Beach gravel on sandstone, Munro Beach, South Westland

Munro Beach lies near Lake Moeraki in South Westland and, like many West Coast beaches, is a rugged and isolated place where swells boom against a steep gravelly foreshore studded with numerous rock stacks and outcrops. Relentless wave action over many years has given rise to exquisitely-shaped sandstone outcrops at the base of the cliffs, shaped by the grinding action of wave-borne beach gravel during each high tide. The beach gravel here is especially abrasive due to its high content of hard white quartz pebbles, brought down to the coast by the numerous rivers that drain westward from the Southern Alps.

As its name suggests, sandstone is stone formed from sand, originally deposited by river or coastal currents, or, in the case of sand dunes, wind. The sandstone that underlies the cliffs here is a typical example that clearly shows its sedimentary origins, its finely layered texture and embedded flow patterns suggesting that this ‘fossilised’ sand probably once formed part of an ancient river bed. Now that it has been exposed by erosion this sandstone is gradually being dismantled again, grain by grain, by the sea. Some of it may well end up in a new sedimentary deposit somewhere on the sea floor nearby, where it may once again end up as part of a new body of sandstone – just as some of its grains are likely to have completed a similar cycle in the distant past.