518 Transncfioiis. 



The gravels, &c., of the Ohau fan and valley deposit, and also of the 

 recent terraces, consist of greywacke, soft slates, fragments of conglomerate, 

 and red and green quartzose material in corresponding proportion to their 

 occurrence in the country rock of the district. 



Since the excavation of its present valley the Ohau River, along the 

 lower part of its course, has raised its bed by the deposition of fine sediment 

 in much the same manner as the Waimakariri River has done in the 

 neighbourhood of Kaiapoi. These alluvial flats, which constitute the fertile 

 lands about the Lower Kuku Stream, have their origin where the terraces 

 of the Ohau merge upon the surface of its fan, and extend from thence 

 to the coast, though there, as elsewhere, their limits are hidden by blown 

 sand. The establishment of these aggraded alluvial flats has prevented 

 the inroads of the dunes on a narrow strip bordering the left bank of the 

 Ohau River. This is the only place within the area shov/n on the accom- 

 panying maps where the fertile land, which elsewhere lies contiguous 

 to the foot of the ranges, approaches to within one mile of the sea- 

 beach. 



The question now presents itself. What caused the Ohau River to ex- 

 cavate its present valley ? A cursory examination of the locality would 

 doubtless lead the observer to the conclusion that the present terraced 

 valley of the Ohau River is the direct result of the (geologically) recent 

 plicatioii of the folds, and consequent slow upheaval, of the Tararua Ranges, 

 and also of the adjacent sea-bed. Such, however, was not the case. It 

 is true that upheaval has taken place, as shown by the occurrence of raised 

 beaches, and it is highly probable that such was due to crustal move- 

 ments in the Tararua and Ruahine Ranges ; but there is evidence to show 

 that the uplift and the formation of the present Ohau Valley were not 

 coinciderit, and therefore the former was not the direct cause of' the latter. 

 The river did not commence the excavation of its valley until the land had 

 well-nigh reached its present elevation.* The Ohau flowed in its north- 

 west channel until the land was within 30 ft. of its present elevation, and 

 after that again it flowed in the same direction as it does now long enough 

 to excavate the sandstone to a depth of about 100 ft. — all before it com- 

 menced to incise the upper portion of its " valley plain." 



These facts point to a very considerable lapse of time between the 

 beginning of the uplift and the commencement of the erosion of the Ohau 

 Valley, so that a more satisfactory explanation of the latter operation 

 must now be sought for. 



The theory recently put forward by an eminent Canterbury geologistf 

 appears to conform with the facts of the case under notice. He considers 

 that one of the causes which will enable a river to excavate its channel 

 is " the failure of the supply of waste " — that is, the failure of the supply 

 of detritus ^^'ith which the river is aggrading its bed or which it is trans- 

 porting to lower levels. Concisely, the theory is as follows : Other factors 

 remaining constant, a cessation or diminution of the supply of transportable 



* Bj' " the excavation of its valle_y " is meant the erosion by the Ohau River of 

 its former deposits — i.e., the " valley plain " and fan accumulations. Being due to 

 causes other than those now being considered, the origin of the second valley in the 

 raised-beach sandstone does not come into the present discussion. 



t R. Speight : " Some Aspects of the Terrace-development in the Valleys of the 

 C'anterbnrj Rivers," Trans. N.Z. Inst., vol. 40, p. 16. 



