By the late PROFESSOR J. A. BARTRUM, M.Sc.
THE LAYING OF THE GEOLOGICAL FOUNDATION OF THE AREA
About one hundred and fifty million years ago, during the Trias-Jura period, there would be no sign of land where now our familiar mountains rise. Beneath this sea, sands and muds were accumulating and mingling with lavas and ashes of rhyolitic nature emitted by volcanoes that had burst into eruption from the shallow sea-floor in the area where the Coromandel Peninsula now stands.
"Old Mother Earth" is a restless body and periodically upheaves her mighty carcase, causing lands and mountain ranges slowly to uprise in some regions and seas to spread over others. Thus, at the end of a long period of sedimentation, great crumples of newly deposited rocks accumulated here to be long and vigorously attacked by streams and other erosive agents until the newly-born land was ultimately worn down to an area of low relief.
The rocks of this land are not visible near Te Aroha itself, but appear as hard, dark-grayish sand-stones (greywackes) and allied rocks in the hills west of the great Piako plains, as well as near Thames and Tapu and areas still further north and in the hills north of Cambridge, but undoubtedly form the basement of Te Aroha area buried beneath much younger rocks.
But now in late Cretaceous times there was an earth-heave of opposite character to the earlier, and seas spread over the greater part of North Auckland Peninsula, depositing beds of which the most characteristic and widespread are the hydraulic limestones used for the manufacture of cement at Portland. It is uncertain whether Coromandel Peninsula was ever submerged at this time beneath the sea, but if so, the deposits then formed were later forced above sea-level by earth-throes and stripped by erosive agents from the ancient basement. Whatever the case, about earlier middle Tertiary times a low-lying greywacke land spread far and wide, with hollows in which vegetation accumulated in thick masses which later were covered, as the land sank beneath the sea, by mud-stones, sand-stones and other sediments and were thus compressed and altered to form coals. These latter include those mined in the lower Waikato, and are represented, along with sandstones and other attendant beds, by the coals at Miranda and at Surrey Redoubt in the Mangatangi Valley. Beds of this character and age are not exposed in the Te Aroha area, though it is not improbable that they exist buried deep beneath thick piles of younger volcanic material.
VAST IGNEOUS ACTIVITY OF TERTIARY ERA
Just after the deposition of the coal and other Tertiary beds, events began which had the utmost importance in the building of the ranges east of Te Aroha. Mighty volcanoes came into action, pouring far and wide their streams of lava and erupting vast quantities of debris, especially in the Thames and Coromandel districts, during their long period of activity, and thus giving rise to rocks that have had immense importance in the mining history of the goldfields, and continue to have great value todayforroad-surfacing purposes.
The vulcanism appears at the commencement largely to have been concentrated along some great zone of fracture in the earth's superficial layers, such as reappeared at a much later date and gave birth to an ancestral Firth of Thames, for, although the volcanic rocks of this "First Period", as it is called, may in the past have extended far east of the present shores of the Coromandel Peninsula, yet westward they are very narrowly limited.
The "First Period" rocks are mainly of the type called "andesite", generally dark-bluish-gray fine-grained rocks speckled with crystals of various kinds, but, here and there over large areas, their characters have been profoundly altered by vapours or solutions rising from the parent mass of heated igneous material below and they have become some-what blotchy greenish-gray rocks often rich in glittering crystalsof iron-pyrites and not seldom leached by percolating surface waters to a relatively soft whitish material. These altered andesites are technically called "propylites", and, along with others called "dacites" possessing close alliance to rhyolites and also greatly altered, form the "country" of the metalliferous veins of the goldfields.
It must not be thought that these volcanic eruptions proceeded continuously without pause. Many long halts intervened between spasmsofactivity, allowing soils to form from the erupted rocks and vegetation to grow, whilst muds and sands accumulated along with beds of plant debris in swampy areas and lakes enclosed amidst bordering volcanic masses. The collected vegetation has given rise to coaly beds of limited extent such as have been prospected at Dickey Flat in the Waitawheta Valley across the range from Te Aroha, whilst others as much as 6 feet in thickness outcrop in Waimata Stream 5 miles west of Athenree, though here they probably originated at a later date than those at Dickey Flat and at the Rahu Saddle near Karangahake.
It is uncertain exactly at what date the alteration of the "First Period" volcanic rocks took place, nor does it matter particularly for the present purpose. The tendency has existed to assign the altered rocks and their obvious contemporary associates to the "First Period", and similar but unaltered andesitic lavas and fragmental masses, developed in great profusion along with occasional interbedded sediments, to a "Second Period" of eruption, but the validity of this separation is somewhat doubtful, however convenient it may be as a means of mapping areas of rocks likely to contain veins of economic importance.
ORIGIN OF METALLIFEROUS VEINS
What is of prime significance as concerns the altered rocks, is that the process that caused their alteration probably gave birth also to the valuable lodes of the goldfields.
Fluids ascended from deep-lying reservoirs of igneous rock along regular series of sub-parallel fissures developed by the movement, one with respect to another of adjacent rock-masses as a consequence of earth stresses. In and alongside of these fault fractures, the uprising fluids deposited the various chemical substances they had brought up from below, thus forming the metalliferous lodes, whilst at the same time they caused the wide-spread alteration of adjacent rocks wherever they were sufficiently shatteredby the earth-movements to allow the passage of the solutions or gases.
This, then, was the origin of the deposits that have been mined in Wairongomai [Waiorongomai - E] Valley and near the well-known Tui mine about a mile north of the Trig. Station, and similarly of the frowning "Great Buck Reef" of Wairongomai [Waiorongomai - E] which trends approximately north and south for nearly three miles. Since the veins were formed, erosion has lowered the original surface very considerably and downwards-travelling waters have materially altered the primary constitution of the vein-filling, especially at higher levels.
"THIRD PERIOD" VOLCANIC ERUPTIONS
The extrusion of the vast pile of rocks commonly referred to the "Second Period" of vulcanism did not close the cycle of igneous activity, for, after a pause, volcanoes again unleased their fury and towards the end of the Tertiary began the extensive ejection of ashes and outpouring of lavas that led finally to the greatest mass of volcanic accumulations that we have in New Zealand.
These "Third Period" rocks are known as rhyolites, and are generally much lighter in colour than those of earlier periods of eruption. They are developed mainly on the east side of Cape Colville Range and south from Katikati build great plateaus largely of pumiceous debris which continue south and unite with the immense plateaus of similar material in the Central Volcanic Region.
BIRTH OF MODERN TOPOGRAPHY
Should we take an aeroplane and, crossing the Hangawera Hills near Tahuna on an eastern flight, steer straight through Te Aroha and ontoKatikati, we should be struck forcibly first with the comparatively rapid descent to the level of the plains of the land-surface near Tahuna, then with its steep rise at Te Aroha into the lofty range at the back of the town and again with the equally abrupt descent to the lowlands near Katikati. We should see beneath us, in fact, four great narrow earth-blocks with approximate north-north-west trend - first the Hangawera Hills Block composed of ancient (Trias-Jura) greywackes with merely local capping of volcanic or other younger (Tertiary) beds; second the low-lying Hauraki Plains Block with its almost-modern sediments; third the elevated Tertiary volcanic mass of Cape Colville Range and finally the depressed Katikati area built of deposits little, if at all, older than the earliest of those forming the Hauraki Plains Block.
Had we, however, travelled along the same latitudinal line at a time well before the dawn of human history, but at no far-distant geological date, not long after the earlier of the long series of rhyolitic ("Third Period") eruptions had broken out on the east, we should have looked in vain for such vigorous topographic differentiation. The area now occupied by the Hangawera Hills would probably have appeared as a somewhat dissected plateau with a fairly extensive cover of flat-lying sandstones and other Tertiary beds associated with coal, such as that at Miranda, whilst east of this we should have crossed a rugged though probably not lofty region of volcanic rocks, already, however, with the symmetry of any cones built around centres of eruption destroyed by erosion.
This state of affairs was soon to change, for, towards the close of the Tertiary, strains set up within the earth began to find relief, and great fractures developed along which some parts of the surface were elevated and others lowered.
Manyof these fractures - the major ones - were directed approximately north-north-west south-south-east, though there were others in roughly transverse directions. The upland block west of Hauraki Plains in common with that of Cape Colville Range, began to be raised relatively to the subsiding blocks represented respectively by the Firth of Thames and the plains and lowlands south of it and by those near Katikati and Tauranga, As the strained rigid rocks snapped, earthquakes would arise in swarms, just as when similar movements occurred near Murchison.
The depressed block of the Hauraki "graben" or "rift valley" lying west of Cape Colville Range was bounded both east and west by great systems of fractures, that on the eastern side even today being readily traceable along its sinuous course, so striking is the abruptness with which the hills of the relatively up-thrown block rise above the plains. The fractures seem to have developed earliest in the north and to have crept slowly south, where the mineral springs of Puriri, Te Aroha and Okauia still rise along the openings they provide. Vigorous streams crossing the mountain face have done much by their erosion to lessen its steepness over a great part of its extent, especially north of Te Aroha and between Gordon and Okauia, their influence is less clearly shown. In fact the Bald Spur at Te Aroha and the similar "steps" in the mountain wall where the old Tui track begins at Okauia, probably represent almost unmodified small earth-splinters separated from the more elevated major block by fractures that have distributed the total relative uplift between them.
It is of distinct interest to note the change of topography that begins south of Okauia, for, instead of the steep slopes that occur further north, over which Wairere Stream tumbles in its lofty falls, there is instead the long gentle decline from the highlands by which the railway to Rotorua climbs on to the great plateau near Mamaku. This appears to indicate that the lowering of the depressed western area, which has been effected further north by faulting, has here been the result of gentle down-flexing.
LATER EVOLUTION OF MODERN TOPOGRAPHY
The birth of the modern topography was signalised by the development of the wide trough of Hauraki graben between its bordering highlands. At its northern end the trough is now occupied by the sea, and the occurrence at Maukoro of a sea beach a few feet above swamp level shows that formerly the shore-line of the Firth of Thames was many miles further south than now. How much further it once extended is not known at present. The subsidence of the floor of the graben is sure to have been gradual, for movement along its marginal fractures was probably a slow process; this would have allowed continuous southward advance of the sea but for one important fact. This was that the Waikato River now found its way through Hinuera Valley into the Firth of Thames, and, fed with debris supplied by the immense rhyolitic eruptions proceeding in the Central Plateau, poured vast quantities of pumice sand and gravel into the Firth, building adeltasimilar to, but on a far grander scale than that of the Waihou and Piako Rivers to-day.
Swamps would occupy the up-river portions of the delta and lakes and lagoons be dotted here and there, whilst, as subsidence of the floor of the graben continued more or less spasmodically, earlier sediments would be covered by later series, the buried forests, drift-wood, gravels, muds, and sands disclosed by the bores for wells all representing different phases of these deltaic deposits. Moreover, as the delta grew seawards, earlier deposits of all kinds would be buried by later, so that the alternations of layers finally became very numerous, whilst progressive sinking of the region allowed a thickness of material to accumulate probably well in excess of 1200 feet - to which depth it has been proved by bores.
There is evidence that suggests, however, that the process of filling the rift valley had extraordinary vicissitudes. Near Clevedon and east from there, beginning at Orere Point, terraces built largely of pumice continue south along the west shore of the Firth of Thanes at least as far south as Brighton. The pumice reaches a height of not less than 100 feet above sea-level in the highest of the terraces, but these are somewhat local in their occurrence, and the most persistent bench is one about 35 feet or 40 feet above sea-level. Pebbles of greywacke in pebble bands associated with the pumice are so greatly decomposed as to indicate that these terraces represent relics of an early filling of the graben that was far more extensive than that of to-day, and, which was deposited when the land was probably considerably lower with respect to sea-level than now. These pumice beds were spread by the ancient Waikato (then flowing through the Hinuera gap and following the upper course of the Waitoa) not only in the Firth of Thames but also over far-flung low-lying areas in the middle Waikato Basin, around Hamilton and even further south than Te Awamutu, where remnants of them are found as high as 460 feet above sea-level, whilst the high downs at Walton and the divide between the Piako and Waikato Basins are constituted by these deposits.
Following this stage of extensive deposition, however, the land rose at least 150 feet higher than now, and in consequence the Waikato River excavated a wide trough, removing the northern portion of its earlier delta and leaving as relics of it only the fringe of terrace-pumice that now exists along the western shore of the Firth, whilst elsewhere rolling hills were carved from the pumice upland.
The uplift was followed by a movement of depression that appears to have been general throughout New Zealand and to have been responsible for the formation of many of the wonderful harbours of North Auckland and elsewhere with their intricately embayed shore-lines. As a phase of the submergence the sea entered the newly-carved trough, pushing the southern shore-line of the Firth at least as far south as Maukoro, 18 miles south of its present location, as is shown by the existence there of the beach that already has been mentioned.
Somewhere about this period, also, asDr. Henderson has shown, the Waikato appears to have become so overburdened with debris that it was compelled to build up its bed until it spilled over from its route by means of Hinuera into a new course through Maungatautari to Cambridge. Thence it followed approximately its present route west, and finally entered a large estuary then existing between Waikato Heads and Manukau Heads, which it proceeded rapidly to fill.
In their bulletin on the Huntly-Kawhia area, Dr. Henderson and Mr. Grange show that, at this stage of its history, the Waikato built a gently sloping fan which spread outwards from Cambridge almost to the furthest confines of the middle Waikato Basin and which often blocked drainage against obstructing hills and produced lakes such as that at Hamilton. After it left the Firth of Thames, also, the major modern streams entering there have been busy building a confluent delta which has advanced no less than 18 miles since the sea washed the slopes of Maukoro, whilst others have hurried down gravels from the bordering highlands and built fans such as those so conspicuous north and south of Te Aroha.
Thus nature has paved the way for the great dairying industry of today. And, as the Piako and Waihou delta has crept slowly out into the Firth, the Waihou River has been compelled to wander to and fro in sluggish meanders within a steep-walled trench carved in earlier times below the surface of the adjoining plains, whilst even the vigorous and enduring Ohinemuri, conqueror of Cape Colville Range which rose slowly athwart its path during the growth of the graben, is now being forced to build up its bed and periodically overflow thence in flood over the neighbouring flats. This is an unavoidable consequence of the growth of deltas.