Adkix. — I'osf-tcr/ /(/)■;/ (Icoloijicdl llixloni of Oliaii 7?irer. 515 



The general ari'cUigemeut of the sandhills is in ridges at right angles 

 to the coast-line. Their culminating point between Paekakariki and the 

 Manawatu River is Moutere, 288 ft., and situated between the northern 

 (^nd of Lake Horowhenua and the sea. The surrounding sandhills have a 

 general altitude of 170ft., and vary in character considerably. Great 

 wastes of bare drifting sand border the coast-line, but further inland the 

 dunes are covered with manuka scrub and grass, so that in these parts the 

 accumulation has now ceased. 



The hollows between the older ridges and hills of drifted sand are 

 occupied by grassy flats, swamps, or lagoons. The last named are very 

 numerous, though their area seldom exceeds a few acres. Papaitonga, 

 the largest of the lagoons, is situated in the wedge-shaped sandstone area 

 lying between the former and the present course of the Ohau River. It 

 occupies a hollow (the former valley of a small stream) on the junction- 

 line of the raised-beach sandstone and the hills of blown sand. Extensive 

 swamps border the western shore of Papaitonga, and a stream connects 

 it with the sea. The smaller island, Motu Ngarara, is artificial, but Kiwi 

 Island is an isolated mound of raised-beach sandstone. 



The Ohau River flowed into the Horowhenua Lake until the latter 

 had attained its maximum area. At this juncture the deflecting force 

 which had caused it to continually erode its left bank compelled the river 

 to vacate its " north-west " channel, and to flow across the plain of raised- 

 beach sandstone in its present position, but on a much higher level. A 

 moment's consideration will show that the deflection was southwards, which 

 in a river flowing west is from its right bank towards the left. This fact 

 gives some indication as to the nature of the force which impelled the Ohau 

 River to continually attack its left bank during the occupation of its 

 " north-west " channel. Ferrel has shown that moving bodies on the 

 earth's surface, in the absence of other controlling forces, are deflected 

 to the left in the Southern Hemisphere. This law has already been applied 

 to the Canterbury rivers,* and there is no doubt that the deflective force 

 produced by the earth's rotation was the agency which caused the Ohau 

 River to alter the direction of its flow. Having reached its present posi- 

 tion, the deflection of the river ceased, owing to the retardation and sup- 

 pression of the deflective force, partly by superficial obstacles- — i.e., the 

 unfavourable configuration of the land-surface, viz., the Poroporo Range 

 — and partly by the awakening of other forces which introduced a new 

 cycle of action into the history of the Ohau River- — viz., the excavation 

 of the valley it now occupies. 



Upon attaining its present position the first act of the Ohau River was 

 to make for itself a valley in the raised-beach sandstone, and to deepen 

 it until the surface of the underlying fan was again denuded, and also 

 slightly incised. This erosion was due to the sandstone having a gentler 

 surface-slope than had that part of the " valley plain " which lay above 

 its limits, and to the tendency of the river to equalize the gradients of the 

 tlifEerent portions of its bed. Soon after this erosion of the sandstone the 

 Ohau River began to channel its former deposits. 



The gradual diminution in the slope of the surfaces of the fan and 

 " valley plain " had an important effect upon the subsequent action of 



* F. W. Hilgendorf : " The Influence of the Earth's Rotation on the Course of the 

 Rivers on the Canterbury Plains," Trans. N.Z. Inst., vol. 39, p. 200. 



17* 



