General Geology and Geomorphology.
The coastal lowland of Western Wellington reaches its maximum width of 28 miles opposite the Manawatu Gorge. Southward it narrows, along a distance of 58 miles, rather regularly as far as Paraparaumu where its width is two miles. For the remaining 6 ½ miles a width of only a mile and a quarter finally tapers to the terminating point at the cliffs of Te Paripari, a little over a mile south of Paekakariki village.
The Quaternary formations of the lowland area hitherto recognized are four in number: fluviatile gravels, marine sandstone, stream alluvium, and blown sand. In areal extent the coast-bordering aeolian dune-belt (the Himatangi Formation, 6,. p. 271), comes first, covering approximately 50 per cent. of the terrain. The marine sandstone (the Otaki Formation, 3, p. 220) comes next, with stream alluvium, of varying age, a close third. The exposure of the fluviatile gravels (the Ohau Formation, 6, p. 269), the basal member of the group, is of limited extent, occurring only adjacent to the debouchures of the Manawatu, Otaki, and Ohau rivers (see Oliver,
5, Maps 1 and 2). In the present paper three more formations are recorded, all of limited thickness and areal extent but including one of considerable importance geologically.
The geomorphology of the above formations is a matter of importance in the elucidation of the development of the coastal lowland*, which is a feature of some complexity, in origin, structure, and surface configuration.
The basal fans, where exposed, present surfaces of regular and characteristic curvature. The preservation of such surfaces despite subsequent marine submergence must be attributed to conditions in which a superabundance of marine sand neutralized the abrasive action of the waves during sea advance. The covering of the basal fans (piedmont gravel plain) was thus accomplished without any discernible modification of their normal surfaces of deposition.
The blanketing of the fan surfaces by marine sand—the Otaki Formation—during the advance of the sea and the evidence of renewed deposition during its retreat as the coastal plain emerged, were considerations leading to the concept of the probable duplex major structure of the Horowhenua coastal plain (2, p. 507). During the time of still-stand, when the Otaki sea-level was at its maximum on the flanks of the Tararua foothills, the superabundance of marine sand piled up by the waves on the shoreline and immediately offshore, produced the steeper profile of the innermost part of the subsequently raised coastal plain. The flatter transverse profile of the remainder of the coastal plain surface towards the present coast—a profile (as determined by detailed measurement and levelling in the latitude of Lake Horowhenua) which has a gentle slope of apparently uninterrupted regularity—appears to indicate continuous and nearly uniform uplift, incidental eustatic rises in sea-level not leaving any permanent record here.
The emergence of the steeper innermost marginal strip of the coastal plain appears to have been followed by a temporary phase of marine abrasion and sea-advance during which the steeper initial profile was reduced and bevelled off, along much of its length, to the existing flatter upland surface conspicuous at Shannon, Levin, Otaki, and elsewhere, abutting on the slopes of the Tararua foothills. However, relict outcrops of the sandstone of the initial inner margin occur perched here and there to indicate the position of the original shoreline of maximum submergence.
It seems that it was the juxtaposition of this wave-bevelled inner portion of the coastal plain (altitude 260ft. east of Levin) with adjacent now isolated and more elevated fragments (in situ)
[Footnote] * The coastal lowland of Western Wellington has been referred to by some writers loosely as a coastal plain. The lowland is, in fact, a complex feature, geologically and geomorphologically, only portion of its area being a true coastal plain, using that term in its technical sense. A coastal plain is defined, following W. M. Davis and others, as a former sea-bottom of rather fine materials of deposition, raised and emergent as the result of secular uplift or of eustatic change in sea-level. A sea-bed surface is capable of emerging intact and unmodified when the energy of wave-action has been neutralized by a sufficient supply of waste which would have the effect of eliminating abrasion of the emerging surface.
of its constituent sandstone (more particularly that at 530ft. situated 3 miles ENE. of Levin) that led Oliver to postulate a fault at that place (5, p. 11; Fig. 39, p. 31; and Map 2, at end). As shown by the foregoing no such upthrust is necessary to explain the discrepancy in altitude in adjacent exposures of sandstone.
Stream alluvium of the district, usually ranging in age from Mid Pleistocene to Recent, but at the southern end of the lowland apparently coeval with the basal major fans, covers considerable areas of both the Horowhenua coastal plain of Otaki Sandstone and the less generally exposed surface of the piedmont plain of Early Pleistocene gravels. The surface of the alluvium takes the form of distinct minor fans and of coalescent fan-form slopes adjacent to hill-ridges, or of terraces and floodplain strips along river and stream courses. Portion of the later gravel deposits are the product of the incision of the upstream continuation of the major fans—the intermont valley-fill—by the former fan-building rivers, and the deposition of the reworked material on the downstream surface of the fans or in the trench since incised in each of them. Farther downstream where the entrenching finally peters out, a low-gradient secondary fan or fan-like deposit has been built forward by each individual river. Where, as in the case of the Otaki River, the trench incised in the main fan terminates close to the present coast, the shingle and gravels of the secondary fan deposit reach the present shoreline and replace to some extent the normal sand beach of this coast. In a manner similar to this the occasional bands and lenses of gravel observed incorporated in the Otaki Sandstone at Shannon and northward of that place (see Oliver, 5, Figs. 19 and 20, and pp. 20, 21, 39), also at Paekakariki, were carried forward and deposited. Such incorporated gravels by no means necessarily indicate oscillations of the shoreline as suggested by Oliver (op. cit., p. 39).