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Volume 63, 1934
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The Nature and Mode of Origin of the Motunau Plain, North Canterbury, New Zealand.

[Read before the Philosophical Institute of Canterbury, August, 1931; received by Editor, 27th September, 1932; issued separately June, 1933.]

Introduction.

The authors have previously made independent general surveys of shore platforms on considerable lengths of the New Zealand coasts, and in the course of this work the problem of determining definite criteria for the recognition of the mode of origin of these features has constantly arisen. It seemed, therefore, helpful to study conjointly one good shore platform in greater detail, and for this purpose the Motunau Plain on the north-east coast of Canterbury was considered the most suitable.

For a discussion of its mode of origin a study has been made of the following:—

(a)

The surface features of the plain.

(b)

The nature and distribution of its covering beds.

(c)

The planed surface of the basement beds.

A general description of the plain has already been given by one of the present authors (Jobberns, 1928, pp. 543–548); and passing reference to it has been made by Hutton (1877, p. 54); Haast (1879, pp. 366–7); McKay (1881, pp. 108–118; 1883, pp. 74–79); and Speight (1912, pp. 222–4; 1918, pp. 93–105).

A. Description of the Plain.

(a) The surface features.

This crescent-shaped remnant of a formerly very much more extensive plain is some 12 miles long, and attains a maximum width of about 2 miles in the vicinity of the Motunau River, with a height of 300–350 feet at the rear. Some 1 ¼ miles off shore, opposite the river mouth, is a remnant of the same plain, namely, Motunau Island, of about 5 acres in summit area.

Most of the plain exhibits the usual regular profile of a shore platform, but there are two striking abnormalities: (1) The absence of defined cliffs at the rear, and (2) the presence of a flat-topped ridge extending along the greater part of the seaward boundary of the plain. As these features might lead the observer to doubt the essentially marine origin of the plain, they were examined more in detail, and fuller reference will be made to them later in this paper.

The plain is dissected by a system of extended and juvenile consequent streams which cross it in deep ravines. The extended streams debouch from deeper gorges in the hills behind the plain, their courses across the latter meandering to some extent and exhibiting flights of terraces which complicate the general topography. They

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may be responsible, too, for the formation of irregular, superficial fanlike deposits of alluvial material near the inner edge of the plain.

Many of the juvenile consequent streams, however, have simply cut back deep and narrow gullies which do not extend the whole width of the plain, so that they have added nothing to its surface cover.

(b) The covering beds.

As the basement rocks in which the platform has been cut have been dealt with in a general way elsewhere, the present writers are more particularly concerned with a description of the covering beds, which they have traced throughout the district, with the specific purpose of examining them from the point of view of their nature, distribution, and mode of origin. As some variation of type has been found at different points along the plain, it will be convenient to describe the deposits as they are seen in the following characteristic localities, viz.: (1) South of the Motunau River; (2) between the Motunau River and Boundary Creek; (3) the north end of the plain; and (4) Motunau Island. For convenience the localities 1, 2, and 3 will be referred to as the Southern, Middle, and Northern Areas respectively.

1.

The Southern Area.

A typical section showing the order of the covering beds has already been noted from the cliffs south-west of the Motunau River (Jobberns, 1928, p. 545). For the purpose of this paper the beds were re-examined in greater detail, and a fuller description is here given. The ascending sequence is as follows:—

(i)

Fossiliferous sands and fine greywacke gravel containing large and small irregular fragments of bored sandstone. Such blocks of bored sandstone occur at this horizon only, and were apparently bored in position. The thickness of the fossiliferous layer varies from a few inches up to about 4 feet.

(ii)

Marine gravels consisting of well worn, evenly graded greywacke pebbles from ¼ inch to a maximum of 1 ½ inches diameter. These are sometimes interstratified with sandy bands in varying proportions. The sand is very fine, and was probably derived from the underlying soft sandy rocks in which the platform has been cut. Whatever may be the origin of the upper members of the covering series, these sandy gravels, together with the fossiliferous band below, appear to represent an original marine cover on the cut platform.

(iii)

Coarser grained beds consisting almost entirely of pebbles of Amuri limestone, not so evenly graded as the underlying greywacke gravels. The change from the greywacke to the limestone facies is everywhere very abrupt.

(iv)

A layer, sometimes attaining a thickness of 6–8 feet of distinctly angular greywacke fragments with occasional angular Amuri limestone pebbles. Taking the pebbles

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    • singly, they appear to be of an alluvial type, but their arrangement and grading seems to have taken place under marine conditions. When one considers that the source of supply could have been very close at hand, and that therefore it is likely that they were not rolled to any extent, the angular nature of the pebbles does not seem to be proof of alluvial conditions of deposit.

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  • (v)

    A surface cap of fine sandy clay varying in thickness from a few inches up to 5–6 feet.

    The distribution of the above set of beds presents several points of interest. Unfortunately, the sections through the beds are exposed only near the tops of the high sea-cliffs and in the equally inaccessible gorges of the streams dissecting the plain. As far as could be ascertained, the fossiliferous stratum extends to a point on the coast about 1 ½ miles west of the Motunau River. It was traced inland also about ¼ mile up the first stream west of the river. The marine greywacke layer immediately overlying the fossiliferous band has a much wider distribution, and was found, becoming somewhat coarser in texture, to extend to the head of this stream, which, however, has not cut back as far as the rear of the plain. Southward the greywacke type occurs in small local pockets only. A section through the surface-cover visible on a slumped face of the sea-cliffs at a point 1 ¾ miles west of the Motunau River, showed the limestone type to have completely taken the place of the marine greywacke.

    The limestone layer extends over most of the area where sections are exposed south of the Motunau River, but in no case could it be found to reach to the inner edge of the plain. Therefore, though more or less sparsely scattered limestone pebbles are found in the covering beds in other parts of the plain, the beds consisting entirely of them are restricted to the southern area. This distribution suggests an entirely local origin, and the fact, that in no case was this facies found at the rear of the plain, suggests the possibility of its deposition during a definite pause in the uplift.

    The present writers are agreed that, while the Motunau River and the large stream in the extreme south of the plain may have supplied the material of the limestone layer, the arrangement of the pebbles and the continuity of the bed across the front of this part of the plain suggest its deposition under marine conditions. In this connection, too, sections in the first stream south of the river are especially interesting, for this stream cannot possibly have deposited any of the material itself, as it does not extend to the rear of the plain.

    A possible alternative source of supply of the limestone is a longshore drift of material derived by cliffing of the limestone range of Montserrat only a few miles to the south. The very restricted distribution of the deposit, however, favours a hypothesis of more local origin probably connected with the cutting of the Vulcan Gorge by the Motunau River.

    The angular greywacke facies, with only scattered limestone pebbles, forming the upper part of the sequence, is also best developed in the southern area. The supply of this, too, may be connected with rejuvenation of the Motunau River progressing head-wards. Thus the distribution of the material across the front of this part of the plain may be regarded as the work of the sea, to which it was brought by the rivers now traversing the plain.

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  • 2. The Middle Area.

    Good sections are exposed intermittently in the north bank of the Motunau River near its mouth, in the sea-cliffs, in the banks of the large stream near the Motunau homestead, and in Boundary Creek. The basal fossiliferous stratum is present at the mouth of the Motunau River, where a characteristic feature of the rock surface on which it rests is the frequent occurrence of bored holes in which shells of the marine mollusc. Anchomasa similis were found. In the sea-cliffs north of the Motunau homestead, and extending ½ mile up the middle creek, the distinctive fossils of this horizon were again found in situ.

    At the Vulcan Gorge also, on the planed surface of the Mt. Brown beds at the extreme inner edge of the platform, fragmentary fossils are found in one small pocket. These latter are, however, not well preserved, and no definite pronouncement as to their recent origin could be made. Associated with the fossiliferous band at the mouth of the Motunau River are fairly large blocks of an impure shelly limestone (apparently derived from a stratum in the underlying basement beds) interspersed with small rounded greywacke boulders.

    The marine greywacke type immediately overlying the fossiliferous band is widely distributed through this area—notably at the mouth of the Motunau River, in the cliffs facing the sea between the Motunau homestead and Boundary Creek, in the middle creek to within ½ mile of the inner edge of the plain, and at the Vulcan Gorge.

    Above the marine greywacke at the mouth of the Motunau River, and along the sea-cliffs towards Motunau homestead, well stratified sands containing richly fossiliferous bands are present. These bands are characterised by Chione stutchburyi, Amphibola crenata, and Macomona liliana. This assemblage of fossils in such a restricted area strongly suggests local estuarine conditions of deposition.

    The Amuri limestone facies appears again in the sea-cliffs where it overlies the estuarine sandy beds, but it is restricted to the neighbourhood of the Motunau River.

    In this Middle Area of the plain the angular greywacke and upper sandy horizons are not well represented, but they were observed at some accessible points in the neighbourhood of Boundary Creek.

    3. The Northern Area.

    This section of the plain is well dissected by several vigorous streams. Four of them, Boundary Creek, Manuka Creek, Black Birch Creek, and Stonyhurst Creek, have cut through the hills in the rear of the plain, but tributaries to these major streams, and some shorter consequent streams, are entirely within the plain itself. Good sections are not numerous in any of the streams, being frequently obscured by bush.

    Near the north end of the plain, at the mouth of Stonyhurst Creek the covering beds consist of:—

    (i)

    The typical marine greywacke facies, more than usually sandy in parts, and containing, at or near the base, scattered marine shells succeeded by

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  • (ii)

    Yellow sandy beds, and

    (iii)

    Greywacke gravels of a more angular nature with Amuri limestone pebbles sparingly throughout.

    This is the only locality where the covering beds were found to vary notably in thickness, and to rest on a surface which is not approximately flat. The base on which they lie is concave in the axis of an older syncline affecting the basement beds. This syncline shows the beds of the eastern limb dipping very steeply inland, while those of the western limb are much flatter. Renewal of synclinal folding appears to have warped the cut surface of the basement beds into a trough in which the covering beds are thicker than usual, the upper surface remaining, however, fairly even.

    In the sea-cliffs, south of Black Birch Creek, the marine greywacke gravel in a matrix of fine sand forms a bed 20 feet thick, with occasional fragments of bored sandstone at its base. This gravel passes upward into fine sand which is succeeded by more angular greywacke gravel with fragments of Amuri limestone. This last thins out to the south. Southwards along the coast the cliffs increase slightly in height on account of the occurrence of thick beds of yellow sandy clay forming an extra surface accumulation. The level of the cut platform appears remarkably uniform throughout. In places a thick sandy bed is present below the layer of marine greywacke gravels.

    A good section through the covering beds is exposed in the north bank of Black Birch Creek at the inner edge of the plain. This shows the usual marine greywacke pebble beds extending to the inner edge of the plain and then disappearing abruptly. As this deposit does not occur on either bank of the stream farther inland, stronger evidence is afforded that its deposition was not the work of the stream. The height here of the extreme inner edge of the plain is 330–350 feet, and the sea must have reached inland to this point as at the Vulcan Gorge.

    A good section is also available in a shingle pit on the south bank of Manuka Creek near the point where the Happy Valley road enters the plain. This section shows the presence of the usual well-graded, marine greywacke pebbles, which again are not visible farther upstream than the inner edge of the Motunau Plain, showing that the sea must here, too, have extended across the whole width of the plain.

    4. Motunau Island.

    Unfortunately, on account of weather conditions, the present writers were able to make only a very hasty examination of this area. On the planed edges of the south-easterly dipping basement beds rests a thick cover consisting of:—

    (i)

    Sandy material containing Macomona liliana and Chione stutchburyi similar to the deposits already described from the north bank of the Motunau River, and embodying, at the base of the series, occassional blocks of bored sandstone, followed by

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  • (ii)

    Beds in which pebbles of Amuri limestone predominate, and

    (iii)

    Angular greywacke pebbles in a matrix of sand. Sometimes this bed is consolidated (probably by lime) into a hard mass.

(c) The cut platform.

The base on which the covering beds rest was found throughout the area to exhibit a remarkably even profile, and a surface devoid of even minor irregularities except at two points, viz.:—(i) Just south of the Motunau River, and (ii) at the mouth of the Stonyhurst Creek.

Close examination of the cut platform was made. Firstly, to see if the nature of its profile would afford any explanation of certain surface irregularities of the plain; and, secondly, to examine the possibility of its having been cut by agencies other than the sea.

At the extreme south end of the plain there is a notable increase in the height of the surface, but the cut platform as exhibited in the sea-cliffs is almost perfectly horizontal. This shows the increase of height to be due in this case to an extra accumulation of covering material, and it appears most obvious to suggest that it is of the nature of alluvial fan detritus deposited after the uplift of the plain.

Just south of the Motunau River, however, there appears to be some connection between a slight upwarping of the cut platform and the overlying beds, with the formation of the definitely higher frontal portion of the plain exhibited there. This peculiar feature of the plain is discussed in a subsequent section of the paper.

In the major stream valleys it is difficult to obtain continuous upstream sections, but in the sea-cliffs, when allowance is made for variation in the amount of cliff recession from point to point, the cut platform appears to be almost perfectly plane. A more obvious warping of the cut platform at the mouth of Stonyhurst Creek has already been referred to. This warping, however, must have taken place either prior to or during the deposition of the covering beds.

Therefore, whatever local superficial irregularities the plain may present, these appear in most cases to have no connection with irregularities of the cut surface on which the covering beds have been laid. The present writers have been impressed by the general very remarkable uniformity displayed by the cut platform throughout.

The rocks of Motunau Island into which the marine platform has been cut comprise two unconformable formations, an older series of sandy beds similar to those beneath the Motunau Plain, and dipping at about 20° to the S.S.E., and a younger series, the lowest portion of which is a hard shelly limestone containing sandstone fragments. This offers considerable resistance to erosion, and forms the seaward face of the island and a number of reefs extending shoreward.

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(d) The inner edge.

Normally a plain of this type is backed by well-defined cliffs, and an attempt has already been made to explain the absence of them from this locality by supposing that the steeper slopes of original cliffs had been reduced to those of the dip slopes of the harder Tertiary rocks which form the hills at the rear of the plain (Jobberns, 1928, p. 546). The complete absence of well-preserved cliffs has led the present writers to consider whether, after all, agencies other than the sea may have cut the platform, as is discussed subsequently in this paper. Reference has also been previously made (Jobberns, loc. cit.) to the marked surface irregularity of the rear portion of the plain.

After re-examination of the whole length of the inner edge of the plain the conclusion has now been reached that, in general, the inequalities may be explained as arising from (i) residual spurs; (ii) alluvial fans; and (iii) dune deposits. It should be noted, however, that such conclusions are based on observation of the position and general appearance of the features, as there are practically no sections to reveal the nature of the material of which they are made. Both writers are agreed that when the plain is viewed as a whole, these inequalities of its inner edge are merely superficial and of minor importance.

(e) The elevated front.

This feature has also been referred to previously (Jobberns, 1928, p. 546), where it was suggested that an apparent longitudinal depression in the middle portion of the plain might be due to a current channel in the original cut platform, or might be a superimposed drainage feature connected with warping during uplift. In reconsidering the whole question, the present writers have given particular attention to the area just south of the Motunau River. Here an elevated flat-topped front portion of the plain is most strikingly developed, but the feature continues as far north as Black Birch Creek, and a possible remnant of it appears again in the extreme north end of the plain, beyond the mouth of Stonyhurst Creek. The following hypotheses as to its origin were considered in the field:—

(1)

Primary—i.e., due to a current channel having existed in the original cut platform.

(2)

Secondary—(i) A superimposed erosion feature.

(ii) A tectonic feature.

Examination of the section through the raised front portion in the sea-cliffs south of the mouth of the Motunau River revealed the following facts which seem to be best explained by assuming a tectonic origin.

Firstly, the surface of the platform cut in the basement beds slopes locally slightly inland.

Secondly, the covering beds also slope slightly inland parallel to the cut surface.

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Thirdly, while the front of the plain is higher than the main middle portion, its surface presents a similar seaward slope (which is in alignment with that of the top of Motunau Island). This definitely gives the impression that the front of the plain has been raised differentially.

If this differential movement was connected with faulting or monoclinal folding there should be some evidence of it in disturbance of the basement rocks of the platform. Therefore the best explanation seems to be that gentle upwarping of the front part of the plain has taken place in a direction parallel to the present coast, and this explanation accords with all the facts noted above. This warping probably occurred at some stage in the uplift of the plain, and subsequently to the establishment of the present system of parallel consequent drainage channels.

There seems to be no evidence for regarding the longitudinal depression as having been occupied at any time by a single stream flowing parallel to the coast.

B. The Mode of Origin of The Plain.

A levelled surface of the type exhibited in the Motunau Plain may conceivably be formed by any of the following agencies:—

(a)

By lateral planation by rivers flowing parallel to the existing coast.

(b)

By the coalescence of the flood plains of numerous streams flowing at right angles to the existing coast.

(c)

By normal marine planation.

One of the present writers (King, 1930, p. 520) has set out certain criteria for the discrimination between coastal plains of marine and those of alluvial origin. These criteria have been considered in relation to the Motunau Plain, and for the purpose of general comparison of types a brief examination was made of the broad raised flood plain of the neighbouring Greta Valley. Therefore the writers have discussed in the field the possibility of the Motunau Plain having had an alluvial origin of either of the above types, viz.:—

(a) By one river parallel to the present coast.

The Greta Valley presents a broad uplifted flood-plain into which the formerly meandering river has become deeply entrenched. It is evident that if all the seaward ranges were removed by marine erosion, so that the present plain became bounded by the resulting shoreline, it would present many features comparable with a normal uplifted plain of marine erosion. Its profile now is biconcave, but a normal seaward sloping profile would be produced by the cutting away of approximately half the present plain. Indeed, the present Greta Valley actually presents a more even surface than do many observed marine platforms. The superficial

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deposits, however, consist in the main of angular and sub-angular gravels, ill-sorted, and, for the most part, not well stratified. The plain also has a general down-valley slope so that if it were made to appear level, back tilting through exactly the angle of this slope would be necessary. Its western edge is determined by the dip slopes of the rocks of the locality, in which entirely structural feature it closely resembles the inner edge of the Motunau Plain.

From the general appearance and position of the latter plain, it appears incredible that whole seaward ranges have been cut away. Moreover, the plain is very wide, and even on Motunau Island the surface still slopes seawards, so that if it originated in this manner it must have been at least 6 miles wide. Furthermore, it definitely has not a general longitudinal slope in one direction, being, if anything, highest in the middle. In any case there is not now any river in the locality sufficiently large to cut such a terrace.

(b) By the coalescence of the flood plains of numerous streams at right angles to the coast.

A characteristic feature of the rivers of this part of Canterbury is their anteconsequent nature, in which they resemble the Hurunui River, which flows across the present grain of the country (Speight, 1918, p. 100). The Motunau River and five lesser streams of the locality are of this type, inasmuch as they flow in deep gorges through the hills in the rear of the plain and across the strike of the country.

In general, these hills represent merely seaward dipping outcrops of the harder members of the local Notocene sequence (Jobberns, 1928, p. 543).

It is possible that the peneplanation of the uplifted surface of such a land might be carried on by streams such as these, leaving the harder rocks standing as hills and planing the softer beds in front of them. In such a case the deposits on the planed surface would be entirely alluvial, and the inner edge of the plain would probably present a sinuous outline instead of the present remarkably smooth curve. Furthermore, the sea coast sections show the surface of the cut platform extending as an almost perfectly horizontal line from one stream channel to the next. It is very unlikely, though not impossible, that such remarkable surface uniformity would be displayed by a platform cut by streams in the manner suggested above.

Indeed, if either of the above hypotheses be accepted to account for the cutting of the platform, it becomes necessary to postulate subsequent depression, accompanied by complete removal of any alluvial cover, to enable marine beds and fossils to accumulate on it.

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(c) By normal marine agency.

It will now be convenient to correlate the more important of the facts recorded in the first part of this paper, and at the same time to consider in turn the criteria already mentioned (King, 1930, p. 520).

(i)

General appearance, extent, position, and relative proportions of the terrace.

The position of the plain adjacent to the present coast, its broadly arcuate inner edge, the ratio of its length to its breadth, and its general appearance in relation to the rest of the country suggest immediately to the observer that it has emerged from the sea.

(ii)

Character and distribution of the covering deposits.

The most strikingly developed and widely distributed of the covering beds is the band of well rounded, evenly graded, well stratified, and sometimes fossiliferous greywacke gravel. This has been recorded from almost all parts of the plain where good sections are available, and it has been shown to be present in three places distributed along the entire length of the inner edge of the plain at a height of 350 feet, where it disappears abruptly.

The pebbles, though usually somewhat flattened, were not of a discoidal form, but both writers regard the deposit as a whole to be typically marine. Moreover, this band usually lies on or near the surface of the basement beds, so that it was most probably deposited in position by the sea, which also cut the platform. With regard to the increasing angularity of the upper members of the covering series in many places, this is only what might be expected to result from elevation, causing rejuvenation of streams and increasing greatly the supply of waste to the sea. If elevation proceeded at any appreciably rapid rate, this material would be left on the platform as evidence merely of the lack of time for its shaping and sorting by the sea. Its texture and its angular nature certainly suggest an alluvial origin, but its distribution just as strongly suggests the work of the sea.

(iii)

The slope of the rear edge of the terrace.

Except for minor surface inequalities, the inner edge of the platform is remarkably level from end to end through a distance of 12 miles. This must be taken as strong evidence of marine origin, and an indication of a very regular uplift.

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(iv)

Presence or absence of cliffing at the rear.

In this respect the evidence for a marine origin is not strong. Well preserved cliffs are not present. It has been suggested previously that this is a function of the structure of the country (Jobberns, 1928, p. 545). Apart from this, however, ample evidence has been quoted to show that the sea has in the past reached to the inner edge of the terrace. Therefore either steep cliffs were never formed (possibly owing to the breadth of the platform reducing the cutting power of the sea), or such cliffs as were present have since been graded so that they now coincide with the dip slopes of the hills behind the plain.

(v)

Presence of “island” interfluves.

Such features are not present. The irregularities of surface relief at the front of the plain cannot be regarded as of this nature, and they have been herein attributed to other causes.

(vi)

Presence of marine shells.

The presence, on an uplifted platform, of marine shells which can be proved to be in position must be regarded as incontrovertible evidence that the platform has emerged from the sea.

It is possible to derive a fossil fauna from older rocks eroded by a river and deposited in the covering beds of a platform. Also it may be possible to have more recent shells deposited on a river cut platform which has been depressed and re-elevated. In this latter case, however, it is extremely improbable that the fossils would be deposited immediately on the cut surface of the platform from which a former alluvial cover would have to be stripped.

In the case of the Motunau Plain, fossiliferous beds are widely distributed, lying immediately on the surface of the cut platform, in which the marine borer Anchomasa similis is found actually in the holes which it excavated. This seems almost conclusive proof that the sea not only deposited the covering beds, but also cut the platform.

C. Conclusion.

The Motunau Plain is exceptional in that it is the only high-level terrace on the coasts so far examined in detail by either of the present writers where recent marine shells have been found in position. McKay (1886, p. 126; 1890, p. 181) recorded Recent marine shells at Amuri Bluff at a height of 500 feet, but these have not subsequently been found. It seems, therefore, that in this general absence

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of fossils other criteria must be relied on, and can with reasonable certitude be used for the determination of the mode of origin of the other shore platforms of our coasts.

The foregoing discussion raises the question of what may be regarded as reasonable proof that the sea planed a terrace which has emerged from it, or that a coastal terrace has ever been submerged.

Though the observer viewing a coastal platform as a whole may be convinced that it is of marine origin, only in rare and very fortunate cases can he state a series of points which prove conclusively that the sea actually cut the platform; and it therefore seems unreasonable to the present writers that a full and definite proof should be expected for every platform.

There are, on the coasts of New Zealand, many extensive benches which have practically no remaining surface cover to afford evidence of a former submergence. There are, too, in parts of the country distant from the sea, various types of bench-like level surfaces in the higher districts. With these latter also, definite evidence is so scanty that one can again adopt only the simplest explanation suitable to the local circumstances.

In the case of the Motunau Plain, proof of emergence of the land through a height of 350 feet is obtained from the surface cover and its fossil content; but it may be argued that the surface was planed by other agents of erosion before being submerged to receive this marine fossiliferous cover. Even the occurrence of the shells of Anchomasa similis in the holes it bored in the surface of the platform is not absolute proof that the sea cut the platform.

In view of the fact, however, that other theories require such a nice adjustment of successive drownings and uplifts, and appear so forced in their application it seems practically certain that the sea was not only responsible for the arrangement and formation of the covering beds, but also carved the platform on which they rest. In this respect the Motunau Plain offers much stronger evidence of an entirely marine origin than does any other platform examined by either of the present writers.

It is therefore hoped that this paper will provoke discussion which may lead to clearer and more definite ideas of the nature and mode of origin of our coastal terraces.

Appendix.
Lists of Fossils From The Covering Beds, Motunau Plain.

In 1927 Mr. E. W. Bennett, late of the Canterbury Museum, examined a collection of fossil mollusca from the covering beds of the Motunau Plain submitted to him by one of the authors (Jobberns), and stated that they might all be referred to Recent species. This collection has, unfortunately, been subsequently dispersed, and was not available for re-study. Towards the end of 1929 another collection was made by Dr. D. W. Johnson and the late Mr. S. J. H. Sylvester and lodged for examination with Dr. J. Marwick of the N.Z. Geological Survey. Lists from this collection have not been

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published, but Dr. Marwick has very kindly permitted the present writers to add a supplementary list of his determinations to their own.

Locality 1.—North Bank, Motunau River, one chain from the mouth.

Upper Horizon.

  • C. elotrochus tiaratus (Q. & G.).

  • Zediloma digna Finlay.

  • Mucrinops urbanior Finlay.

  • Lepsithais lacunosa (Brug.).

  • Mytilus planulatus Lamk.

  • Aulacomya maoriana (Iredale).

  • Mactra discors (Gray).

  • Spisula aequilateralis (Desh.).

  • Tawera spissa (Q. & G.).

Middle or estuarine Horizon.

  • Amphibola crenata (Martyn).

  • Macomona liliana (Iredale).

  • Chione stutchburyi (Gray).

Lowest Horizon.

  • Anchomasa similis (Gray).

Locality 2.—Top of Sea Cliffs, ten chains south of the mouth of the Motunau River.

  • Coelotrochus tiaratus (Q. & G.)

  • Micrelenchus tenebrosus (A. Adams).

  • Mucrinops urbanior Finlay.

  • Dardanula roseola (Iredale).

  • Stiracolpus symmetricus (Hutton).

  • Maoricolpus of. roseus (Q. & G.).

  • Zegalerus tenuis (Gray).

  • Sigapatella novae-zelandiae (Lesson).

  • Buccinulum aff. littorinoides.

  • Buccinulum kaikouraense Powell.

  • Xymene plebejus (Hutton).

  • Axymene cf. turbator Finlay.

  • Axymene corticatus (Hutton).

  • Lepsithäis lacunosa (Brug.).

  • Zemitrella choava (Reeve).

  • Zemitrella pseudomarginata (Suter).

  • Alcithoe sp.

  • Baryspira depressa (Sow).

  • Austrodrillia chordata (Suter).

  • Melatoma buchanani (Hutton).

  • Guraleus sinclairi (E. A. Smith).

  • Kerguelenia australis (Q. & G.).

  • Dentalium sp. indet.

  • Aulacomya maoriana (Iredale).

  • Limatula maoria Finlay.

  • Ostrea sinuata Lamk.

  • Venericardia purpurata (Desh.).

  • Mactra ovata (Gray).

  • Tawera spissa (Q. & G.).

  • Paphirus largillierti (Phil.).

  • Notirus reflexus (Gray).

  • Gari stangeri (Gray).

In addition to some of the above listed species, the following were identified by Dr. J. Marwick from the collection made by Dr. Johnson and the late Mr. Sylvester.

Locality 2.

  • Notoacmaea daedala (Suter).

  • Micrelenchus sp.

  • Conjectura glabella (Murdoch).

  • Subonoba 2 n. spp.

  • Estea cf. zosterophila (Webster).

  • Estea cf. impressa (Hutton).

  • Estea cf. subfusca (Hutton).

  • Turbonilla sp.

  • Odostomia densilirata Suter.

  • Odostomia n. sp. aff. huttoni.

  • Terebratella inconspicua Sow.

  • Nucula hartvigiana (Pfr.).

  • Lissarca cf. exilis Suter.

  • Hochstetteria cf. meleagrina Bern.

  • Cosa filholi (Bern).

  • Chlamys cf. celator Finlay.

  • Venericardia (Pleuromeris) n. sp.

  • Verticipronus cf. mytilus Hedley.

  • Condylocardia crassicosta Bern.

  • Notolepton sanguineum (Hutton).

  • Lasaea minutissima Iredale.

Comparison of the faunas indicates that Locality 2 is the same as the Upper Horizon at Locality 1. Thus ten chains west of the mouth of the Motunau River the estuarine or Macomona facies below the marine cover is not present. The ancient estuary extended eastwards, however, at least as far as Motunau Island (q.v.) and for

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some distance northward, where the estuarine beds with their characteristic fossils outcrop continuously along the top of the present marine cliffs.

Comparison was made with the present distribution of all the species recorded, but in view of the incompleteness of our knowledge of this department no safe conclusions could be drawn. It seems tolerably certain, however, that the fauna is very close to the existing forms on the littoral at Motunau to-day.

List of Papers Referred To.

Haast, J. Von, 1879. Geology of Canterbury and Westland.

Hutton, F. W., 1877. Report on the Geology of the North-east Portion of the South Island of New Zealand, from Cook Strait to the Rakaia. Rep. Geol. Expl. during 1873–74; No. 8, pp. 27–58.

Jobberns, G., 1928. The Raised Beaches of the North-east Coast of the South Island of New Zealand. Trans. N.Z. Inst., vol. 59, pp. 508–570.

King L. C., 1930. Raised Beaches and other Features of the South-east Coast of the North Island of New Zealand. Trans. N.Z. Inst., vol. 61, pp. 498–523.

McKay, A., 1881. On the Motunau District, Ashley County. Rep. Geol. Expl. during 1879–80; pp. 108–118.

McKay, A., 1883. On a Deposit of Moa Bones near Motunau, North Canterbury. Rep. Geol. Expl. during 1882; pp. 74–79.

Speight, R., 1912. A Preliminary Account of the Lower Waipara Gorge. Trans. N.Z. Inst., vol. 44, pp. 221–223.

Speight, R., 1918. Structural and Glacial Features of the Hurunui Valley. Trans. N.Z. Inst., vol. 50, pp. 93–105.