110 Transactions. 



of the trunk stream. These correspond to successive levels in the valley of 

 the trunk stream — namely, benches cut in the sandstone side of the valley 

 as it was deepened ; the surface of the built flood-plain ; and subsequent 

 levels cut in the flood-plain alluvium. Along the margin of the ranges the 

 lesser streams have deposited cones and steep slopes of alluvium ; these 

 attain their maximum development between Waikanae and Te Horo, and 

 north of Tokomaru. 



Sufficient has been said to show that the topography of much of the 

 coastal plain is c|uite youthful, and that only small tracts here and there 

 have reached a mature or submature stage. On the other hand, two 

 portions exist in which, by reason of their position and the vagaries of 

 the drainage-lines, modification by stream erosion has been even less pro- 

 nounced than elsewhere, and their topography is strikingly youthful. These 

 portions, both situated at the foot of the ranges — one at Palmerston North, 

 the other near Shannon — have been somewhat isolated by the larger 

 adjacent streams, and, as their present surface has been but slightly lowered 

 from the original level, they stand out plateau-like above the surrounding 

 country. The comparatively few gullies intersecting them are as a rule 

 deep, steep-sided, and narrow, the wide flat-topped divides terminating 

 abruptly at the top of the gully- walls. A peculiar feature is that the 

 streams draining these gullies seem to have reached a more mature stage 

 than the topography. The topography is undoubtedly youthful, as shown 

 by the wide, flat-topped uplands, but the streams themselves are sluggish, 

 and often meander on flat, graded bottoms, frequently swampy. The 

 explanation is probably the porous character of the formation, which retards 

 the development of the gullies and consequently of the topography, while 

 the streams have meanwhile attained a temporary base-level. 



The Deformation of the Southern End of the Horowhenua 



Coastal Plain. 



Until recently the writer entertained the belief that the Horowhenua 

 coastal plain terminated, and that its inner margin descended to sea-level, in 

 a south-westerly direction, coincidently with the present complex lowland, 

 at a point about two miles south of Paekakariki. Doubt was cast on this 

 supposition by the compilation of the above table, which shows, inter alia, 

 the altitudes of the inner margin of the coastal plain southwards from the 

 vicinity of the Manawatu Gorge. As tabulated, these altitudes form a 

 descending series (marked by a certain step-like irregularity), proving the 

 differential character of the uplift that raised the coastal plain. The method 

 adopted to determine the height of the inner margin of the coastal plain 

 was to select suitable localities along its length and ascertain the altitude 

 of the highest traceable beds in those localities. The irregularity of the 

 altitudes obtained is explained by the denudation of the beds even in the 

 selected localities 



On the Palmerston-Pahiatua Road, eight miles south of the Manawatu 

 Oorge, the coastal plain appears to have attained its maximum uplift, and 

 here the beds of its inner margin are found 770 ft. above the present sea- 

 level. Proceeding southwards, the beds on the inner margin are found at 

 decreasing altitudes until Te Horo is reached, near which place the sand- 

 stone beds are found at a height of 320 ft. If, as previously supposed, the 

 inner margin of the coastal plain descended to sea-level at Paekakariki, the 

 average slope of that margin would have been about 13 ft. per mile, and 

 the greatest altitude it attained at Te Horo would have been 200 ft. At 



