Cotton. — Fossil Plains of North Otago. 429 



Art. XXXV.— The Fossil Plains of North Otago* 



Bv C. A. Cotton, D.Sc, F.G.S., Victoria University College, Wellington. 



[Read before the Wellington Philosophical Society, 27th October, 1915 ; received by Editors, 

 30th December, 1916 ; issued separately, 30th November, 1917.] 



Plates XXX, XXXI. 



A fossil is a thing "dug up," and to the palaeontologist the term "fossil" 

 indicates something buried a very long time ago and dug up very recently. 

 By the term fossil plain, therefore, may be understood a plain which, 

 after coming into existence as a plain of erosion, has been buried by sedi- 

 ment and long afterwards re-exposed by renewed erosion. The sloping 

 plateau surfaces to which the name is here applied are the most striking 

 features in the character-profiles of North Otago. f 



As a rule, in that district a fossil plain forms one side of the unsym- 

 metrical valley developed in a fault-angle. On the one side is a fault-scarp 

 more or less dissected ; and on the other is the tilted surface of the next 

 earth-block, from which the weak cover has been stripped, revealing the 

 planed surface of the undermass. 



As is more fully explained in another paper, J the dissection of the sur- 

 faces, though they are inclined, is generally shallow, because of the close 

 spacing of numerous consequent § streams, which, combined with the small 

 rainfall, gives such streams steeply sloping profiles even when fully graded. 

 Only a few larger streams trench deeply. Thus the fossil plain, once 

 stripped, is a stable form under the climatic conditions prevailing in North 

 Otago, and this accounts for its common occurrence. 



The Shag Valley Fault-angle. 



The fault-angle depression known as the Shag Valley is an excellent 

 example of the type. This depression is really a branch of the Central 

 Otago depression-system, and it connects that system with the greatest 

 of all depressions — the Pacific Ocean. The divide between the head of the 

 south-eastward-flowing Shag River and the streams flowing westward to the 

 Maniototo Plain is situated in an area of mature topography developed on 

 covering strata strengthened by the presence of abundant volcanic rock. 

 There is thus, as it were, an artificial separation of the Shag Valley de- 

 pression from the Maniototo depression, the most easterly member of the 

 Central Otago system proper, which would be much less prominent but 

 for this local strengthening of the overmass. 



The fault-scarp of the Kakanui Range, which forms the north-eastern 

 boundary of the Maniototo depression, continues south-eastward along the 

 front of that portion of the highland block termed the Horse Range, and 

 thus forms the boundary of the Shag Valley depression also, being here 

 opposed by one of the most perfect sloping plateaux in Otago- — a fossil plain 

 (see fig. 1, and Plate XXX, fig. 1) — which descends at a low angle north- 

 eastward and eastward, passing, towards the sea and in the seaward portion 



* The subject-matter of this article formed part of a paper on " Block Mountains 

 in New Zealand," read before the Wellington Philosophical Society, part of which has 

 been published in the American Journal of Science (vol. 44, 1917, pp. 249-93). 



f C. A. Cotton, The Structure and Later Geological History of New Zealand, 

 Geol. Mag., dec. 6, vol. 3, 1916, pp. 243-49, 314-20 (see p. 315). 



% hoc. cit. (1917), pp. 256-58. 



§ More strictly termed " superposed consequent " at the present stage of their 

 history. 



