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appropriately named the Pikikiruna fault-scarp (see fig. 2). The Piki- 

 kiruna block, which is tilted towards the east, is much dissected on its 

 eastern or back slope, the recently drowned margin of which forms the 

 indented western shore of Tasman Bay. 



>.iSll 



Fig. 2. — View looking south along the PiMkinma fault-scarp, which bounds the Takaka 

 Vallej- on the east. In the centre is seen a hcg-back of Tertiary limestone, which 

 is turned up along the fault. 



Tasman Bay itself no doubt had its origin i-n the subsidence of an 

 earth-block, while the lowlands forming its southerly continuation have 

 the appearance of a fault-angle depression, now much modified by 

 erosion, bounded on the east by the scarp of the Richmond or Waimea 

 fault* and on the west by the eastern boundary, probably in great part 

 a dissected back slope, of the great block or complex of blocks constitut- 

 ing the highlands of Mount Arthur, the Mount Arthur tableland, and 

 the neighbouring ranges, which are more or less continuous with the 

 Haupiri and Pikikiruna blocks towards the north. The structure of 

 the covering strata as interpreted in a series of sections by McKay,! 

 however, indicates a considerable complication of the block movements 

 in this neighbourhood by folding. 



The Aorere-Gouland Depression. 



The Aorere River in the lower seventeen miles of its course, in 

 which it flows north-eastward, is guided by the fault-angle depression 

 previously referred to as the Aorere Valley, a name which it is convenient 

 to restrict arbitrarily to this obviously consequent portion of the whole 

 river-valley. At the head, or south-western end, of the Aorere Valley 

 the river enters it from the south, emerging from a deep, narrow, and 

 steep-sided valley between high mountains — a valley which, unlike the 

 other, appears to owe the whole of its depth and width to erosion, which 

 is perhaps consequent but possibly insequent, and which may be con- 

 veniently designated the " Upper Aorere Valley." 



Tlie Aorere Valley depression is bounded on the north-western side 

 by the fault-scarp front of the Wakamarama block, and on the south- 

 eastern side by the tilted surface of a poi-tion of the Haupiri block. 

 To the north-east it is open to the sea, while at the south-western end. 



* See Bell, Clarke, and Marshall, " The Geology of the 'Dun Mountain Subdivision. 

 Nelson," N.Z. Geol. Surv., Bull. No. 12, p. 12, 1911. 



t A. McKay, " The Baton River and Wangapeka Districts and Mount Arthur Range,' ' 

 Geol. Surv. of kz.. Reports of Geol. Expl. dur. 1878-79, p. 122, 1879. 



