An Eroded Coastline

By R. Speight

this idea, assuming that the contour maps are correct. Even with the augmentation mentioned previously, the Ashburton was a much smaller river than the Rakaia, and it now lies in a very shallow trough between the Rakaia and Rangitata. The threshhold where it issues from the mountain area is not as high as that of the Rakaia, so that its grade conferred no advantage on the river as a waste-carrier. There may have been some coalescence or overlapping in the early and middle stages of construction, but I cannot see evidence of such action at a later date. The banks of the Hinds lie at a generally higher level than those of the Ashburton in its upper and middle reaches. Only in its lower reaches does it lie in the hollow between the fans, and at its mouth it lies on the southern slope of the Ashburton fan. The Rangitata runs along the crest of its fan except near the mouth, and even this discrepancy may be due to the marked erosion of the southern bank as suggested by Hilgendorf. If any coalescence of the fans of the three main streams took place when they had reached maturity, the junction lies off the present coastline and has been removed by the sea.

Special attention has been paid to the mode of construction and the form of the plains since the coastal cliff, to be referred to frequently later on, owes many of its features directly to the structure and form of the plains, especially its changing height, and, of course, its lithological composition. This cliff gives a cross-section right along their front, and its varying height is directly dependent on the form of the plains in its vicinity.

3. The Coastal Features

These are evidently derived from a swamp deposit in position on the submerged seaward extension of the plain beneath the beach and within the range of erosion by the waves. Other similar outfalls for the drainage occur further north in the direction of Taumutu.

These facts clearly indicate a lowering of this stretch of coastline, which may be part of the extended movement of depression referred to earlier, but it must be remembered that, extending west from Taumutu some six miles to the north, there is a clearly defined old barrier beach, whose crest is now about twenty-five feet above present sea-level (Speight, 1930, p. 163). This clearly indicates that the land was formerly lower than at present, and that it is passing through a period of instability.

A depression similar to that just mentioned occurs at the southern end of the plains as indicated by the drowned valleys and submerged forests near Timaru, and this instability at both ends of the beach under consideration suggests a possible explanation of the present higher level of the central portion, viz., that the northern and southern ends have been lowered while the central portion has remained stable or actually have been warped upwards, the axis of the anticline so formed corresponding with the ridge of higher land passing southeast through Dromore-Chertsey as mentioned earlier. The faults and fault-traces, some oriented on N.W.–S.E. lines involving recent gravels which occur in the mountain area in the vicinity, indicate some crustal instability. However, the only feature of the area adjacent to the coastline which might support such an hypothesis is the existence of high river terraces at the mouths of the Wakanui and Ashburton, and their decline in height upstream. This can be explained most satisfactorily by the rapid marine erosion of the incompetent beds and the consequent adjustment of the grade of the rivers near the coast. The absence of coastline evidence discounts the hypothesis.

Mention should be made in this connection of the marked recent erosion of the New Brighton sand-spit on the north side of Banks Peninsula. This erosion seems to be related in some way to a possible depression of the land, although tidal records at the adjacent port of Lyttelton do not support the contention. However, the flooded lower course of streams like the Avon and Heathcote discharging into Sumner Estuary do give some evidence for such a movement—evidence which is masked on reaches of the sea-shore near the Waimakariri mouth by material poured in by that river and deposited on the sea-bed by a southward-moving current, which becomes progressively less and less effective as the distance from the river-mouth increases. Thus the end of the spit receives no such addition and erosion is not checked or masked.

II. Rakaia to Ashburton River

South of the Rakaia the coastline is marked by a definite cliff, usually vertical or nearly so, formed of loosely consolidated gravel and sand, and therefore incompetent to resist attack from the sea. At a point two and a-half miles south of the river mouth it is twenty feet in height, and when followed south it gradually increases with increase in the height of the surface of the plain, till at the mouth of the Ashburton it is nearly seventy feet in height (Plate, Figs. 3 and 4). The country south of the Rakaia is relatively dry; streams

near the coast is determined by the susceptibility of incompetent beds to both marine and stream erosion, and the river is able to reduce its bed in the lower reaches and enter the sea at grade in spite of the rapid erosion by the sea. But this effect diminishes upstream till it disappears about six miles from the shore. From this point upstream the grade is the same as before and corresponds roughly to that of the plains. In the case of the Rakaia and Rangitata, the terraces are lowest at the mouths of the rivers and increase in height upstream.

III. Ashburton River to the Rangitata River

The moist marginal fringe of the plain continues south of the Ashburton River, and is most fully developed in the vicinity of Longbeach and Lowcliff. In parts it is definitely swampy, and springs and small permanent streams are frequent, the largest being the Hinds River. Haast referred to this (loc. cit., p. 399) as lying between the Ashburton and Rakaia—the last-named river should be the Rangitata—and that it was drained artificially through shingle beds in front. However, the Hinds drains to a part of the coast where the cliffs are lower than at any other spot with the exception of those in the immediate vicinity of the Rakaia and those near Loweliff.

The reason for the swampy nature of the land is not quite clear to me. It may arise from the fact that the area lies between the seaward extension of the fans of the Ashburton and Rangitata, but this does not explain its existence entirely. A sub-surface layer of impervious material probably underlies it, and intercepts in its passage to the sea the leakage from three rivers in close proximity, viz., the Hinds, Ashburton and Rangitata, as well as ground water from the part of the plains lying to the north-west. In wet seasons, that is, when the annual rainfall lies between thirty-two and thirty-five inches, the springs become active in the area behind the coast, and the amount of surface water presents a serious problem. It remains to be seen to what extent the Rangitata Irrigation Scheme and the consequent increase of surface water which it brings to the plains will affect the drainage of this area.

The sea-front of this stretch is also determined by a cliff. At the northern end it is just over sixty feet in height (Plate 2, Fig. 5), while at Lougbeach, six miles to the south-west, it is forty-five feet, at the north side of the mouth of the Hinds about thirty feet. When followed further it falls in height so that for long distances near Lowcliff it does not exceed fifteen feet, and in places does not reach ten feet, and is hardly worthy of the name of “cliff.” Further south again it rises, and at the mouth of the Rangitata it is thirty-five feet. So that the statement on the contoured map recently issued by the Survey Department that the coastal cliffs on this stretch rise to from forty to sixty feet needs some revision.

In this stretch the coastal cliff is cut by numerous gullies, some of which are occupied by permanent streams, as, for example, that near Longbeach homestead; another such discharges into the lagoon at the mouth of the Hinds River. Many are short and usually dry. Those occupied by permanent streams enter the sea at grade, but the dry ones frequently end in a cliff as is the case further north.

4. Estimate Of Rate Of Erosion

It is very difficult to estimate the rate of recession of the coastline since the beginning of settlement of the area. Some farmers on exposed stretches say that they have lost along their sea-frontage about two feet per year, and the ends of fences terminating in the cliff testify by their overhang of the edge that this estimate may be reasonable. On the other hand, some farmers say that they have experienced little or no loss. The end of the road at the Raikaia Mouth Settlement has been moved about a chain since the place was first inhabited, and an old well lies in the lagoon fronting the present shore. The road along the cliff or bank to the north has been cut back by the waves, and the same has happened on the stretch of coast near Lowcliff, where the road has had to be moved back about a chain.

Considering the large amount of gravel brought down by the rivers it might be expected that the coast near their mouths would show some amount of prograding, but this does not occur or the forward, progression is very slight. In every case a barrier beach of shingle is disposed across the mouth of the river, generally abutting against the base of the cliffs on the south side (Fig. 5), but overlapping them to the north (Figs. 2 and 4), and behind this barrier beach of shingle lies a lagoon into which the river discharges, but with the barrier breached at some point to allow the surplus water to escape to sea. Those near the Rakaia and Hinds have already been referred to, the northward extension of the former being at present very noteworthy. The overlap of the barrier on the north side of the major rivers is no doubt due to the movement north of coastal material during heavy southerly seas. It might be thought that this barrier would protect the base of the cliffs, and so it does normally, but when the river mouth has moved so that it lies opposite the cliffs, there is no protection by the barrier and the sea enters the lagoon to break against their base and to erode them rapidly. This goes on till the position of the river-mouth changes and a new breach in the barrier beach is made so the part of the cliff formerly eroded has again the protection afforded by the barrier. While the river makes its excursion in the northerly direction a considerable amount of erosion of the northern bank occurs owing to attacks by the river itself as distinct from the action of the sea, as a result of which the northern bank of the river is set further back than the southern bank, seeing that the lagoons are generally developed to the north. But during very heavy storms the waves do break right across the barrier and attack the shore behind the lagoon.

The certainty that the cliffs are being rapidly eroded at present suggests the question of the width of the strip which has been removed from the margin of the plain in the past, but the answer cannot be given with any definiteness. First of all, if the slope of the surface of the plain had continued seaward at the same angle at that of the last half mile between Chertsey and Wakanui, viz., twenty-six feet to the mile, it would have intersected the surface of the sea about two and

a-half miles from the present shore. If the inclination of the plains had been flatter the line of intersection would have been further out to sea, and of course, had the angle been steeper—a most unlikely condition—it would have been nearer the shore. These figures assume that the land has stationary. Whatever has happened to the area between Wakanui and Chertsey, it is certain that depression of the land has taken place at the two ends of the coast as has been mentioned earlier. The even form of the sea-bed—to be referred to later—extending as it does for some thirty miles off the present shore, does suggest a lowering of the land. The late Professor W. M. Davis puts (personal communication) a maximum width of two miles that can be eroded from a coastline without change in level of the land, so that it is possible that the depression which affected both ends of the area also affected the middle portion.

5. Features Of The Sea-bed

The question now arises as to the destination of the products of erosion of the coast and those poured in by the rivers. Wherever a cliff occurs it is buttressed by gravel transported by the waves. Except for the north side of the mouths of the rivers this lies right against the base of the cliff. All the evidence points to a general movement to the north and this accounts for the great accumulations between Lakes Ellesmere and Forsyth and on the southern shore of Banks Peninsula. Although these accumulations at the northward end of the beach are very great, they must represent only a fraction taken from the shore and deposited offshore. The material at the top of the beach is usually coarse and that at lower level increasingly finer, but it is only rarely that sand appears and then almost entirely at low water. The slope of the beach is usually about five degrees, but steeper at its upper levels.

The only source of information concerning offshort conditions is that furnished by the latest Admiralty chart (1944) of the sea in the vicinity. This shows a gently sloping sea-bed extending for a distance of over thirty miles with no irregularities whatsoever. The record shows that it consists chiefly of sand with sandy mud and shelly sand in minor proportions. The eleven fathom line runs almost parallel with the coast and four miles distant from it, so that the bed has an initial slope of sixteen feet to the mile, compared with twenty-six feet to the mile of the plain between Chertsey and the sea. The thirty fathom line is sixteen miles from the shore and the slope very similar, viz., sixteen feet to the mile. The thirty fathom line is thirty miles from the shore and the slope of the bed in this strip nine and a half feet to the mile, so that there is a slight flattening. However, the next four miles show a steeper slope, and the depth increases by another eleven fathoms, so that this line marks the edge of the continental shelf, if it exists. I have given in another article (1943, p. 17) evidence that depths of about forty fathoms are to be found one hundred miles from Banks Peninsula on a direct line to the Chathams. It should be noted that according to the chart the depth of the sea is very irregular off that peninsula and similar depths are to be found only five miles from the coast. The depth of the sea off the plains and the nature of the bed are accurately portrayed in a diagram in Professor Cotton's Geomorphology (1943, p. 413) and his explanation of deposits offshore apply entirely in this case.