Auckland's Long Past: Region Was Uplifted From The Sea
DATES BACK MILLIONS OF YEARS
AUCKLANDERS rarely pause to! **• think of the possible age of the stretch of land on which they dwell, and if they do they probably take the view that it came into existence perhaps a few thousand years ago, and leave it at that.
Among geologists, however, it is common knowledge that the local history of Auckland began, not thousands, or even tens of thousands, but millions of years ago. Mr. E. J. Searle, of the'department of Geology, Auckland University College, says the beginning was nearly 180,000,000 years ago, at the start of the age of reptiles, when the Auckland district was part of a great slowly sinking trough which was beneath the seas bordering some large continental area. The slow movement continued for a matter of 50,000,000 years. During this time a great mass of land waste accumulated on the slowly sinking floor.
At Kawhia, on the west coast, there are indications, says Mr. Searle, that the floor on which this part of New Zealand stands must have sunk to a depth of nearly six miles, for sediments there are more than 30,000 feet thick.
When the accumulation of land waste came to an end, earth movements produced a wrinkle on the floor of the ocean, and the area of the North Island was for the first time uplifted from beneath the seas. That was somewhere about 130,000,000 years ago. Rocks formed from the sediments now make up most* of the main mountain ranges of the North Island, and are found close to Auckland in the Hunua; Ranges, on Waiheke Island, and on other islands in the Hauraki Gulf.
Limestone Rock Formed
"The newly born land mass was then subjected to erosion and worn down to an almost featureless plain," continues Mr. Searle. "Near the end of the age of reptiles it was once again flooded by the oceans and a new series of sediments deposited on the warm surface of the old terrain. The land waste which now gathered was enriched by the skeletons of myriads of microscopic animals, called forams, and the limestone rock which to-day covers much of North Auckland was formed. Once more the land was uplifted, and as a result of earth pressure its materials were compacted and folded. In the area near Silverdale great bodies of liquid rock which settled into the sediments include the serpentine used to-day for mixing with superphosphate fertiliser."
Again the surface was broken up by heavy erosion, to such an extent, says Mr. Searle, that in places near where Auckland stands the limestone was completely removed from the surface of the older strata. In mid tertiary times—that is a mere 35,000,000 years ago—the region was submerged for the third time. The subsidence began slowly with the formation of swampy basins in which plant remains accumulated to produce coal at Drury and Bombay, while later in the shallow seas the deposits yielded sandstone and mudstone, which, from their appearance in the cliffs round the harbour, are a typical Waitemata characteristic.
Occasional outbursts of volcanic activity led to the formation of volcanic grit beds similar to that exposed in the cliffs near the Parnell
baths. Probably the land mass from which the material was carried was situated some distance to the northwest of the city, for a distinctive bed in the series contains pebbles and boulders of igneous rock, forming a mass 600 feet thick at Kaukapakapa. Nearer the city, however, the beds are thinner and the boulders smaller.
Then came the final phase in the accumulation of the Waitemata sediments, accompanied by outbursts of activity from many centres in the shallow water near the present West Coast. Material from these eruptions settled partly under the water and partly above the surface. Much of -it.seen to-day is the result of ancient lava flows* broken up and partly rounded by the waves.
Once again, Mother Earth gave a great heave, the land appeared, the seas drained off, erosion began afresh and the rocks were worn down to a plain-like surface, distinguishable to-day in districts north of the city. Only the more resistant volcanic debris formed at the end of the period of accumulation withstood the erosion, to form to-day the Waitakere Ranges 'and other elevated parts.
Under Enormous Pressure
Pressure again changed the shape of New Zealand, but this time, instead of producing folding of the strata, it caused the whole countryside to become fractured and broken into great blocks. Some were tossed or tilted up, others sank down. Thus was produced the characteristic basin and range topography of the New Zealand landscape. The Coromandel Ranges and the Hunua Hills, for example, are uplifted blocks, while the Hauraki Plains, the Firth of Thames and the Manukau lowlands are depressed blocks.
Papakura Valley occupies the angle made by the tilting down of one block against another. At this time the Waikato River was discharging through a great estuary which stretched from the Waikato Heads to the north head of the Manukau, and a great plain of pumice silts brought down from the mighty volcanic outbursts in the centre of the island, which poured out more than a hundred cubic miles of volcanic materials, was taking shape. The great estuary was closed by the building of a series of great dunes across its mouth, and the Waikato River from then on was restricted to the southern outlet.
: While the world was experiencing the rigours of the great ice age, about half a million years ago, the local Auckland area suffered a series of movements, but in the main there was a progressive uplift, interrupted by occasional standstill periods or slight reversals. A pause occurred when the land was about 100 ft lower than at present, and a wide bench was carved.
"This bench may be recognised today in the level ridges of the city and northern suburbs," says Mr. Searle. "There followed a rapid uplift which caused streams to gouge out deep valleys, subsequently filled in by clays and silts, as at New Lynn and Hobsonville. A further pause occurred when the land stood at about 40ft lower than now, and broad benches ware again carved by the old Waitemata River, to produce the flat areas of Avondale, Point Chevalier and Hobsonville."
Modern volcanic activity then took a hand. The earliest eruptions formed the large craters in Shcal Bay, after which another spasm of uplift produced deep, trench-like valleys, down one of which flowed the lava to form Black Reef, Point Chevalier.
Volcanic eruptions, during the great "boil-up," -occurred from no fewer than 63 points near the city, and produced lava flows, explosion craters and the scoria cones which so enrich Auckland's landscape.
The last major geological change was a downward movement which caused the sea to flood into the river valleys, to form the Manukau Harbour and largely determine the modern geography of Auckland and its environs.
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Auckland's Long Past: Region Was Uplifted From The Sea, Auckland Star, Volume LXXVI, Issue 170, 20 July 1945
Auckland's Long Past: Region Was Uplifted From The Sea Auckland Star, Volume LXXVI, Issue 170, 20 July 1945
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