Adkin. — P()xt-ter(i<ini (rtoloi/iral llishnii of Oluni River. 508 



When the deposition of the chiy alluvium had ceassd — the riverd having 

 attained a state of equilibrium — lateral erosion came into play, and the 

 Ohau River began to cut into the southern end of the Mill Range at the 

 point where it flows from its upper gorges on to the sloping " valley plain." 

 The river cut away and removed about 20 chains from the end of this range, 

 reducing its constituent rocks to the level of the valley deposit. Tt was at 

 this time that the Arapaepae Range was shorn of the spurs on its eastern 

 flanlc* For the most part these spurs were merely truncated, but in the 

 narrow part of the Ohau Valley, near the fluviatile vent, they were entirely 

 removed. This partial destruction of the Arapaepae Range was the joint 

 work of the Ohau and its two }nain tributaries, but in the Makahika Valley 

 it was accomplished by that river alone, though, as will be shown later, 

 in a remoter period. As a result of the lateral erosion the area of plain 

 within the Ohau Valley was very considerably increased, and thenceforth 

 consisted partly of the fluviatile deposit, and partly of equally elevated 

 and graded platforms of the country rock. The spurs of the range lying 

 between the Makaretu Valley and the fluviatile vent have also suffered 

 truncation, but in this instance the operation must have occurred either 

 during the excavation of the primary valley of the Ohau River or not 

 later than the period in which the " valley plain " and fan were 

 built up. 



As shown by the annexed table, the average superficial dip of the Ohau 

 "valley plain" was much steeper than that of its fan, the dip of the 

 former averaging 110-76 ft. and the latter 64-88 ft. per mile. Though this 

 is so, the progressive diminution of the slope between the upper limits of the 

 " plain " and the lower part of the fan is not regular, but is characterized 

 by declivities of varying steepness. The dissimilar gradients of the " vallev 

 plain " are, of course, clue to the respective width and narrowness of the 

 valley itself. The gentler surface-slope of the '" plain " in the Lower Maka- 

 hika Valley and in the narrow connection immediately above the apex 

 of the fan (see table) shows that in these places the deposit exactly corre- 

 sponds to those which sometimes form in valleys of uniform width ; but 

 where the Ohau Valley widens out (round about the junction of the Maka- 

 hika and Makaretu Rivers) the contained deposit endeavoured to assume a 

 fanlike structure, and there the surface-slojie is much steeper. 



The dissimilar gradients of the fan seem to have been produced in 

 another fashion. They lead one to suspect that the final additions of 

 material to the fan differed in character from the main mass of that forma- 

 tion. In building up its fan the Ohau River transported detritus to posi- 

 tions progressively more distant to the apex of the former. This had the 

 effect of not only raising the surface of the fan, but also of extending its 

 outer edges upon the surface of the Pliocene plain. As the fan aj^proached 

 completion the river's power of transportation diminished, the area of the 

 fan ceased to be extended, and the final additions to its mass were deposited 

 nearer and nearer the fluviatile vent. The alternate steeper slopes of the 

 surface of the fan are the respective limits of detritus laid down in periods 

 of transportation of lessening intensity. 



* The truncated ends of the spurs closely resemble fault .scarps, and might easily 

 be mistaken for such. There is, however, no direct or exclusive evidence for such an 

 origin, and fluviatile lateral erosion is a most satisfactory explanation. The origin of 

 the truncation of the spurs is. for the most part, closely connecttMl with the early Pleis- 

 tocene fluviatile de])osits. 



