Fossil turret shells in sandstone, Palliser Bay, Wairarapa

Turret shells are commonly found around New Zealand’s coast, preferring fine sandy environments on the sea floor below the reach of violent wave action. Under favourable conditions these small gastropods can reach densities of several thousand per square metre, their shells making up a considerable proportion of the sea floor itself. They are also often found fossilised in sandstone, a sedimentary rock formed from sea floor sand that was buried millions of years ago and eventually compressed into solid rock. Over many thousands of years this rock was then raised above sea level by movements along active fault lines, eventually being re-eroded from coastal cliffs and gullies so that blocks of this shell-rich stone can now be found on present-day beaches.

The sandstone in this photograph was found on the Palliser Bay coast, east of Wellington, where most of the local cliffs are made up of soft sedimentary rocks. Sandstone layers are common here, some of them containing many thousands of fossilised turret shells. As these rocks are eroded away by wave action the shells are also gradually worn away, showing perfect cross-sections of their spiralling, sand-filled interiors.