The Salt That Built a Spine of Ironstone: Western Australia's Fortescue Marsh Banded Iron

A hundred kilometres south of Port Hedland, the earth turns red. Not the red of soil, but the red of rust—layer upon layer of iron oxide exposed to air for two and a half billion years. The Fortescue Marsh lies at the centre of this landscape, a saltpan the size of Sydney that fills with water only once a decade, ringed by ridges of banded ironstone that contain more iron than any other deposit on Earth.

The Salt That Shapes Stone

The Hamersley Range is famous for its banded iron formations—alternating layers of chert and hematite that precipitated from ancient seawater when cyanobacteria first flooded the atmosphere with oxygen. But the ridges that surround the Fortescue Marsh tell a different story. Here, the iron formations have been sculpted by salt.

Groundwater rising through the rock dissolves iron-bearing minerals at depth. When it reaches the surface, evaporation leaves behind a crust of salt and iron oxide. The salt crystals grow, wedge into cracks, and pry apart the hardest stone. This process—salt weathering—has been grinding down the ridges for millions of years, creating a landscape of shattered rock pavements and angular boulders.

The ironstone ridges themselves act as dams. They trap sediment and groundwater in the basin behind them, creating a saltpan that accumulates evaporite minerals over tens of thousands of years. The salt crust on the marsh is up to two metres thick in places, underlain by brine that is ten times saltier than seawater.

A Billion-Year Battery

The banded iron formations of the Fortescue Marsh region are part of the Brockman Iron Formation, deposited between 2.5 and 2.4 billion years ago. Each centimetre of banded iron represents roughly a thousand years of chemical precipitation. The entire formation is more than 300 metres thick.

The salt that now breaks the ironstone once fell from a sky that had never seen rain—it was locked in the ocean until the first continents rose and the water learned to cycle.

When the cyanobacteria pumped oxygen into the atmosphere, the dissolved iron in the oceans reacted instantly, precipitating out as rust and settling to the seafloor in rhythmic bands. The process stopped once the free iron was exhausted, leaving behind a billion-year battery of stored oxygen. The salt on the marsh today carries the signature of those ancient seas—the same sodium and chlorine that once floated in the Archaean ocean.

The ridges have been exposed and re-exposed multiple times over the eons. Each time the continent rose, the salt worked deeper into the rock. Each time the climate dried, the salt crust thickened.

The Marsh That Remembers

When rain does come to the Fortescue Marsh—maybe three or four times a decade—the saltpan transforms into a shallow lake that stretches 80 kilometres from end to end. The water dissolves the salt crust, and the brine teems with microscopic algae that can tolerate salinities ten times that of the ocean.

Banded stilts fly thousands of kilometres to breed here during these rare flood events. The birds arrive within days of the water, build nests on the salt crust, and raise their chicks before the lake evaporates again. The entire cycle—flood, breeding, evaporation—takes less than six months.

When the water disappears, the salt reforms. New crystals grow in the cracks of the ironstone ridges, continuing the slow work of breaking the hardest rock on Earth. The process has no end. The salt will keep rising, keep splitting, keep sculpting, until the ridges themselves are ground to dust and washed back to the sea from which they came.

The Fortescue Marsh is not a static landscape. It is a machine that runs on salt and time, turning billion-year-old iron into a surface that shifts with every flood and every drought. The red ridges stand guard around it, slowly crumbling, returning their iron to the cycle that first brought it out of the Archaean ocean.