The Gap, South Piha

The cliffs of South Piha have been eroded by the sea out of layered volcanic conglomerate (Piha Formation). This conglomerate was deposited on the upper submarine slopes of the growing Waitakere Volcano about 19-18 million years ago. Lenses of current-swept sandstone show that strong bottom currents swung around the volcano and moved fine sediment along on the sea floor.
Several million years after the conglomerate was deposited, buried by more sediment and hardened into rock, it was intruded by sheets of molten andesite magma. This magma pushed its way upwards along planar joints - some magma may have erupted at the surface as lava flows and the magma left in the joints cooled and solidified as jointed dikes. Dikes can be seen at the Taitomo Island sea tunnel and two cut across The Gap. At The Gap, one dike is younger than the other and has intruded right through the older dike and displaced it slightly on one side. Over 15 million years since these rocks were deposited and the magma injected, the Waitakere Volcano has been progressively eroded down to what we see today in the Waitakere Ranges.
In recent times (last 7000 years) sea waves have pounded the cliffs and eroded along planes of weakness such as joint planes. This has created narrow sea caves and at The Blowhole (to the south of The Gap) one of these caves has eroded right through the ridge forming a sea tunnel which the small stream flows into. Another more prominent sea tunnel has been eroded right through Taitomo Island and is used by fishermen and surfies to get access to the exposed side of the island. Here the tunnel eroded along a narrow planar dike of jointed andesite. Waves coming into the growing sea cave compressed air that pushed its way into the joints of the dike and loosened them, until blocks of the hard andesite dike dropped out and progessively the tunnel was excavated.
In some places the conglomerate is cut through by prominent planar joints. These are planes of weakness. On the northeast side of Taitomo Island, sea erosion had undercut the cliff and large blocks of conglomerate above, gave way along the joint plane and slid down to collect in a heap at the foot of the planar cliff face.
Tafoni is the result of a kind of erosion that produces honeycomb depressions on cliff faces well above the waves. You can see some on the cliffs below Tasman Lookout. Here salt spray wets the rock and when it dries salt crystals grow and loosen sand grains in the rock which are blown away by the wind. Over many years these depressions erode back into sizeable caverns.

Can you see the jointed dikes of solid andesite cutting through the conglomerate? Can you see which of the two at The Gap is the younger one?
Look around for examples of honeycomb weathering (tafoni) high in the cliffs.
Can you find the track that takes you south to The Blowhole?
If the tide is low, walk one way around the foot of the cliffs to The Gap and return the other way along the track that goes up and over Tasman Lookout.