After years of puzzling, geologist Bruce Hayward reckons he’s solved the mysterious sandstone swirls in a road cutting on Auckland’s west coast. They’re moa footprints.

Photo: RNZ / Alison Ballance

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Bruce believes the rocks beneath our feet are a book to be read. And for him, a road cutting is a beautiful cross section.

“A lot of the insights that geologists have into the structure of the rocks can be seen in road cuttings. They’re not seen on the surface of the countryside.

“Where the road has cut through we can see a slice through the rocks and may be able to read some of the history of what’s gone on here.”

A closer look at a road cutting

For an astute observer like Bruce, it’s the small details that tell powerful stories.

So what does he see in what appears to a very typical Auckland road cutting?

Bruce identifies the rock as soft sandstone, laid down as a coastal belt of sand dunes between one to one and half million years ago. It is white and orange in colour, from the white quartz sands and ‘rusted’ black titomagnetite sands that are common on the coast.

The lower part of the cutting has cross-bedding, which is the front of the sand dune as it was advancing. “The sand grains rolled down the front.”

On top of that cross-bedding there is a lens about two or three metres thick that contains horizontally laminated sands, which Bruce says have clearly been deposited in water.

His interpretation is that this was the edge of a small dune lake, and there is clear evidence of ripples that formed in the sand in response to wind blowing across the water.

Photo: Supplied / Wikimedia, John Megahan

The moa footprints

What has fascinated Bruce for more than 20 years are small disturbed areas in the cutting where the stripy layers deform and swirl.

Bruce points out that they would have been formed by something heavy pushing down and compressing soft damp sand at the edge of the lake.

“Something’s come along and pushed down on the layers of sediment. The depression that was created has been preserved and then whatever caused it has been lifted out.”

Bruce’s eureka moment came a few years ago when geologist Greg Browne from GNS Science published a paper detailing his discovery of dinosaur footprints in much older rocks on the northwest coast of the South Island.

“In cross-section they look very like what we’re seeing here.”

Bruce sent photos of his possible footprints to Greg who immediately agreed that they were indeed footprints, although far too young to be dinosaurs.

Photo: RNZ / Alison Ballance

Instead, Bruce thinks they were made by the largest creature living at the time – the now extinct giant moa.

“It clearly had to be a big heavy bird, and although I can’t prove it was a moa, that’s most likely.”

He imagines a moa coming to drink at the lake edge and its feet sinking into the moist sand. These footprints were immediately filled in by further layers of sand which preserved their shape.

Other records of moa footprints

Bruce says these are not the first moa footprints to be discovered. There are about ten other records found in mud and ash deposits, “but they’re all seen in plan view [from above] so you can see the three toes.”

“Many of these records were found a century or more ago, and some were dug out and collected.” Some of them have already eroded away.”

But because no one had ever seen a moa footprint as a cross-section, Bruce says that although the road cutting is 50 or 60 years old, “no one had recognised that it was really special.”

To protect the moa footprints, Bruce won’t say exactly where the road cutting is. Several footprints have already been ruined, and he’s anxious to preserve the rest, even as natural erosion and algae slowly obscure some of them. He’s also got his fingers crossed that there won’t be any more damage from people carving graffiti into the soft sandstone.

Bruce is an independent geologist at Geomarine Research, and is the author of several books, including Volcanoes of Auckland (2011).

He has featured on Our Changing World previously talking about Auckland’s buried volcanic forest and the discovery of four previously unrecognised volcanoes in Auckland.

Bruce’s latest book has just been published by the Geoscience Society of New Zealand. Out of the Ocean, Into the Fire: the history in the rocks, fossils and landforms of Auckland, Northland and Coromandel (2017).