These pinnacles are formed from the soft gravels of the area. They are 12-14 million years old, and over the millennia, the river Putangirua has eroded deep gullies ('badlands) down which the rain pours, washing away the gravel. Large boulders shed the water, and protect the gravel immediately below them, creating these pinnacles. When they lose the capping rock (as many have), they erode quickly.
I climbed up the 'bush track', about a n hour of fairly steep going, climbing maybe 100m. At the top a look out has been built off the cliff, providing this splendid view.