516 Traiimrtions. 



the Ohau River, As has been shown in the appended table, this slope 

 is very similar to a thalweg, or erosion-curve, being relatively steep in its 

 upper parts, but gradually becoming flatter (with some irregularity) as 

 it is descended. At that time the chief endeavour of the river was ap- 

 parently to reduce the steepness of its bed ; and on this account, and as a 

 result of the general progressive diminution of the superficial dip of its 

 former deposits, the upper and middle parts of the " valley plain " were 

 the first to be incised. The material from this excavation was washed 

 down, and spread over the surfaces of the lower part of the " valley plain " 

 and the upper part of the fan ; and it was also deposited to the depth 

 of some 20 ft. on the floor of the valley in the raised-beach sandstone. 

 Lateral erosion on the part of the river then came into play, and in the 

 last-mentioned locality the sides of the valley were cut back — down to the 

 level of the material just previously deposited. This lateral erosion was 

 effected in such a manner as to cause the resultant flood-plain to have a 

 convex surface when viewed longitudinally. In forming this part of the 

 flood-plain the river cut sideways, on its right bank, not only through 

 the sandstone side of the valley, but it also notched (laterally) the slope 

 of the underlying fan. Vertical erosion again attaining an ascendency, 

 another flood-plain was formed somewhat below the level of the first, and 

 this time extending from the upper edge of the " valley plain " to some 

 distance down the slope of the fan. Subsequently, periods of lateral 

 erosion and the formation of successive flood-plains alternated with periods 

 of vertical erosion, and in cases when the flood-plains were not totally 

 destroyed by the further action of the river their fragments were left to 

 form the terraces which are now so common in the several portions of the 

 Ohau Valley. 



Owing to its straightness and uniform width, the Lower Makahika Valley, 

 at the commencement of the erosion of the early Pleistocene deposits, 

 was extremely liable to be swept from side to side by floods, so that, 

 with the exception- of two or three fragments which now indicate the 

 altitude of the original surface, the Makahika branch of the " valley plain " 

 was destroyed and washed away. As the excavation progressed- — -lateral 

 erosion being in part superseded by vertical — the lower flood-plains were 

 less affected by the action of the river than the original surface was, and 

 are therefore more extensively preserved. The truncation of the spurs in 

 the Makahika Valley appears to have been effected in the Tertiary epoch, 

 since thick masses of early Pleistocene deposits can be seen abutting upon 

 their truncated ends. 



An examination of the escarpments of the terraces which have been 

 cut in the " valley plain " shows that there are of the latter two distinct 

 types. Li one the terrace-face consists either of shingle, gravel, &c., from 

 summit to base, or has a considerable thickness of these materials rest- 

 ing on the country rock, while in the other type the country rock is 

 surmounted by only a comparatively thin layer of water-worn detritus. 

 The first type of terrace shows that the Ohau has incised its Pleistocene 

 deposits (which fill the primary, or pre-Quaternary, valley of the river) 

 either in the middle or at one side of the valley, while those of the second 

 type indicate that the river has cut laterally and then vertically into the 

 bounding hills, and, in forming the subsequent flood-plains, deposited a 

 veneer of gravel and shingle upon the successive platforms which it carved 

 in the ancient base rocks. 



