Putangirua Pinnacles are at the head of Putangirua Stream, on the Whatarangi Road to Cape Palliser, 6 miles south-east of Lake Ferry, Wairarapa Valley. Fossiliferous muddy sandstones and siltstones of late Miocene age are exposed in the lower course of the Putangirua Stream, and up stream from them (1 1/3 miles up stream from Whatarangi Road) are thick gravel deposits, called the Putangirua Conglomerate. These actually underlie the fossiliferous strata, but appear up stream because both beds have been tilted in a down-stream direction. The same fossiliferous beds occur in Hurupi Stream (next stream towards Lake Ferry), but the gravels are not well exposed.
Though the gravels are of late Miocene age (about 12–14 million years ago), they are soft and easily eroded and the Putangirua Stream has excavated headwards to form a series of deep gullies (“badlands”) down which rain wash is funnelled vertically. The soft matrix of the gravels is readily removed by the rain wash, but where large boulders occur they shed the water and protect the gravel immediately underlying. After many years of such essentially vertical erosion a series of pinnacles (“earth pillars”) has been produced, each with a capping boulder or local patch of hardened gravel. Many of the present-day pillars have lost their capping rocks and are probably being actively eroded.
by Graeme Roy Stevens, M.SC.(N.Z.), PH.D.(CANTAB.), Paleontologist, New Zealand Geological Survey, Lower Hutt.