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Extended notes on the Geology of the Eastern Bay of Plenty.
Contributed by Dr David Kear of Ohope Beach.
Photos by Andrew Whyte.
 Ohope
boasts sand in great quantities - for housing, gardening, sunning, swimming,
golfing and much more.
Only the cliffs at the western headland are made of something much harder
- "greywacke".
Its most prominent feature is the presence of hundreds of "bedding
planes" - features that were once flat sea beds like that offshore
from Ohope today.
They have been tipped on end by "plate tectonic" forces during
the 140 million years since they were simply sand on an old sea floor.
They have been hardened as well as contorted, and form the photogenic
steep cliffs, as well as the lower part of the pathway to Otarawairere.
The shoreline rocks over there provide many intertidal pools - homes to
small crabs and very small fish. Otarawairere
has a shelving shelly beach - good for picnics and swimming. A
solitary fossil belemnite (a "cigar shell") has been collected
from the beach rocks there by Joan Kear, an Ohope resident.
It dates from the Jurassic Period, a time when modern New Zealand was
almost wholly under the sea, and dinosaurs wandered lands far away.
 The
present day Ohope beach is sandy, with a flat to moderate bottom profile
which attracts a horde of board surfers in the deeper water, and swimmers
and body-surfers in the shallower - particularly in the patrolled beach
at the Surf Club.
The sand is dark in colour, because most sand grains are simply small
rounded particles of dark greywacke.
Some may have come from the West End cliffs, but most comes as gravel
down the Whakatane and Waimana Rivers, when they are in turbulent flood.
The brownish water at flood times shows up then well offshore. Both
it, and its load of sand and wood of every shape and size, drift slowly
landwards, to build up and litter the beach. The
lighter-coloured sand grains on the beach are of pumice - often seen as
a thin surface layer on the darker sand below.
The pumice was brought to the coast down the Rangitaiki and Tarawera Rivers
from the highly active volcanic centres of Taupo and Okataina. The
sand is moved to Ohope by the predominantly eastwards near-shore current.
At some times, the direction changes to westwards, and green mangrove
"seeds" from Ohiwa Harbour are then seen commonly on the beach.
 Many
shell species are exposed between the tidal limits - reflecting the 200
or so species of shellfish and other sea dwellers that live near the Ohope
shore. Tuatua (Amphidesma
subtriangulatum) is the most common of the species gathered from knee-deep
and shallower water for barbeques, hangi and fritters.
Open shells accumulate after such meals, and after gulls have dropped
them from a great height on to the hard wet sand, and feasted on the broken
remains. Oyster
catchers are also busy.
Older generations remember when the tiatua-like, but larger, Toheroa (Amphidesma
ventricosa) was also collected at Ohope.
Its shells are found in local middens. Other local shells include the
large circular "Dollar Shells" (Dosinia anus), the Kaikai
or "Large Ostrich Foot" (Struthiolaria papulosa - a fat
spiral shell with a thick entrance area which survives after the rest
has disappeared), the slim spiral "Turret Shells" (Maoricolpus
roseus), and the wide variety of scallops, mussels, oysters, and large
and small spiral shells (snail-like gastropods), and the rest.
 The
easterly near-shore currents built the Ohope spit steadily over the last
5,000 years or so - using mostly sand brought to the sea by the Whakatane
and Waimana rivers.
Initially the sand accumulated in the intertidal area.
Occasional storms moved large quantities of sand landwards beyond normal
high tide, and more sand was added by the wind whenever the beaches were
dry. Collectively,
the level was raised to a metre or two above high tide, and the ever-easterly-extending
spit eventually reached Ohiwa Heads. Even
there sand is still moving eastwards along the sea bed - travelling across
the "bar" offshore from the Ohiwa Harbour entrance. That
transported sand has built out the land on the eastern side by well over
150 m since the small Ohiwa settlement there was attacked by the sea in
1976, and subsequently abandoned.
 Ohope
spit forms a barrier to Ohiwa Harbour.
Prior to that it was a near-open bay. Now
it is a predominantly intertidal area - picturesque at high tide and alive
with small snail shells at low.
It is a home to pipi (Amphidesma australis), and flounder.
At about 1500 AD, the sea was close to its present level, and the temperature
was slightly warmer than today's.
Then the "Little Ice Age" struck. Temperature
dropped and caused the Maori crop of taro to fail.
Taro had been brought to New Zealand as a carbohydrate crop - just as
it is used today in "The Islands". Some
still grows in sunnier parts of Northland, but fails to grow the valuable
bulbous stem. In
Britain that same climate change made it no longer possible to make wine
there. The cooler
climate caused the polar ice caps to grow in size and the sea level to
fall as its water became tied up as ice.
This was not to the same extent as in the major Ice ages, when sea level
dropped by over 100 m. Nevertheless it was sufficient for the high tide
mark at Ohope to move some distance offshore.
The sun dried out the new land, originally sea-bed, and the wind blew
the loose sand ashore.
Thus, for something like 200 years up until around 1700 AD, sand-dunes
were formed parallel to the coast along the whole of the Bay of Plenty
shoreline. They
built steadily upwards.
 In
Ohope the dunes are commonly high enough to cut out any view of the sea
- along much of Ocean Road for example.
In the area between Ocean and Harbour Roads, houses built on those dunes
have uninterrupted views of both the sea and Ohiwa Harbour. The
high dunes continue towards the Ohiwa Heads where the Ohope golf course
takes full advantage of the steep country to provide 18 very different
and interesting holes. The
club house rests high on the dunes, with fine views all around.
Major periods of stormy conditions, lasting a few years, have removed
some of the dunes locally. This
natural effect is remembered well in the nineteen-fifties and nineteen-nineties
when the sea reclaimed several tens of metres of land quite quickly in
a few years. Perhaps
this effect is cyclical, each cycle lasting several decades.
In the longer intervening periods, the coast-line builds out more slowly
at around 1 m/year
The Ohope sand deposits include large rounded lumps, of up to small
football size, of white pumice.
That was erupted from Taupo around 200 AD, and floated in from the sea.
There are also
smaller walnut-sized pieces of black pumice, thought to have been the
result of one or more offshore eruptions, one of which has been dated
at around 700 AD. Such
observations help to establish that each cycle of erosion tends to leave
the subsequent coast slightly further seawards than on the previous occasion
- a conclusion that agrees with the fact that the coastline of the Rangitaiki
Plains has built out seawards by up to 10 km in the last 5,000 years.
 Changes
in sea level, relative to the land, are as slow as they are natural. Cooler
periods such as the Little Ice Age, caused sea level to drop.
Also, however, because of relentless "Plate Tectonic" movements
all around the Pacific, some large land areas are forced relentlessly
upwards out of the sea - like the whole eastern area of the North Island
where its "main range" has formed as a result.
At the Wellington end, the land has risen 25 m upwards in the last 5,000
years in four or five massive earthquakes. The
corresponding rate at Ohope is not known.
Most geologists believe that it is much slower here, but the highest point
in the main range is Hikurangi - towards our end of the range.
At Ohope, the major result of these sea level changes has been to raise
some very young deposits up high - the "Ohope Beds".
In their lower parts they appear as marine "papa" in the cuttings
on the lower bends on the Ohope Hill Road. In
the cliffs behind West End, a rich fossil bed is now well above today's
sea level - showing at least 55 m of land uplift or sea level fall. In
1946 a major slip brought down large amounts of this shell bed.
Sir Charles Fleming, New Zealand's chief paleontologist at the time, recognised
more than 200 species of sea dwelling creatures. Most
live offshore from Ohope today, but a few do not, allowing him to date
the bed as 500,000 years old.
The deposition of sand and shell continued after that shellbed was formed,
causing a shallowing of the sea then.
Eventually river gravels were  deposited
on top of the sea sands, and finally a thick deposit of pumice covered
it all. The sea
deposits, gravel and pumice are all exposed along different parts of the
track over to Otarawairere.
In the steep cliffs, at the eastern end of that bay, the horizontally
bedded Ohope Beds can be seen clearly above the contorted beds of greywacke
- one of the most easily seen examples of an "unconformity"
in New Zealand.
The upper surface of the Ohope Beds was originally near-horizontal,
although sloping slightly towards the sea. The
surface has been modified, as erosion has carved stream valleys into it;
but it still looks horizontal when viewed from the sea, both behind West
End and Pohutukawa Avenue.
The pumice in the upper parts of the Ohope Beds was the first indication
that Ohope would become volcanic - receiving pumice ash through the air
from New Zealand's two most active volcanic centres - Taupo and Okataina.
The latter is large and close to Ohope.
It includes Tarawera in the south, and Rotoiti to Rotoma in the north.
The ash deposits themselves show clearly in road cuttings at the top of
the gorge from Whakatane, and at the top of the Ohope Hill. Two of the
20 or so specific beds of ash are around 2 m in thickness - each being
the result of a single eruption.
 Two
volcanoes of a very different type to Okataina can be seen as islands
from Ohope Beach. Whale
Island was born around 50,000 years ago, with a central high area and
two flanking domes being active at various times.
Although it is probably now extinct, hot springs around it are a reminder
of relatively recent activity.White
Island, far offshore, has been active since about 10,000 years ago.
It ejects steam continuously, and ash from time to time. It
is a constant reminder of Nature's strength and persistence.
© Ohope Beach.info 2003
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