The Pinnacles

These pinnacles found north of Perth in Western Australia are approx 1 million years old.  They are limestone pillars which are a sedimentary rock.  They are up to 31/2 metres tall but most are between 1-2 metres tall.  They are made from Tamala limestone which is the same rock that spreads along the South-West coastline of Western Australia.

 During the Pleistocene period waves and wind brought ashore quartz, lime and sand that formed dunes. The grains were then cemented together by calcium carbonate.  Then  leached from the lime sand by rainwater sieving down through it turning the dunes into limestone.

Quartz, lime and sand gets blown in from the ocean.

Plants take root in the sandy soil.  They die along with animals which helps more plants to grow.  The roots of the plants stabilize the surface of the soil and an acidic (acid base) layer of soil develops in the rotten soil.

Now the soil is acidic it speeds up the process of breaking up and dispersing the softer parts of the limestone.  Over a long period of time harder pinnacles of limestone, which is surrounded by the tough quartz sand, remains.

Soft limestone weathers and erodes and is taken away by water running down them, leaving behind quartz sand that wouldn't dissolve.  Eventually only the most resilient columns of limestone remained.  Bush fires and wind carried the plants away leaving the limestone pillars to stand on their own.  Today no plants are growing on top of the pinnacles. 

The pillars of limestone were probably exposed to standing alone 6,000 years ago, and then covered up by shifting sands, before being exposed again in the last few hundred years.  This can still be seen going on today as the pinnacles in the northern part of this area are uncovering due to the southerly winds, but the pinnacles in the southern end of this area are being covering.

Fossilised oval pupal cases of a weevil are commonly found in limestone pillars. (Not shown here).  This is a close up of a pinnacle today. 

If you are visiting the Pinnacles please drop in to the stromatolites at Lake Thetis at the Cerventes. 
 They are very close by, and there are no entrance fees here.

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