westerly direction along the splinter, coming right down to the level of 

 the low terraces bordering the river. While the cover has been removed, 

 the fossil plain is almost untouched by erosion. The fault-scarp at the 

 back of the splinter is well preserved ; that in the front has been sharpened 

 by the river. 



* C. A. Cotton, foe. cit. (1916), fig. 2, p. 316. 



