498 T ranaactions. 



upper surface is inferred from the configuration and character of tlic super- 

 imposed fluviatile deposit about to be described. 



The rivers which at present flow into the sea along the Terawhiti-Egmont 

 coast-line had to traverse the ncAvly uplifted plain, and were then not only 

 of considerable length, but were also of greater volume, due to the greater 

 elevation of the land producing a more abundant rainfall, and incidentally 

 — for the fact does not affect the folloM'ing considerations — a larger drainage- 

 area. Like the others, the Ohau River crossed the Pliocene plain, and its 

 subsequent history nvdy be traced as follows : Upon leaving its vent in 

 the foothills of the Tararua Ranges the Ohau River began to deposit 

 material upon the Pliocene plain as soon as it was uplifted above sea-level. 

 The deposited material took the form of a fan, and — for a while at 

 least — kept pace with the uplift of the plain, as well as reducing the dis- 

 similar gradients of the river-bed to a more uniform slope. The extent 

 of the fan in a direction radial to its apex or summit is as yet unknown, 

 but it certainly exceeds fifteen miles. Its maximum thickness is also not 

 known, but in this respect it has l)een found to exceed 700 ft. At the pre- 

 sent time the apex of the fan is only 345 ft. above sea-level, the average 

 surface-slope being about 65 ft. in the mile, though the dip of its component 

 layers at some depth below its surface is doubtless greater. 



The bulk of this massive and coarse fluviatile deposit consists of some- 

 what irregular alternations of shingle, gravel, boulders, coarse sand (quick- 

 sand when saturated with water at some depth below the surface), and thin 

 bands of claj'. The beds of finer detritus are sometimes found in immediate 

 association with the coarser material, any portion of the whole forming 

 an exact counterpart of what may be seen on the Ohau River bed at the 

 present day. The thin layers of clay which were deposited on many succes- 

 sive levels point to a temporary cessation of deposition on such areas, due 

 to the river changing its course to a more or less distant portion of its 

 fan.* During its formation the Ohau fan, with its great wastes of bare 

 shingle stretching far and wide, must have presented a very desolate ap- 

 pearance. Any vegetation which managed to take root upon its barren 

 slopes would, by the river changing its course, as was its unceasing habit, 

 be completely covered by a stony mass. Swamps appear to have existed 

 in hollows on the surface of the fan, probably on many successive levels, 

 an ancient sAvampy layer 5 ft. in thickness having been met with at a depth 

 of 323 ft. when the first trial artesian bore was sunk on the State farm 

 at Weraroa. Another old land-surface was discovered when a well was 

 dug on Section 38 in the Horowhenua Village Settlement. After passing 

 through from 70 ft. to 80 ft. of gravel and shingle the workmen came upon 

 a swampy layer 1 ft. thick, on the surface of which was found an entire 

 pukatea stump in situ. 



Driftwood has al'oo been found at various depths in the Ohau fan. 

 After heavy rains, driftwood derived from the bush-clad ranges and washed 

 down by the swollen river would be cast ashore along its margin, and subse- 

 quently buried. In several cases when wells have been sunk into the fan 

 this old water-borne timber has been brought to light. Twigs, leaves, 

 and branches were found at a depth of 20 ft. in a well sunk in the Levin 

 Borough about ten years ago. In another well a layer of branches and 

 twigs and also a piece of pukatea timber were found at a good depth. In 



* In tho artesian well.s sunk on the ^^■(•l•aI■oa State Farm the thin day-seams were of 

 very frequent occurrence. 



