Adkin. — Readjustment of Drainage on the Tararuas. 183 



Art. XXV. — Examples of Readjustment oj Drainage on the Tararua 



Western Foothills. 



By G. Leslie Adkin. 



[Read before the Wellington PhilosojMcal Society, 8th October, 1919 ; received by Editor, 

 31st December, 1919 ; issued separately, 15th June, 1920.] 



Plate XL 

 Contents. 



Introductory. 



The Major Physiographic Features of the Tararua Range. 



Topography of the Arapaepae Ridge and of " The Heights " Basin. 



Changes of Drainage in " The Heights " Basin. 



Changes of Drainage on the Poruriri Ridge. 



1. Introductory. 

 In tte long - settled and more - closely - studied countries of the Northern 

 Hemisphere the processes and events of physiographic history genetically 

 connected with the present geographical regime have been more or less 

 thoroughly worked out, but in New Zealand the subject has been dealt 

 with in detail in only a comparatively few isolated areas. During the 

 last few years, however, New Zealand physiography has attracted ever- 

 increasing notice, and has come to be regarded here as elsewhere as being 

 of considerable importance in- the deciphering of the detailed geologic 

 history of a country. I have therefore been led to contribute the follow- 

 ing notes on the physiography of an area which has hitherto received 

 but scant attention? While it is not claimed that the course of events 

 outlined in section 4 is in any way unique, the constricted area in which 

 the diverse changes took place may be deemed somewhat remarkable, 

 comparable to some slight extent with the classical instance of drainage- 

 readjustment in the district round Chur, in Switzerland (Heim). 



2. The Major Physiographic Features of the Tararua Range. 



The Tararua Range is that portion of the structural axis of the North 

 Island extending from the headwaters of the Hutt River to the Manawatu 

 Gorge, a distance of fifty-seven miles. The range consists of a series of 

 parallel and subparallel longitudinal ridges, so disposed that they collectively 

 form an obtuse angle or wide arc convex to the west ; thus it is that 

 from south to north their trend changes from north-north-east to north-east 

 by north. In addition to the longitudinal ridges there is a subsidiary series 

 of transverse ridges which link the former together. 



Another feature of the Tararua Range is its asymmetry. The highest 

 ridge — that bearing the peaks of The Mitre and Mount Holdsworth^lies 

 well to the eastern side of the range, and on that side the altitudes of 

 the foothill ridges decrease more abruptly towards the subjacent lowlands 

 than on the west. In general, both towards east and west, the longi- 

 tudinal ridges decrease in height in succession as the respective piedmont 

 lowland areas are approached, the outermost foothill ridge on either side 

 being usually the lowest of the respective series. The whole is suggestive 

 of a high-standing tilted earth-block, having a steep eastward-facing .scarp 

 and a back-slope declining towards the west. The initial intense com- 

 pression and crumpling of the Trias-Jurassic (Marshall, 1912, pp. 127-29, 

 208) strata (the post-Hokonui deformation) was probably succeeded by 

 peneplanation (Cotton, 1916, p. 246 ; Thomson, 1917, pp. 399-400), and 



