314 Ti-ausactions. 



gravels, which reach a height of 3,000 ft. above sea-level and overlap the 

 Murchison peneplain right to the Kawatiri basin. The Oweka Valley was 

 filled by the gravels of the Old Man Bottom, which as they were pushed 

 farther and farther south overlaid the beds knoMni as the Blue Bottom. 

 These are practically contemporaneous with the Old Man Bottom beds, 

 and were laid down in a shallow bay, to be ultimately covered by the 

 advancing delta of the Oweka and other lesser streams from the alpine 

 peneplain. 



Further elevation now took place, and the present cycle of erosion 

 commenced. The peneplains, lifted more rapidly than the rift-valleys, were 

 now sufficiently high to allow vast snowfields and glaciers to form. They 

 were not at that time so thoroughly dissected as at present, and the surface 

 of high land was much greater than now. The Mount Arthur, Lyell, 

 Victoria, and Paparoa peneplains all testify to the former existence of 

 glaciers, and the alpine peneplain shows traces of a much more exten- 

 sive glaciation than now obtaining. This elevation brought about many 

 changes in the river-systems. The Wairau, eating back along the great 

 Wairau fault, captured the head of the Motueka.* The Oweka had 

 formed a coastal plain, and its mouth had been forced to the north by the 

 prevailing northerly drift. When elevation took place it was handicapped 

 in cutting back by the hard Miocene sandstone immediately underlying the 

 alluvium of its lower course Moreover, the eroding-power near its mouth 

 was greatly lessened by the deposition of the great bulk of the sediment 

 in its sluggish middle course. The Mokihinui and Buller, small consequent 

 streams, broke into and captured the upper portion of the Oweka. The 

 Mokihinui had only the rock-bound source of the Oweka, but the Buller 

 reached the unconsolidated Old Man gravels, and carved its way south, 

 capturing the Inangahua,f which had already been beheaded by the Upper 

 Grey. The stream which cut the Upper Buller Gorge had now a greatly 

 augmented eroding-power, and captured the drainage of the Kawatiri basin, 

 the outlet to the Wairau being blocked by the great glaciers which formed 

 Lakes Rotoroa and Rotoiti. The conquest of this basin was contemplated 

 when the head of the Maruia was transferred from the Upper Grey. J This 

 must have taken place almost within historic times. 



At one time, also, the streams forming the Grey formed a river flowing 

 to the east of its present course. A range of Old Man Bottom hills lies 

 between this old course and the present course of the Grey. 



Summary. 



(1.) West Nelson consists of a series of earth-blocks differentially 

 <>levated. 



(2.) The fault-lines separating the blocks were probably established at 

 the time of the intrusion of the granites, consequent on the orogenic move- 

 ment which formed the alpine chain. 



(3.) The first cycle of erosion, which was very complete, produced the 

 coal series and base-levelled the elevated earth-blocks. The radial drainage 

 obtaining on the western portion of the Mount Arthur peneplain and on 

 the Pikikiruna, Lyell, and Paparoa penplains has probably been evolved 

 from this ancient drainage. 



* Haast : Geol. Explor. of W. Nelson, pp. 3, 90, 91. 



t Haast : hoc. cit, p. 103. Marshall : Geog. of N.Z., pp. 140. 141. 



+ Mackay : Geol. of S.W. Nelson, p. 43. Mines Statement, 1893, p. 175. 



