Maerewhenua Rattle Stones

The rattle stones come from Bushey creek which flows into the Maerewhenua River.

Rattle stones are partly hollow concretions, made where one or more spheroidal shells, usually enriched in rusty-coloured iron oxide, have formed around a central nucleus during weathering of rocks. This may happen in a sandstone made up of grains of quartz, which is insoluble, held together by a cement of calcite (calcium carbonate), which is slowly soluble in water. In time, continuing weathering may leach out the calcite, leaving the remaining sand grains and any iron-oxide-enriched bands inside the outer shell detached and able to rattle.

Rattle stones are not very common, but are well known world-wide. Here in Otago quartz sandstones are found in many places overlying the schists, but usually they lack a soluble cement. Conditions were evidently right, however, at Danseys Pass. Chemists speak of Liesgang rings, which are formed by rhythmic precipitation within some material saturated with fluid. The concretionary layers in rattle stones were formed by the same sort of phenomenon. Weathering of rocks has released iron into solution diffusing through the sandstone. The iron ions in turn became oxidised to form insoluble iron oxide and are precipitated to form concretionary layers.