496 



Tronsactions. 



Art. XLI V. — The Post-tertiary Geological History of the Ohau River and 

 of the Adjacent Coastal Plain, Horoivhenua County, North Island. 



By George Leslie Adkin. 



[Read before the Wellington Philosophical Society, 5th October, 1910. \ 

 Plates XV-XIX. 



The principal waterway of the central portion of the Horowhenua County 

 is the Ohau River, situated between the Manawatu on the northern bound- 

 ary of the former, and the Otaki and Waikanae in the south. Though 



a small stream, with 

 a length of only 

 twenty miles, its 

 valley and the ad- 

 jacent country pre- 

 sent many features 

 of great scientific in- 

 terest. The total ex- 

 tent of its drainage- 

 a r e a is sixty - four 

 square miles, forty 

 square miles of which 

 is mountainous, the 

 remainder being that 

 portion of the ad- 

 jacent coastal plain 

 which is drained 

 by the Ohau. Its 

 largest tributaries 

 are the Makahika 

 and M a k a r e t u 

 Streams ; others — 

 viz., the Blackwater, 

 Kuku, &c. — though 

 inferior in volume, 

 are by no means in- 

 considerable. At the 

 point where the 

 Ohau River crosses 

 it the present coastal 

 plain has a width of 

 nine miles, but it is 

 wider toward the 

 north, and narrower 

 in the south. 

 The Upper Ohau River, flowing in a valley which is alternately longi- 

 tudinal and diacUnal, lies among the western subsidiary ridges of the 

 Tararua Ranges. In this vicinity these latter constitute a wild stretch of 

 country varying in altitude from 500 ft. to over 5,000 ft., and characterized 

 by parallel tectonic ridges, and, in the diaclinal valleys, by tortuous rock- 



Fio. 1.- 



-LocALiTY Plan of Akea described and 



MAPPED. 



