The many layers of sediment laid down in the Whanganui Basin record three to four million years of geological history, attracting fossickers and fossil hunters alike, Emeritus scientist Dr Hamish Campbell tells Laurel Stowell.
Dr Hamish Campbell is speaking to a group of about 30 who are looking west from the top of Whanganui's Durie Hill.
"I hope you like that you are sitting on top of a pile of very young sediments," he says.
It's a field trip organised by Whanganui Regional Museum and Campbell's talk the previous night drew 153 to the museum lecture theatre.
He's asking people to look at the landscape through a lens of geological time.
What's visible is at least four or five flat-topped terraces, visually complicated by trees and houses.
Each rises to the next with what was once sea cliff. In total there are about 13 of them within 20km of the current sea coast, Campbell says.
They were made, long ago, by the sea.
"You need a process to grind any topography flat. The top 40m of the sea where the wave action is a great flattening machine."
In geological terms, the city is in what's called the Whanganui Basin.
It is oval in shape, 100km wide and 180km long. Half of it is onshore and the other half is under water. It is all less than five million years old - young in geological terms.
Its base is greywacke, a hard old sedimentary rock. The basin is slowly sinking or sagging, because of a buckling action as the heavier Pacific tectonic plate is pushed under the Australian plate.
"The crust is actually being drawn down and creating a dish-like structure. It's a place for sediment to accumulate."
The dish or basin is filling up as fast as it is pulled down. The filling is by rivers adding more layers of sediment that washes off the higher ground to the east. The ground is higher because it is being pushed up by the same collision of tectonic plates.
The eastern edge of the basin is the North Island's axial ranges, the Ruahines, Kawekas and Kaimanawas.
All the rivers drain from the higher east toward the west. The Whanganui River has followed the same course for a long time, Campbell says, probably along a fault line.
"This river is capable of enormous floods."
There are about 5000 metres of sediment caught in the basin, and the first were laid down well before the volcanoes of the central plateau rose.
As the basin sank and the layers built up there were a series of ice ages.
During them much of the earth's water was frozen into ice and sea levels dropped so far that, if there had been humans here, they could have walked from Whanganui to Nelson.
The ice ages were cold and windy, the rivers carried huge loads of sediment toward the sea and the winds blew huge quantities of it back inland in dunes and drifts.
After Durie Hill, the field trip moved to Ototoka Beach, its sandy floor the marine terrace that is forming now. Above it is the Rapanui Terrace, a surface that is 120,000 years old.
The cliffs at Ototoka are made up of layers and layers of sediment, some containing peddles, rocks or shells and other fossilised sea life. It's like cutting into a very complex layer cake.
Under the beach sand are about 3000 metres of older sediment layers.
The basin contains about 56 sequences of sediment, Campbell says. Each starts with a shell bed, buried and sealed into place when a flood brings down a big load of sediment.
The first sign of volcanic influence in the cliff comes 1.7 million years ago, when Taupō erupted in 232 AD and 55 cubic kilometres of silica-rich material was ejected.
The crater of Taupō is the hole filled by Lake Taupō. It's the most dangerous volcano on the planet, Campbell says. It erupts about once every 1000 years.
"It's a high-pressure system waiting to burst. Then it will be all on and it will be messy, but we have no idea how big it's going to be."
The most likely trigger for an eruption will be an earthquake, which Campbells calls "the snapping of solids" and allows hot rock under the earth to emerge.
The Taranaki volcano is much more recent. It's only 7000 years old and last erupted in 1755. Its lava comes from deeper in the earth and has a lower silica content. It makes for the rich Taranaki volcanic loams.
The orange layers in the Ototoka cliff form when iron-bearing minerals are washed down through porous soil and react with oxygen, forming "a natural rust". Thick layers of this can become glued together, usually through biological action in a swamp, and become a rock-like ironstone.
Ironstone is one of the few things that resembles rock in our basin. The other is what we call shellrock, a mass of shells glued together by "a natural cement". It is the calcium carbonate they contain, and they are preserved by burying. If the rock is more than 50 per cent calcium carbonate it is technically a limestone.
Talk of rock brought on a question: what is it exactly?
To know that, you have to understand chemistry, Campbell says.
He recommends Nick Mortimer's "brilliant" introduction to A Photographic Guide to the Rocks and Minerals of New Zealand, a book he wrote with Margaret Low.
Rocks can contain any amount of the 94 natural elements in the periodic table. But they are mostly made out of oxygen, Campbell said.
The main rocks of the earth's crust are more than 75 per cent silica. Basalt rock comes from deeper under the earth and has less than 52 per cent silica.
It's heavier, and makes up much of the oceanic crust.
"The best soils in New Zealand are basalt soils, like at Pukekohe and Kerikeri," Campbell says.
The type of rock formed by an eruption depends on what substances are erupted.
"Every time there's a volcanic eruption it's a new batch, like cooking."
Asked about pakohe or argillite, the hard black stone found in parts of the Whanganui River, Campbell says it was a mudstone or siltstone, squashed and baked for a very long time by a body of really hot rock.
"It's essentially a natural ceramic."
Asked about the totara stumps that still stand in the Waitōtara River, Campbell says the trees would have grown sometime in the last tens of thousands of years, when the sea level was lower. They have been preserved until now by burying.
"To get preservation you do need to eliminate the drivers of decay. Number one is oxygen, but also flowing water, bacteria and radiation."
The many layers of the Whanganui Basin were mapped in the 1940s by Charles Fleming, and described in his 1953 book The Geology of Wanganui Subdivision.
Fleming was later knighted and worked at the New Zealand Geological Survey, now GNS Science. Campbell knew him for a decade, and got his job after he retired in 1977.
A palaeontologist, Campbell studies fossils and how they relate to evolution. His specialty is the Permian and Triassic periods, between 299 million and 201 million years ago.
His great-great-great-grandfather, Robert Campbell, came to Whanganui in 1847 with the 65th Regiment. He retains a family link with the district.