Art. LXI.—On the Alluvial Deposits of the Lower Waikato, and the Formation of Islands by the River.
[Read before the Auckland Institute, 29th May, 1871.]
The alluvial deposits of the Lower Waikato, between Taupiri and Mercer, consist of reddish-yellow loam, resting on pumice, gravel, and sand.
The pumice deposit is generally pretty regularly stratified, and can often be divided into three subordinate divisions. The lowest of these is composed of coarse white quartz sand without much pumice, and contains layers of magnetic iron sand. The middle one is pumice gravel, the stones often being of the size of peas or beans; while the upper deposit is similar to the lowest but with more pumice, and also with pieces of obsidian. The sand is for the most part formed of slightly rolled transparent crystals of quartz. All parts of this formation are sometimes bound together into a hard rock by a ferruginous cement. It is not found much below Rangiriri. It is quite evident that nearly the whole of the materials that form this deposit have been brought down from the volcanie region in the centre of the island, and it must therefore have been formed when the land south of Taupiri was above the level of the sea. The absence of fossils is easily accounted for by supposing it to be an old river bed, for no shells inhabit the centre of the river, and none are found on the present sand-banks; they only live at the margins of the river where the bottom is more muddy, and where alone vegetation grows. Above the pumice formation lies a fine reddish-yellow loam, with occasionally a few rounded stones of pumice, so much decomposed that they are easily cut through with a spade. I have never found any shells in it, but in places it passes into a whitish sand containing the following species of diatoms, viz.:—Epithemia, sp.; Himantidium pectinale, Kutz; Amphicampa, sp.; Surirella craticula, Ehr.; Cyclotella, sp.; Orthosira punctata, Sm.; Stephanodiscus, sp.; Cocconema cymbiforme, Ehr.? Navicula affinis, Ehr.; Navicula, sp.; Pinnularia major, Sm.; P. viridis, Sm.; Pinnularia, 2 sp.; Pinnularia interrupta, Sm.; Stauroneis phœnicenteron, Ehr.; and two others, the genera of which I have not been able to determine. These diatoms decidedly prove the fresh water origin of the loam.
This loam generally rests on a tolerably even surface of the pumice sand, but in places a good deal of contemporaneous erosion and filling up, and re-arrangement of the pumice sand, appears to have been going on during its deposition.
Above the loam a vegetable soil more or less darkly coloured by humus, and from 0 to 4 feet in thickness is generally found. This soil sometimes rests abruptly on the loam but more often passes gradually down into it. It contains in various places, and at various depths down to 18 inches from the surface, collections of marine pipi-shells (Mesodesma chemnitzii) and burnt wood, evidently the remains of Maori feasts, for they still occasionally bring up these shells from the mouth of the river. No trace of holes having been dug for Maori ovens can be seen, although if any digging or turning over of the soil had taken place it could not fail to be detected in the clean sections of the river bank. “We must therefore conclude that these remains show the level of the river bank at the time when the shells were cooked. The ordinary floods of the Waikato seldom overflow the banks of the river, although they fill up large swamps beyond the banks; and even when they do flow over they seldom remain at that height more than three or four days, and they deposit very little mud during that time; but in August and September of last year a heavy flood occurred, which was the highest that had been known for 30 years, and it covered the banks for about three weeks. During this time a layer of fine yellow loam about three quarters of an inch thick was deposited, but the traffic on the bank destroyed a good deal of it again. Of course when the banks were lower the floods would be oftener, and perhaps remain longer, but I think that an inch of deposit in 20 years would be a very liberal allowance, and at this rate some of the Maori feasts would date back 360 years. The next series of beds, in descending order, is the clays and sands which form the greater part of the hills between Rangiriri and Mere-mere, and on the opposite side of the river; this also contains no fossil shells, but only plants and beds of lignite with wood, and it is probably of fresh water origin also. There appears therefore to be no geological evidence of the sea having been in the Lower Waikato valley since the upheaval of the Waitemata series; that is since it has had any existence. I therefore think that the fact of the presence of several littoral plants in the Lower Waikato basin, brought forward last year by Mr. Kirk,* may be best explained by supposing that they have spread down the river from the Middle Waikato basin after the formation of the Taupiri gorge.
It is, I believe, the commonly received opinion that islands in a river have generally been caused by the stream gradually cutting through its bank at a sharp bend, and leaving what was before a point as an island. Without committing myself to the opinion that this is never the case, I may state that it is seldom, if ever, the way in which islands are formed. If it were so, we ought to see rivers running round a deep re-entering bend without a salient point projecting into it, and islands ought often to be shorter in the direction
[Footnote] * See Trans. N. Z. Inst., Vol. III., p. 147.
of the river than across it, and these cases are certainly rare, if indeed they exist at all. The river also should always swell out round an island, whereas islands are often seen in parts of a river where it is no broader than it is either above or below it; and it would also be difficult in this way to account for two islands lying parallel and abreast of one another—a case of common occurrence, especially near the mouths of rivers. Besides, the greatest strength of the current is never in the bends. A sharp bend blocks up the stream, and reduces its velocity; and it is at the point where the water is able to get away that the strongest current is found. Any one who has been used to paddling up a river in a canoe will, I am sure, bear me out in this. The wear on the bank therefore is greatest at the point and gets less and less towards the bend, and the river tends to wash away the point and not to detach it as an island. The great flood of last August, which has been already mentioned, has left behind it two sand-banks which bid fair to become permanent islands, and which well exemplify the usual way in which these islands are formed. The sand-banks of the Waikato are constantly shifting their places, and moving down the stream, on account of the sand being pushed along the bottom of the river. This only happens when they are covered by water, for when the river is low, and the banks are above it, the strength of the current is not sufficient to wash them away, and they retain their position until the next flood. The high flood of last spring raised two of these banks—one opposite to Rangiriri and the other about four miles above it, opposite to Armitage's farm—so high that no subsequent flood covered them until the beginning of April, 1871, so that they were about seven months in the spring and summer above water. During this time they became so covered with vegetation that the flood of last April, which covered them, had no further effect than to raise them still higher by depositing more sand and mud round the plants, and it is evident that if this is continued they will soon only be covered by the highest floods, and will thus resemble the ordinary islands of the river.
I visited the island opposite to Rangiriri on the 17th of last April, and collected on it the following plants:—Nasturtium palustre, abundant; Pelargonium clandestinum, common; Geranium microphyllum; Trifolium pratense; T. minus; Coprosma robusta, one seedling; Cotula coronopifolia, abundant; Erigeron canadense, abundant; Gnaphalium luteo-album; G. involucratum; Soncus oleraceus; Plantago major, abundant; Rumex obtusifolius, abundant; Juncus communis; Cyperus ustulatus; Isolepis setosus; Agrostis œmula, common; Arundo conspicua, one old plant only which had been floated down the river, but was now standing upright and growing well; Holcus mollis; Agrostis Billardieri, and Poa australis v. lœvis.
This island was then about 250 yards long by 50 broad, the highest point being the end pointing down the river, as is always the case with sand-banks.
On the east side the water deepened quickly, but on the west side it was shallow for a considerable distance. It is situated just opposite to the main entrance of the Waikari Lake.
The second new island is situated a little above Armitage's farm; it is smaller now than the first, but has been much reduced in size by a large coal barge having been stranded on the upper part, and the disturbance caused by getting her off has washed a large part of it away.
It is evident that if these banks are prevented from moving by the roots of the plants growing on them, every flood must increase their height until ultimately they are rarely under water; so that all that is wanted to form an island is that some unusually high flood should so raise a sand-bank that it is not covered for several months during the season that seeds can germinate, and so allow the plants to grow sufficiently large for their roots to hold the sand together the next time that it is submerged. Floods in spring might therefore often cause islands, while floods in autumn might fail to do so.
These remarks on the formation of islands by rivers must not be taken as applying to rocky masses in a river bed, nor the upper portions of any river, but only to the lower portions, where a large river flows tranquilly through an alluvial plain.