Art. XLVI.—On the Drift Beds of Wakapuaka and Port Hills, with Remarks on the Boulder Bank and its Formation.
[Read before the Nelson Philosophical Society, 3rd March, 1884.]
I have chosen this locality for my paper from its proximity to the township, so that members who take an interest in geological science may have no difficulty in visiting the district, which may tend to invite discussion on a future occasion, and assist in verifying, or otherwise, the conclusions I may have come to on the subject.
Drift in geological language means portions of the earth's crust removed by forces from one place and deposited in another, some from comparatively short distances and others very wide apart. These forces are of various kinds, the most important of which are, Fire, Water, Ice, and Wind—Fire exhibited in volcanic action, which is powerfully exemplified on the west coast of the North Island, from Mount Egmont to Wanganui. This belt of country is chiefly composed of pounded pumice stone, in some places many feet in depth, which has been erupted from volcanoes in that district and drifted by wind currents all along the belt, now producing the luxuriant grasses which exist there for the feeding of cattle and sheep. Water, the next force I have noted for effecting changes on the earth's surface, is by far the most important, being constant and continuous. The smallest rippling stream to the largest river are daily engaged in transporting matter from a higher to a lower level, as instanced on a large scale—the Canterbury Plains, the soil of which is the degraded high lands brought down by the force of water to nearly sea-level. Ice glaciers you are all aware are forces which carry in their course large quantities of rock and debris and deposit them at a low level in what are called “moraines.” The last noted is wind force, which drifts lighter materials, such as fine scoria and pumice dust in volcanic countries, in some cases for hundreds of miles from the mountains from which they have been erupted.
With these preliminary remarks on drift beds and the forces which occasion them, I now come to boulder drift on a small scale, seen on the Wakapuaka Road and Port Hills.
This drift shows itself on what we may call a raised beach, commencing at the town boundary, and exposed on bare faces for several miles towards Wakapuaka. This beach is raised at a high angle, in some places not less than 45 degrees. In the clay drift on these faces we find stone boulders stuck through it, somewhat like plums in a pudding. These boulders are generally round in form and smooth in surface, showing they have been carried from a distance, getting their round shape and smooth surface from
I do not think I have anything further to say about the boulder drift on the line of the Wakapuaka Road. It would be a very pleasant excursion for any of the members of this Society to take a hammer in hand, traverse the district, and find the parent rocks from which these boulders have come, and endeavour to trace the course the force of water took in bringing them to the position in which we now find them.
I may, however, mention here, that as you get higher up on the line of this raised beach, you meet with beds of shells from a few inches to a foot under the surface, at a considerable elevation above sea-level. When these beds were first exposed on the slopes, the impression of the settlers who exposed them with the spade or plough was, that the Maoris must have brought them; but when we find them assuming the same striæ and curved shapes which the tidal current gives them at the present time in our lagoon, or what we call our mud flat, there is no difficulty in coming to the conclusion that they were deposited at sea-level, and subsequently raised by forces from underneath.
I shall now offer a few remarks on a natural object conspicuous among our surroundings here, viz., the “Boulder Bank.” How was it formed, and what were the forces employed in building it? The first consideration is, where did the material come from of which it is built? If we travel to the extreme north end of the bank, by Mr. Mackay's property, we will find a bold rocky bluff, nearly vertical, with the sea lashing on its base at flood tide. This rock is local, and does not extend far beyond this point, and is of the class of mineral called syenite, composed chiefly of quartz, hornblende, and felspar, very hard and crystalline in its character. Now if we leave this bluff and walk down towards Nelson on the seaside and examine the boulders which form the bank, we will find them mostly all of the same character as the perpendicular bluff before mentioned. This bluff has been acted upon by the sun's rays on its face on the one hand, and by the sea and spray and rain on the other, whereby its cohesion is loosened, when it gradually falls down into the sea in smaller or larger pieces. These fragments of rock get disturbed by the swell of the sea and by a powerful current which sets down from that point, are then gradually rounded at their sharp edges by rubbing against one another, when they are easily carried forward by the swell and current, and rolled up on the bank where we now find them. It is observable also that the further they get on their journey the less they get in bulk from the grinding process they have to undergo in transit.
Another feature in this bank is the straight line—north and south—on which it is built. How has this straight line been preserved throughout? We find there are two forces in operation, an outer and inner current, the swell of the sea and a strong tidal current on one side, which heaves up the
known as the trap series, largely distributed over the earth's surface, and as they contribute in a larger degree than any other class of rocks to the economy of vegetable and animal life, I may be excused in alluding here to their properties in this respect. In their composition they contain in considerable quantity potash and other salts necessary to the growth of plant life, and when decomposed by the sun's heat and rain, which destroy their cohesion, the pulverized particles are washed down into the valleys and form our best soils for growing grass and cereals. It is possibly one of the best tests in looking after new country for settlement, to examine the character of the rocks surrounding the country you are traversing, and you may rest assured that if this trap series is the prevailing rock the valleys will be rich and fertile. We have a very good example of this in that part of our township called The Wood, where we have that rich sedimentary deposit, where we grow hops and vegetables. That district has at some former period been a lagoon of comparatively still water, the force of the Maitai current being broken by the bluff at Mr. Huddleston's property, and the sediment from the degraded trap dykes I have before mentioned, finding its way in solution into this lagoon, gradually deposited itself into a rich alluvium, which proves so fertile for cereal crops. I conclude, therefore, that this alluvium is chiefly a decayed trap deposit, because no other rock in the Maitai series from its composition could give out soil of this description. The dyke from which this deposit has chiefly come runs from Mr. Huddleston's Bluff, past the back of Mr. Sharp's house, and again shows itself on Mr. Curtis's property, on the Wakapuaka Road, the worn-down face of this ridge being the deposit I have now mentioned.
Probably our greatest misfortune in the Province of Nelson is, that we have so little of this class of rock, but are surrounded on all sides right on to the west coast by slates, schists, and rocks of a quartzose character, and on the east by the Dun Mountain Range, by slates and a variety of rocks of a magnesian class, extending from D'Urville Island in the north, to the Top-house at the head of the Wairau Valley—all these rocks I have mentioned being from their composition unfavourable to plant life, and the alluvium from them being considered country of a third or fourth class. We are not, however, left without some comfort in having at different points in our surroundings rocks of a more fertile class, such as a volcanic trap, granites, and limestones of various kinds, and some others containing felspar in considerable quantity, which has given the district some rich alluvial soil. We are very apt, in travelling through a country, to pass along giving very little heed to the character of the rocks surrounding us on every side, but there is little doubt that the fertility or non-fertility of a country depends very much on the character of the rocks by which it is intersected.
rolling about on their journey to where we find them. If these surmises are correct, then some force must have been at work to bring them to their present position. This force the observing geologist concludes has been water, bringing down fragments from the parent rock, also detritus of a lighter material in the form of clay, and depositing it at sea-level. However, we find this deposit, as I have before remarked, far above the level of the sea, consequently some other force must have been at work to raise it to the elevation at which it now stands.
If we travel up the Maitai, a short distance before reaching the slate formation we find a range of greenstone dykes running in a north and south direction, and to them I presume may be attributed the upheaval of this raised beach. Where these dykes obtruded themselves, the elevation of this boulder drift was the sequence. I may mention here that the boulders in this drift are of a different mineralogical character from those brought down by the River Maitai, being mostly hard indurated sandstone and conglomerates, while the Maitai drift partakes more of rocks of a crystalline character, such as serpentine, hypersthene, dunite, and various hornblendic rocks, with a mixture of slate, so we must look for another outlet for this Wakapuaka drift than the Maitai. If we examine the back country north of the Maitai, and onwards towards the Happy Valley outlet, we will find the same sandstones and conglomerates in sitū and they are at the present time being brought down by the Ludd Stream, which flows through that district, so that we may fairly conclude that this drift has been carried from the degraded rocks of the higher country behind, and subsequently raised by the ejecting of the greenstone.
The Port Hills are all drift matter, overlying the sandstone which is exposed at the cliffs, part of which has been carried from the eastern ranges and part from the west, and is very easily defined. The western drift is well shown on the ridge at the back of Major Richmond's property, containing granite boulders in the clay, and as no granite is found on this side, in sitū, we may conclude that this drift material came from the west, granite there being the prevailing rock; and on examining the Port Hill faces on the Nelson or township side, they partake more of the material which might be expected to come from the Wakapuaka Ranges; so that water-force must have been at work both on the east and west side of these drift beds.
In mostly all sedimentary strata of any considerable antiquity we find faults occurring, that is to say, we seldom find them in the same horizontal position in which they were deposited, but either at a less or more abrupt angle, having been subjected to internal forces from underneath. These greenstone dykes I have been speaking of are one of a large family generally
larger boulders, and a lesser current flowing up on the inner side, thereby disturbing and heaving up the smaller shingle. It will be observed also that the inner side is characterized by small gravel and the outer by larger boulders; so that the bank itself may be said to be the centre of two actions, an outer and inner current.
These two currents are performing the same work that a couple of navvies would do in building a breakwater of the same materials, the one heaving up the boulders on the one side and the other shovelling up the smaller material on the other, the only difference being a question of time as to the completion of the work. It has been suggested by some observers that there may have been at some early period a ridge of rock extending the whole distance, and that the present Arrow Rock at the entrance to our harbour is the termination of this supposed ridge, the other portion having been degraded and worn down, and the present bank resting on its base.
This is a probable theory and may be correct, and certainly would go far in facilitating the building-up of the bank, it having got a solid foundation to rest upon, but I have never heard of its being verified by sinking or otherwise. I am however of opinion that the forces at present in operation, however inadequately I may have described them, are quite sufficient to form the bank without the help of this rocky base to rest upon. This is a subject it would be interesting to get proofs of, and probably some enquirer of this society who may have time and inclination to institute a research in this direction may communicate his views on some future occasion.
I have said in the course of my remarks that this bank is chiefly composed of fragments from this syenitic bluff, and would hazard the opinion that, if this very hard crystalline rock had not been in the position we now find it, there would have been no Boulder Bank, or, had the bluff been sandstone or any other rock of a softer and more friable material, the forces acting upon it would speedily have reduced it to sand or mud, which would not have had the resistance to form a breakwater—in fact, no other rock in the district would have been fitted for the work; and when the supply of fresh material from this source is exhausted, or ceases to be rolled down to supply the degraded waste going on in the bank below, the decay of the bank itself will take place—there are some appearances of this going on at the present time. A large bank of shingle has been and is being formed at the base of this syenitic dyke, the degraded fragments of which no longer drop into the sea to be rounded and carried down, but are left where they fall among this shingle, consequently the bank below is not receiving the same amount of fresh material necessary to its support in supplying the waste going on in consequence of the action of the sea, thereby a breach in the bank itself may sooner or later be the consequence and the sea find its
way into the inner lagoon from the upper end of the bank. Should this happen, it would of course speedily change the character of the district, and might have important effects on the lower part of the township itself. I do not state this as an alarmist, as artificial means might be employed to support the weak points in the bank as they occur; but there is no doubt that inroad at the present time is being made by the heavy swell which strikes the bank in its upper portion, and symptoms of degrading influences are already showing themselves.
Another geological feature in this district, and connected with drift material, is the basin or mud flat between the main land and boulder bank, which is being silted up, and will at no very distant period be dry land and covered with natural pasture—“provided no intervening circumstances occur”—without any artificial aid more than the forces employed at the present time. Since I first lived in the district, nearly forty years ago, I perceive a considerable change in this direction; from 1 foot to 3 feet in some places of mineral deposit has been laid down in the upper part of the lagoon during that period. I also perceive a very considerable change in the character of the material now deposited from what it was at a former period, which was then more of an argillaceous decomposed vegetable ooze, in which you sunk to the boot-tops in crossing over to the Boulder Bank at ebb tide. Now we have over that a firm layer of clay deposit on which you may walk over on the upper part of the flat in a pair of slippers without soiling them. This is accounted for by the settlers disturbing the surrounding country in their farming operations, which the rains and intersecting rivulets bring down to the sea-level, the silt being more of a consistent clay material than the deposit of a former period. This is a very good example of how sedimentary strata are determined by surrounding circumstances. In sinking a vertical shaft in the earth's crust, we find one layer overlying another, having different mineral characteristics. These lie over one another like leaves in a book, and it is the business of the geologist to turn over these leaves, where he finds a true and accurate history of what the surrounding country presented from time to time—the animals and plants then alive upon its surface, and external events faithfully photographed for æons of years in our earth's history.
I may before closing this paper mention a few of the historical drift beds now in course of formation, although I have no doubt you are all acquainted with them in the course of your reading. The lower basin of the Mississippi shows a drift bed at the present time to the extent of 700 miles in length, formed from the sediment brought down this river and its tributaries, from the lands and degraded mountain ranges in the upper districts of the country. This bed is being daily added to, and no doubt at
some time in the future will have filled up the entire Gulf of Mexico. The Nile Valley basin, 500 miles in length, is the mineral and vegetable ooze brought down by that river from the Abyssinian Mountains and interior of Africa, which is the great fertilizer of Egypt, a country which would be comparatively worthless without its agency. It is calculated that not less than sixty millions of tons of solid matter is yearly brought down this river, rendering the waters of the Mediterranean turbid for a distance of 30 miles from its mouth. These are two prominent examples of water-drift, and all other rivers on the earth's surface are similarly employed in effecting the same ends and purposes.
I have not remarked previously on moraines or ice-drift beds. A remarkable bed of this description is seen on the Norfolk coast in England in what is called the boulder clay, extending many miles. It puzzled geologists for a long time to account for its formation, the boulders in the clay being entirely of a different character from any rocks existing on the English coast, till the glacial theory was suggested and established, when it was afterwards proved and verified that these boulders and clay had their parent rocks on the mountains in Scandinavia, and had been brought over by glacial ice and deposited where they now lie. As to volcanic drift, we have as examples the buried cities of Pompeii and Herculaneum, and we are informed from the accounts of the very recent volcanic eruptions in the island of Java and Straits of Sunda, that scoria and ashes in immense quantities were ejected from the volcanic island of Krakatoa, some of which was drifted by wind-force as far as Cheriton, 250 miles distant. From the remarks made, we may infer that rock masses form a very important part in life's economy.