Adkix. — Posf-fi rf/arif (reoJoffictil ///sfori/ of 0/iaii Hirer. 519 



rock debris within a catchment-area will cause the contained river to deepen 

 its channel. In this district the ascertainable facts appear to support and 

 confirm this view. 



Though at the present time the basin of the Ohau River contains only 

 a few insignificant areas rising above the timber-line, in the early Pleis- 

 tocene, when the river was building up its fan, a very considerable pro- 

 portion of the mountains among its sources and upper reaches was bare 

 of arboreal vegetation. The former lower altitude of the timber-line in 

 relation to the sources and upper reaches of the Ohau River was, of course, 

 due to the greater elevation of this part of the country at that time. 



A coveriiig of dense forest acts as a shield and filter — controlling the 

 inipetuosity of the water following heavy rain ; binding the soil together 

 aiid protecting the underlying rocks ; allowing only the finest silt to be 

 transported to lower levels ; and, perhaps most important of all, prevent- 

 ing the formation of landslips. A striking, though by no means rare, ex- 

 ample of the last-mentioned attribute of a forest covering was observable 

 from Levin on Friday, the 3rd September, 1909. Dry weather followed 

 by rain ■ — a very favourable series of circumstances — produced their 

 usual effects on the mountains ; but, though innumerable slips were visible 

 on those ridges of the Tararuas lying above the limits of the bush, not 

 more than half a dozen were to be seen upon the nearer forest-clad 

 ranges.* 



It follows, then, that in the early Pleistocene, when the land hereabouts 

 stood at least 1,000 ft. higher than now, the areas bare of forest suffered 

 severely from the action of the elements— rain-storms swept away the 

 soil ; the alternate action of sun aiid frost shattered the rocks thus 

 exposed ; and landslips and freshets carried the resultant rock debris 

 to the valley -bottoms, furnishing the Ohau and its tributaries with an 

 abundant supply of " waste " whercMath to build up its deposits in the 

 lowlands. 



This state of affairs was brought to a gradual close by the following 

 subsidence of the land. It would appear that the upward extension and 

 growth of the forest did not keep pace with the subsidence, because, had it 

 done so, the excavation of the present Ohau Valley would have commenced 

 in the middle Pleistocene — the period of recent minimum elevation of this 

 part of the country. It seems, therefore, that the forest was slow in adapt- 

 ing itself to, and taking advantage of, the new conditions, and by failing 

 to check the supply of " waste " delayed the excavation of the valley to 

 the Recent geological period^ — almost to the present day. 



In the foregoing an attempt has been made to account for the varied 

 physical features of the district, the various geological formations, and the 

 peculiarities of surface-configuration. As previously stated, the absence 

 of fossils will no doubt force investigators to fall back on stratigraphical 

 evidence and the e\'idence of the slow crustal movements — first epeirogenic, 

 then orogenic — when attempting to place the formations of the district in 

 their correct position in the geological scale. The problems of the district 

 are by no means exhausted, but, as those still unsolved apparently do not 

 affect the continuity of the chain of events, their non-solution is in the 

 present instance of no great importance. 



* That the nearer ranges are not immune from landslips is shown by the fact that 

 when the bush is felled they are particularly liable to be disfigured by these hideous 

 scars. 



