WHAT'S HERE?
Lava flows that fill a crater blasted through the older submarine volcanic conglomerate rocks.
Rhyolitic ash layers within windblown dune sands forming the higher parts of the hills.
One of longest sea tunnel networks in New Zealand
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LEARN ABOUT IT
These rocks have been deeply buried and hardened and cut by two sets of weak joint planes. Erosion has removed all the overlying rocks and at the present time marine erosion is preferentially eroding along the weak joints forming two sets of intersecting sea tunnels through Erangi Pt. Some parts of these can be explored at low tide from entrances at the north end of Bethells Beach. Go to the head of the one which has been naturally daylighted (roof collapse) and ascend its head and follow a trail round to the left into another eroding joint which is an eroded gut with a small beach at its head. This system of intersecting sea caves and tunnels is one of the longest in New Zealand.
In the cliffs at the north end of O'Neill Bay the conglomerate rock is replaced by cliffs made of lava flows separated by breccia layers. These lava flows appear to have filled a crater that was blasted through the conglomerate late in the life of the Waitakere Volcano (about 17-16 million years ago). This is one of a number of volcanic centres that form an alignment down the west coast today.
In the last 15 million years, most of the Waitakere Volcano has been eroded down to a stump out beneath the Tasman Sea. Sand carried down to the west coast by an ancient Waikato River has been thrown up over the eroded surface on the Waitakere Volcano rocks and can be seen forming the upper two thirds of the sloping cliffs behind O'Neill Bay and the top of Erangi Pt. Within this sequence of windblown sands there are several white layers of wind-blown rhyolitic ash that was blown here from eruptions in the centre of the North Island about 1 million years ago.
HAVE A LOOK!
Can you see evidence of strong sea floor currents in the volcanic conglomerate in the cliffs in the middle of O'Neill Bay?
Can you see the lava flows that fill the crater at the north end of O'Neill Bay?
How many white ash layers can you see in the sandstones forming the slopes above O'Neill Bay?
Park in the Bethells Beach carpark beside the surf club at the end of Bethells Beach Rd.
Only attempt this trip if the tide is within 3-4 hours of low tide.
Do not attempt to cross the Waitakere River if it is in flood or the tide is high or the water more than 25 cm deep.
Be careful of large waves surging up onto the rocks, beach and especially surging into the Erangi Pt sea tunnels.
From the carpark walk down to the beach. Do not attempt to cross the Waitakere River when in flood or when the tide is high. Only accessible when tide is within 3-4 hours of low tide. On the beach choose a shallow place to cross the Waitakere River (usually 10-25 cm deep) and walk further up the beach to the rocks at the north end. Later take the track over the sand dunes on the point over to O'Neill Bay. Descend over the wooden stile to the beach and walk up the beach to the rocks at the north end. 2-3 hours return.