512 Transactions. 



Since the excavation of the shallow channel it now occupies the Kopu 

 taroa has, of course, been confined to its principal valley, and again pours- 

 the whole of its waters into the Manawatu River. 



The alluvium of the Koputaroa and its tributaries, being fairly 

 typical of the deposits of the other streams which convey the drainage 

 of the Tararua foothills across the coastal plain, is worthy of some 

 notice. In surface form there is a marked contrast between the deposits 

 of the Koputaroa and those of its tributaries. This difference is due to the 

 dissimilarity in the gradients of their beds, the bed of the* trunk stream 

 having at that time, as now, a much gentler slope than had those of its 

 tributaries. The Koputaroa spread its alluvium from one side of its valley 

 to the other in gently sloping sheets, w^hile its tributaries formed lateralh- 

 coalescing fanlike deposits, each deposit having its apex in the gully of the 

 stream to w'hich it owes its origin. The deposits of the tributary streams 

 are not true fans, because they were modified by the presence of the lateral 

 spurs of the Arapaepae Range, M^hich separated their upper portions. 



Besides having a gentler surface-slope, the alluvium of the Koputaroa 

 differs from that of any one of its tributaries in other respects ; the area 

 it occupies is greater, though its thickness is less ; and its contained rock 

 debris consists of well-rounded pebbles arid boulders, and not, as do the 

 tributary deposits, of angular fragments. The alluvium of the Koputaroa 

 itself consists of masses of gravel, grit, and boulders, surmounted by patches 

 of yellow clay. The total thickness probably does not anywhere exceed 

 30 ft., and, though the thickness of the lower portion is fairly constant, 

 the upper clayey division varies — in some places swelling out to a depth of 

 5 ft. or more, and in others thinning away to the upper surface of the mider- 

 lying gravel. These masses of superficial clay frequently contain numbers 

 of sporadic stones of various size. Of these the largest observed was a 

 greywacke boulder, roughly rhomboidal in form, but nevertheless well 

 water-worn. It lay just below the surface of the clay, and was found to 

 weigh 3201b., its greater diameter being 28 in., its lesser 20 in. 



From recent observations made in Victoria by Mr. Guppy it has been 

 found that muddy w^ater, having a greater specific gravity than that w^hich 

 is free from suspended particles, is able, even when flowing at a moderate 

 velocity, to transport boulders of considerable size. From this it will be 

 seen that the presence of large sporadic stones in a deposit of fine silt, and 

 their transportation, by a stream of no great magnitude, to positions some 

 miles from the parent rock, are phenomena not difficult to be accounted 

 for. These facts seem to indicate that when the Kuputaroa Stream first 

 began to deposit material it was a muddy torrent washing down large 

 numbers of various-sized stones ; but after it had raised its bed by de- 

 position its flood-waters spread, in the absence of any w^ell-defined channel- 

 over the whole of the floor of its valley, and, while depositing clay, also 

 rolled dowii and disturbed the comparatively few pebbles and boulders 

 which happened to come within the range of its action. 



The Ohau River flowed in, and gradually widened, its " north-west "' 

 valley, until the sea had receded, in this vicinity, to about five miles fronr 

 the ranges, when a bar, or perhaps sandbanks, were formed at the mouthi 

 of the river. By the action of wind and waves, combined with the uplift 

 of the land, the bar or sandbanks were raised above sea-level, and sand- 

 dunes were piled up, at first obstructing and afterwards completely blocking: 

 the river at the point where it entered the sea. 



