h. Changes in Level of the Land.
The following is a table showing the probable changes in level in the locality:
Higher land when valleys on the peninsula were eroded.
Depression, accompanied by invasion of valley floors by the sea.
Practical still-stand, no major movement when spit was constructed.
Depression till the spit was awash and barrier beach constructed on the lake margin, and the boring of the stumps now exposed on the seaward margin. At this time shore platforms were cut on the ends of the spurs along the Tai Tapu-Little River Road.
Elevation of from 12 to 15 feet, the present level.
It will now be advisable to compare this sequence of events with any chronology of movements of the land indicated in the adjacent districts. To the south there is definite evidence of a downward movement of the land in the neighbourhood of Timaru, the drowned river stream valleys incised in the edge of the Timaru Downs, as well as the presence of submerged forests at Pareora and to the north of Timaru indicating such a movement clearly. Further north the dominant feature of the sea coast, that is, the sea cliff north and south of the mouth of the Ashburton River, and specially between it and the Rakaia, postulates long continued erosion, with little change in level of the land. There may be a slight downward movement, but an upward movement is not ruled out, since the attack on a somewhat easily eroded shore line might rapidly obliterate any shore platform or marine terrace that might have been formed. Between Taumutu and the Rakaia mouth there are lagoons ponded back behind a barrier beach, but these can be explained without any demand for a sinking sea-coast, although some slight movement in that direction may have occurred. These lagoons and other pieces of swampy land may quite well represent shallow salt-water areas that have formed behind a barrier beach, and have been partly filled with detrital matter so that they are slightly above sea-level and are now fresh, if containing water, or have been turned into swampy land.
On the northern side of Banks Peninsula there is evidence of recent uplift of from 12 to 15 feet as suggested by Haast, Speight and Jobberns. This will naturally correlate with the uplift inferred in
connection with the spit. There is also evidence for a higher level of the land from the records of the artesian wells of Christchurch in that peat beds occur at various horizons down to 600 feet beneath the present sea-level. The peat must have been formed on a land surface, so its present level demands depression of the land since it was formed. Inter-stratified with the peat and gravel beds are others containing the molluse Chione stutchburyi, a marine and estuarine form now living chiefly between tides. These shells are also found in the bores put down in places like Lower Riccarton, Ladbrooks and Greenpark, and no doubt they occur also at Halswell and Tai Tapu between these three places, but the wells in the latter districts have been driven with a plug in the pipes, and so no record of the beds passed through is available. The evidence from the first three places in conjunction with that from the Christchurch area proves the former presence of a strait passing round the western margin of Banks Peninsula, as suggested by Haast (1879, p. 400) on the evidence afforded by surface phenomena. This strait must have been a comparatively narrow one as there is no record of shells having been found further out on the plains than the places mentioned and the bores have been mostly through shingle. Thus there is no record of shells in the deep bore at Islington (732 feet) which is about 8 miles north-west of the base of the peninsula hills. However, the interstratification of gravels with marine deposits right up to their foot indicates that at times antecedent to the present the plains tied, or almost tied, the volcanic mass of the peninsula to the mainland. The gravels must have been laid down on a land surface or in immediate proximity to a beach, so that the gravels inter-stratified with marine beds indicate a former higher level of the land, while the sea-shells indicate an invasion of the sea as the land sank.
The shells which are found at a depth of 300 feet in the Christchurch area indicate either a deep sea, or, as is more probable since the molluse is a shallow water form—a progressive lowering of the land, so the alternation of land and sea over the area, evidenced by the inter-stratification of land and marine beds, was repeated several times. Some of the shells brought up from the artesian wells are very stout in substance, and this is a feature shown by shells living on open beaches. So it is probable that the water where they lived was not very well sheltered, and therefore there was no effective protection against the seas sweeping up from the south round the western side of the peninsula. The closing of the southern entrance to this strait by the Ellesmere Spit no doubt took place during a period of relative still-stand of the coast, when the depth of water to the south of the peninsula was such that the transport of material could take place across the strait, and the drift did not follow the line of its western margin. This depth might have been due to change in level of the land or to the progradation of the shore line. It is unlikely that the depth of water at this time exceeded 30 feet, that is, the approximate thickness of the gravel forming the spit.
The fact that the land was much higher than at present within fairly recent times is evidenced by the coastal features of Banks Peninsula. The drowned valleys which mark the whole seaward fringe, and the alluvium-filled valleys facing the plains all demand a
lowering of the land to account for them. No precise determination of the amount of lowering that has taken place can be made from considerations affecting them, except that the minimum amount must be measured by the depths of the seaward end of the valleys. Now the depth of water at the entrance to Akaroa Harbour is 18 fathoms, so there must have been a minimum subsidence of the land amounting to just over 100 feet, but this estimate does not allow for the fact that the harbour may be partly filled with sediment washed from the neighbouring hills, or that the land may have extended some distance further seaward. The only occurrence that I know of where the depth of alluvium has been determined is in the case of the valley behind the town of Sumner where, on boring for water, solid rock was struck at a depth of about 200 feet, so that there is thus indicated a further amount of subsidence. This submergence must have taken place before the spit was constructed. I cannot see any evidence for the additional submergence of 800 feet postulated by Jobberns (1928, pp. 556-7). The presence of a level shelf over half a mile broad on the northerly slope of Mount Herbert, if due to wave action must have taken a long time to form, and the absence of similar shelves in the neighbourhood where they should occur, especially on the southern slopes of the Peninsula, makes me view the conclusion with suspicion. It seems to me far more probable that this flat surface is determined by the hard layer of basaltic rock which forms its floor. Then again if the waves have cut this bench, why is there an entire absence of any trace of benching on the spur on the opposite side of the Charteris Bay Valley. The presence of the ridge on the western side of Gebbies Pass of height somewhat accordant with the shelf on the western side of Charteris Bay appears to me a mere accident. The top of this ridge is formed by the highest exposure of the underlying substratum of greywacke and related rocks, while the steeper ground to the west is formed of the overlying volcanics. There are no benches or beaches at accordant levels on the southern facing of the peninsula such as might have been expected had the land stood still long enough for a shelf to be cut back a distance of over half a mile in the comparatively sheltered waters of the northern side of Mount Herbert. In any case a shore platform should also be accompanied by a train of phenomena and in the absence of any of these, or of all but one, a conclusion as to its existence must be viewed with suspicion. Further, if the land had been depressed till this bench was submerged all the loess which caps the spurs on the southern side of the peninsula should have been removed unless the time of the depression is moved back to pre-glacial, that is, to pre-Pleistocene times, and also there should be signs of long still-stand of the land at the 800 foot level on the Canterbury Plains and on the foot-hills adjacent thereto—but these are entirely absent. Even if the shore platform were cut in pre-glacial times some traces of it should be in existence, especially as the last phase of the glaciation was posterior to the formation of the plains, for moraine lies on top of the gravels at the Rakaia Gorge and in the Ashburton Valley.
I must also express my present disagreement with a suggestion of my own advanced many years ago (1908, p. 32), that the ends of the spurs reaching down on the southern side of Banks Peninsula
owed their flat extremities to marine erosion when the land stood at a lower level. I am sorry that I must apply the same test as given above and conclude that there has been no definite submergence and subsequent uplift of the coast to the extent demanded by the height of the ends of the spurs. I submitted my original contention to Professor W. M. Davis when he was on a visit here, and he said that he saw no evidence of this submergence, and the subsequent uplift, nor does he mention the point in his recent work on Coral Islands, although he takes Banks Peninsula as one of his types of depressed and eroded islands (1928, pp. 151-4). It seems more satisfactory to attribute the flat terminations of the spurs to the existence of a flat or flatter capping of hard rock, such as does occur on the outskirts of the volcanic mass, especially where the lavas are fluid as basalts usually are. There are no beaches on these flat spurs to support the contention that the land form is due to marine erosion. The covering of loess would naturally be deposited when the land was at a higher level and there has been no depression to wash this off since it was blown from the front of the glaciers and from the streams issuing from them in Pleistocene times.