A forest of stone
In Nambung National Park, about 200 kilometers north of Perth, thousands of limestone pillars rise from yellow sand. This is the Pinnacles Desert. The pillars vary in shape and size, with some reaching up to 3.5 meters in height. They are composed of aeolianite, a type of limestone formed from wind-blown, lime-rich sands derived from ancient sea shells. This material is part of a larger geological formation called the Tamala Limestone, which was deposited during the Pleistocene Epoch. While the age is debated, formation likely began around 25,000 to 30,000 years ago. The exact process that sculpted these thousands of individual pillars from a bed of limestone is a subject of scientific debate, with three main hypotheses competing for acceptance.
Competing explanations
The first major theory centers on plant roots. In this scenario, ancient vegetation stabilized the sand dunes. As plants died, their root systems created channels deep into the limestone. Water percolated through these channels, precipitating hard calcrete, a type of cemented calcium carbonate, around the decaying root structures. Wind erosion later stripped away the softer, surrounding limestone, leaving the more resistant, root-cast pillars standing. This explanation connects the Pinnacles' formation to a time when the area had a more humid climate and greater vegetation cover.
A second hypothesis proposes a process of extensive solutional weathering, or karstification. This model suggests that a hard caprock of calcrete formed over the limestone. Acidic rainwater then seeped through cracks in this caprock, dissolving the softer limestone below and carving out channels and depressions called solution pipes. The pinnacles are the more resistant remnants left behind after this widespread subsurface erosion. Many pinnacles have harder caps, which are interpreted as remnants of this original protective layer that slowed the erosion of the column beneath.
A third, more complex theory suggests the pinnacles formed in place through soil-forming processes without requiring either ancient tree roots or a uniform caprock. In this view, cycles of wetting and drying caused calcium carbonate to move and re-precipitate within the soil profile, cementing columns of sand which were later exposed by erosion. All three theories acknowledge the foundational role of the lime-rich sand and the subsequent removal of surrounding material by wind and water to expose the pillars we see today.