How the Pinnacles Formed

Thousands of limestone pillars standing in a flat sand plain is not a normal thing for a landscape to do, and the question every visitor asks at the car park is how they got there. The park authority publishes the answer in detail, and it is a sequence rather than an event. Guided tours of the Pinnacles Desert include a stop at the Discovery Centre where the same story is told with the rock in front of you.

The stone is old seashell

Start with what the pillars are made of, because it is not what most people guess. They are limestone, and the limestone came from the sea — shell debris from marine organisms, ground down and blown inland by wind to form high mobile dunes. Nothing about the material is volcanic and nothing about it was pushed up from below. It arrived, one grain at a time, on the wind.

Tamala Limestone, and how it got inland

The unit has a name: the Tamala Limestone, deposited across the middle to late Pleistocene, 1,500,000 to 10,000 years ago. It is one of three sand and limestone systems that run parallel to this coast, each laid down seaward of the last as the sea moved. The Pinnacles sit in the middle one, in the yellow Spearwood sands, and the same limestone runs for hundreds of kilometres north and south of here under the coastal plain.

Stage one — leaching, and a hard grey lid

Rain fell on the dunes. Soil formed over the remaining quartz sand and turned acidic, which accelerated the leaching of the limestone below it. As that water moved down it redeposited calcium carbonate, and a hard layer of calcrete set over the softer limestone underneath.

That grey cap is still there, and it is visible on many of the pillars today as a distinctly different-looking crown on top of a paler column. It is the single most important thing in the whole sequence: without a hard lid, everything below it would have gone.

Stage two — roots cut the channels

Channels formed in the calcrete, and plant roots exploited them. Water followed the roots down through those channels, leached away the softer limestone beneath, and the channels themselves gradually filled with quartz sand. Where a root had been, a pipe of sand now ran.

This is the stage that decided which pillars exist. Subsurface erosion carried on underground, hollowing out everything the water could reach, until only the most resilient columns of cemented limestone were left standing inside a body of loose sand.

Stage three — the wind takes the sand away

The pillars were already there, buried. What produced the landscape you drive through is the last step: the prevailing winds blew the overlying quartz sand away and exposed them. The desert is not a place where something was built. It is a place where everything soft was removed and the hard parts were left standing.

That also explains why the field has no named individual formations, no summit and no single famous rock. There is no one pillar because there was never one event — there is a whole plain of them, at every stage of exposure.

The sequence in one table

Rhizoliths: the process caught halfway

In the sand between the pillars there are rhizoliths — fossilised plant roots, cemented hard by the same process. They are low, knobbled and easy to walk straight past, and they are the most interesting thing in the field, because they show the first stage of pinnacle formation still in progress.

How old are the Pinnacles?

The honest answer has two halves. The limestone unit is Pleistocene, deposited between about 1.5 million and 10,000 years ago, and that is the park authority’s own dating. The pillars — the columns as you see them now, standing clear of the sand — are much younger than the stone they are made of, and the figure widely published by Australia’s Coral Coast, the regional tourism authority, is 25,000 to 30,000 years.

Those two numbers are not in conflict. One dates the material and one dates the exposure, and the exposure is still happening. Sand still moves across the field, and pillars are still being uncovered and reburied.

Why they are still changing

Nothing here is finished. Wind still moves the yellow sand, water still works on the softer limestone, and the calcrete caps still fail eventually. A field photographed in 1960 and photographed today is not identical, and the tallest pillars, at up to 4 metres, are simply the ones whose caps have held longest.

The people who worked it out

The Pinnacles remained relatively unknown to non-Aboriginal Western Australia until the late 1960s, when the state agreed to establish a national park over the area; the reserve specifically for the “preservation of natural formations — Pinnacles” was gazetted in 1967. The mechanism above is set out in the park’s management plan and draws on published work by the palaeontologist Ken McNamara.

The country itself is much older ground: the Nambung area was frequented by the Whadjuk and Yued people of the Noongar cultural group, and the park takes its name from the Nambung River, from a word that possibly means crooked or winding. Dutch charts recorded the hummocks on this coast as early as 1658, and one account has the mariners taking the field for a small city a short distance inland.

The published figures behind all of this — the dating, the park’s area, the height of the pillars and the sources for each — are collected on the facts page, and things to do sets out where in the field to look for the calcrete caps and the root casts. Pinnacles Tours lists every departure that stops here.