Art. XXVI.—On the Nelson Boulder Bank.
[Read before the Nelson Philosophical Society, 13th November, 1899.]
About six years ago I had the honour of reading before this Society a paper on the geology of this district. In the discussion which followed the reading of that paper I was asked for an expression of opinion upon the formation of the Boulder Bank. In reply to that question I stated that in all probability the Boulder Bank had been formed by the upheaval of a boulder stratum. Mr. Leslie Reynolds has evidently heard of this theory, for in his report on the proposed harbour improvements he says he can see nothing to support the theory that a reef underlies the bank.
One's own experience of the difficulties of understanding
the geology of a strange district enables one to excuse Mr. Reynolds for not being able to see the facts which support that theory; but when he hastily arrives at conclusions about this wonderful formation, and then bases estimates thereon involving the expenditure of thousands of pounds, I cannot help thinking that a little more caution would have been advisable. An eminent geologist from Australia told me he would require at least three months' residence here before he could express any opinion upon the geology of this district.
The origin of the Boulder Bank is a geological question, and most geologists when dealing with the subject have spoken more or less cautiously, realising the difficulties of the problem. The drift theory from Mackay's Bluff might be the true solution, but there is a good deal of evidence in favour of the other theory, that the Boulder Bank is the upturned edge of a stratum of rock or boulder drift.
I shall now proceed to give the facts upon which this theory is based, and then mention the difficulties which make it almost impossible to accept the opposite theory. In the hill above the Rocks Road may be seen a series of stratified rocks, inclined at very high angles. These rocks incline outwards from the face of the cliff, or, in geological language, they dip easterly. On the beach below the Rocks Road are also rocks standing almost on edge. In some places they have been planed down almost to a dead level by the action of the sea, and are covered by every tide; but in other parts, where the rocks are harder or less exposed, they are standing up in wall-like ridges above the general level of the beach. These rocks underlie, and are therefore older than those seen in the cliff. This series of rocks may be traced seawards till the Arrow Rock is reached, upon which they evidently lie at a very high angle. Now, it is strikingly apparent that the rocks in the cliff, and also those on the beach, must at one time have extended seawards in the form of an arch, forming a rounded hill. The Arrow Rock must therefore be regarded as the core of this hill, and this hill was not a mere cone, but had extension northwards and southwards. The strike of the inclined rocks on the beach and in the cliff is north-north-east and south-south-west, and it is also a remarkable fact that the Boulder Bank lies in a similar line. Keeping in mind the fact that a hill once covered the Arrow Rock, that this hill had a northward extension, as is shown by the parallelism of the stratified rocks in that locality, it follows naturally enough that a ridge of hills once occupied the present site of the Boulder Bank. The core of this ridge is represented by the Arrow Rock, which so far has resisted the denuding action of the sea. This, of course, is deduction, but it is deduction based upon solid facts which cannot be gain-
said. This ridge of hill, being composed of soft sandstones and clays similar to the rocks in the cliff and on the beach, has, with the exception of what remains at the cliffs, been washed away. The hard core of this ridge, composed of rock similar to the Arrow Rock remains, however, as an upstanding reef, forming the basis of the Boulder Bank.
Having shown that the existence of a reef under the Boulder Bank is highly probable, the next point for consideration is the nature of the rock of which this reef is composed. That the reef is similar to the Arrow Rock has already been remarked, but the Arrow Rock is only a fragment of an extensive belt of rock, and probably does not adequately represent the whole. An inspection of the Arrow Rock shows that it is made up partly of solid syenite rock and partly of syenite boulders firmly cemented together into a conglomerate. There is also in the syenite an intrusive sheet of lava. The boulders referred to form a part of the Arrow Rock, but lying around its base there are numbers of loose boulders which at one time doubtless formed part of the solid mass. These boulders are syenite, and quite similar to some of those found on the bank.
When the Torpedo Corps were improving the entrance to the harbour they blasted away rock made up of syenite boulders, and owing to its stubborn resistance found they could make but very little impression upon it with their charges of gun-cotton. Such, then, is the kind of reef that probably under lies the Boulder Bank—a boulder stratum, underlain by solid syenite, turned up on its edge by the upheaving force that raised the Port Hills.
There is no need to go into the origin of this boulder stratum beyond stating that it is probably the result of glacial action. In the cliffs above the Rocks Road several boulders of syenite have been unearthed. These stones prove conclusively that boulders were being carried in that direction while the rocks in which they are imbedded were being laid down as horizontal strata.
Given such a reef as has been described, then the origin of the Boulder Bank and the formation of Nelson Haven become simple matters, unbeset by any difficulties, and easily understood.
This theory of the Boulder Bank does not preclude the possibility of drift having come from Mackay's Bluff. In all probability—one might almost say certainly—boulders and shingle have drifted from there along the bank. This theory of the underlying reef, however, gives to the Boulder Bank its alignment, and removes some of the difficulties which make it hard to accept the purely drift theory. Some of these difficulties will now be mentioned.
It is a geological certainty that on the bottom of the greater part of the inner harbour—perhaps the whole of it—there are rocks standing practically on end. How far this arrangement of rocks passes beyond the Boulder Bank it is difficult to say, but that they do pass beyond must also be considered a certainty. Now, if the purely drift theory is to be accepted, we must suppose that these rocks were planed down by the action of the sea for at least 20 ft. below low water before any boulders were deposited. This, of course, is not an impossible thing to happen. Geological sections often reveal such an arrangement of rocks on their edges covered by horizontal strata. But in this case there is a difficulty. If these rocks were planed down, say, 20 ft. below the level of the sea, why were the Arrow Rock and the reefs near it not also planed down to the same depth? How would this denuding action reach from Mackay's Bluff to the Arrow Rock and then stop short? The thing is well-nigh impossible; when the sea planes it usually planes pretty smoothly. Then, supposing the planing-down process to the depth indicated did actually take place, what became of the boulders that were being formed at Mackay's Bluff at the same time? Did they stay there till the adjacent rocks had been planed down 20 ft. below sea-level and then start drifting south-west? Not a likely thing to happen. Another difficulty against the acceptance of this theory is the largeness of some of the stones at the extreme south of the bank. The advocates of the drift theory realise the difficulty, and suggest that heavier seas must at one time have prevailed in the bay. Now, how could those heavier seas have been produced—seas mighty enough to roll huge boulders ten miles along a level sea-bottom at a depth of 20 ft. from the surface? Only by so altering the coast-line that Tasman Bay would be more exposed to the ocean than it is at present. To effect this change of coast-line in the direction indicated we should have to give up some of our fundamental ideas of New Zealand geology. It would involve first the submergence and then the reappearance, in comparatively recent geological time, of some of the land by which the bay is at present sheltered. Then, too, the absence of gradation in the size of the stones on the bank, when viewed lengthwise, is another difficulty against the acceptance of the drift theory. It is a fact that some of the stones at the south end of the bank are as large as any to be found at the north end of the bank. If all these stones had been drifted from one place, one would expect to find large stones near the source, and a gradual diminution in size down to gravel or even sand as the distance from the source was increased. But such is not the case on the Boulder Bank.
The remarkable position of the bank, standing as it does some distance away from the shore-line of the inner harbour, is another difficulty. Why were the boulders not driven right in shore by those mighty seas, and piled up along the beach? It is all very well to speak about the opposing forces of the tides in the harbour; but where was that force when the bank was only a mile or two long? It did not then exist.
The last difficulty that I shall mention is the finding of stones on the Boulder Bank that are not to be found at Mackay's Bluff. On that part of the Boulder Bank lying south of the lighthouse there are numerous boulders of red syenite. This red syenite is not to be found at Mackay's Bluff. It, however, forms a part of the Arrow Rock, and the cause of its redness is there also apparent. When speaking of the composition of the Arrow Rock reference was made to an intrusive sheet of lava that had invaded the syenite. This lava contains much iron, and the syenite in contact with it has been stained red by the oxide of iron produced by decomposition of the lava. There are also on this part of the bank boulders of a very fine-grained rock which does not appear at Mackay's Bluff, but is found on the Arrow Rock. Having carefully examined the beach at Mackay's Bluff, the southern end of the Boulder Bank, and the Arrow Rock, I am fully convinced that the points of similarity between the rocks on the Boulder Bank and those of the Arrow Rock are far more striking than the same rocks compared with those of Mackay's Bluff.
Enough has now been said to show that the origin of the Boulder Bank is a question not easily settled simply by observation and reasoning. The sinking of a shaft on the bank, however, would probably set the matter at rest. Before attempting to cut the bank this preliminary precaution should certainly be taken. Although a strong supporter of the underlying-reef theory, it would be gratifying to me to find that the reef did not exist, because the facilities for improving our harbour would be then greater than I now consider them. If, on the other hand, a solid wall of syenite, or of firmly cemented syenite boulders, does underlie the bank, it would be well to know the truth before spending large sums of money on a work that might never be completed.
What has been said about the probable reef-formation of the Boulder Bank applies with equal force to the submerged banks within the harbour. As they too would have to be cut in carrying out the proposed harbour improvements, it would be necessary to test them also by boring to the required depth.