Category: Whanganui

Picking Up Stones While Walking the Puppy: Kai Iwi Beach

I have been back home in Whanganui over the past three weeks (late February to mid-March). During this time, I often visited the nearby Kai Iwi Beach to walk our new puppy, Jasper.

Following Jasper down to the bridge over the Kai Iwi Stream.
On the beach north of the Kai Iwi Stream. Notice how few stones there are!
South of the Kai Iwi Stream.
Small stack of harder clay in the distance.
At a stack even further south.
Jasper on Kai Iwi Beach.
The wet Kai Iwi Beach stones from a 4lb barrel after a 220 grit tumble. The reddish-pink reflections are from the shirt I was wearing when I took the photo (which was taken indoors).

Kai Iwi Beach is well known for its fossils. However, as a tumble polisher of stones, I am more interested in smooth-worn beach pebbles than fossils. Though there are very few stones on the beach, I usually managed to collect a few each walk. Most are what I think are iron-stained quartz, though there is quite a variation among them. I managed to put some through a 220 grit stage in a 4lb barrel (9 days tumbling). This Post features 12 of the stones from that tumble. Some context and background on Kai Iwi Beach and its stones can be found in this TumbleStone Post.

The first two stones appear to be breccia:

Breccia stone, Kai Iwi Beach.
Other side of stone.
Another breccia stone?
Other side of stone.

Four of the mainly orange hued stones:

Interesting texture.
This stone has the small clear pieces of quartz I often find in quartzite stones on other beaches.

Six stones with darker patterns:

Some fine black veins.
Upper half dark.
Suggestion of dark waves.

Thank you, Jasper, for the fossicking opportunities!

“K” is for “Kai Iwi Beach Stone” and “L” is for “Leithfield Beach Petrified Wood”

F is for Fossil Worm Cast
G is for Green Hydrogrossular
H is for Hematite Jasper
I is for Ichnogenus Protovirgularia
J is for (Picture) Jasper

The following are my Posts for “K” and “L” in the alphabetical series of a Facebook Group I belong to, “New Zealand Lapidary, Rocks, Minerals, Fossils”. The first Posts in this Series can be found here.

“K” is for “Kai Iwi Beach Stone” – This beach, about two kilometres from my home, is probably named after the Kai Iwi Stream that emerges here at the coast, a few kilometres north of Whanganui. Kai Iwi Beach is at the small town of Mowhanau, site of the Kai Iwi Beach Holiday Park run by Bruce Taylor (another Group member) and his wife. There are usually not a lot of stones on this beach but I have occasionally been collecting mainly iron-stained quartz to tumble-polish, discovering some nice-looking pebbles, like this one.

K is for Kai Iwi Beach Stone
https://www.google.co.nz/…/@-39.8853384…/data=!3m1!1e3
Kai Iwi Beach, looking north/north-west from near the stream mouth.
Kai Iwi Stream mouth, with pedestrian bridge. Often quite a few stones can be found downstream of the bridge.
Looking back at the pedestrian bridge.
Along the cliffs to the south.
Typical scattering of stones on the beach.
The cliffs to the south.

For more on the Kai Iwi Beach stones I have tumble-polished and more about the beach, see the previous TumbleStone Post here.

“L” is for “Leithfield Beach petrified wood”. I wrote in a post on this Facebook Group on 26 August 2020: “Yesterday I visited Leithfield Beach, just north of Christchurch, for the first time, mainly because at least one person had posted in this Group recently that he had found petrified wood there. I have found very few specimens of petrified wood in my fossicking career and have never been sure of its identification. So I walked Leithfield Beach for a couple of hours while rain showers came across. I was delighted to find five pieces of petrified wood that were obviously petrified wood because they actually looked like wood!”

L is for Leithfield Beach petrified wood
Photo of the piece on the beach when found, taken with my phone.

As Jocelyn Thornton wrote in “Gemstones”, wood can be turned to stone when it is buried in waterlogged sediments carrying dissolved minerals which soak into the wood and replace the organic material. The wood’s cell walls usually act as a “template” for the mineralisation, retaining the wood-like look.

Leithfield Beach, google maps. I park at the north end of the town when making a fossicking visit.
Looking south from the car park I use.
This old storm-water pipe was recently being dismantled. I like to walk down to that area.
Looking north from the car park.
The stones of Leithfield Beach.
Jocelyn Thornton’s introductory piece on petrified wood. The numbers refer to photos – see next pic.
The photo of specimens for Jocelyn Thornton’s introductory piece on petrified wood.

I have yet to repeat my fossicking success of that day in relation to petrified wood.

See here for the next Post in this Series, and here for the Series Index.

The Iron-Stained Stones of Kai-Iwi Beach

Google Maps view of Mowhanau Village (centre), showing Kai-Iwi Stream.
Kai-Iwi Beach, just beyond the Kai-Iwi Stream mouth.
Mobile phone photo of sunset at Kai-Iwi Beach, taken at the end of a stone collecting visit.

I moved to Whanganui from Cambridge just over six months ago. I now live quite close to the seaside village of Mowhanau situated on Kai-Iwi Beach. I have walked on the beach a few times, especially near the Kai-Iwi Stream mouth. I have not been particularly impressed with the few stones to be found amidst the scattering of stones and shells and other bits and pieces in the sand. They seemed to originate in a layer in the cliffs, and some were maybe being brought down by the stream.

Mowhanau Village.
The Kai-Iwi Stream mouth and pedestrian footbridge.
An area for fossicking for stones.
Kai-Iwi Beach, just along from the Kai-Iwi Stream mouth.
The crumbling cliffs.
Stones from a layer in the cliffs.
The scattering of stones on the beach.

Then I took a closer look at the small brown (iron-stained) stones lying here and there on the beach and in the stream. On this closer inspection, they looked like they might repay tumbling as there were hints of patterns within them and they appeared to be made up mainly of quartz, which polishes well. It took three weeks to tumble the first batch. The results surprised me. Here are four of the 60 polished stones.

Washed Kai-Iwi Beach stones, ready to be tumbled.
The 60 tumble-polished Kai_Iwi Beach stones.
Stone 2, side B.
Stone 6, side A.
Stone 7, side A.
Stone 1, side A.

To take a closer look at two of these stones which have very different colours and patterns:

Stone 1, side A.
Detail of Stone 1, side A.
Detail of Stone 1, side A.
Stone 1, side B.
Detail of Stone 1, side B.
Detail of Stone 1, side B.
Stone 2, side A.
Detail of Stone 2, side A.
Stone 2, side B.
Detail of Stone 2, side B.

Here are three stones which are practically orange-coloured:

Stone 3, side A.
Detail of Stone 3, side A.
Detail of Stone 3, side A.
Stone 3, side B.
Detail of Stone 3, side B.
Stone 4, side A.
Detail of Stone 4, side A.
Stone 4, side B.
Detail of Stone 4, side B.
Stone 4, side A.
Detail of Stone 4, side A.
Stone 4, side B.
Detail of Stone 4, side B.

Three Kai-Iwi Beach polished stones of complex construction:

Stone 6, side A.
Detail of Stone 6, side A.
Stone 6, side B.
Detail of Stone 6, side B.
Stone 7, side A.
Detail of Stone 7, side A.
Stone 7, side B, a much darker colour.
Detail of Stone 7, side B.

Some of the darker-coloured stones:

And, to finish off this part of the Post, three of the lighter-coloured stones:

Stone 8, side A.
Detail of Stone 8, side A.
Stone 8, side B, with a dark intrusion.
Detail of Stone 8, side B.

What is the source of these stones? The following is a brief report on my initial investigations. Kai-Iwi Beach is geologically part of the Whanganui Basin. This Basin has been described in “Landscape and Quaternary Environmental Change in New Zealand” (2016) as “a unique global archive”, unique because of the way it reveals “a shallow marine basinal sequence, exposed on land, which spans the entire Quaternary” (the last 2.6 million years). The cliffs and marine terraces along the coast from the mouth of the Whanganui River to the mouth of the Patea River expose a series of “sequences” of geological layering.

Source: https://link.springer.com/chapter/10.2991/978-94-6239-237-3_4
The Mowhanau context. Source: https://ref.coastalrestorationtrust.org.nz/site/assets/files/6757/stratigraphy_of_the_castlecliffian_type_section_10_mid_pleistocene_sequences_from_the_wanganui_coast_new_zealand.pdf
Broad analysis of cliffs northwest of Mowhanau. Source: https://ref.coastalrestorationtrust.org.nz/site/assets/files/6757/stratigraphy_of_the_castlecliffian_type_section_10_mid_pleistocene_sequences_from_the_wanganui_coast_new_zealand.pdf

There are three parts to each sequence, described in a technical manner in a 1999 article in the New Zealand Journal of Geology and Geophysics as follows: The first part is “a basal suite of shore face and inner shelf sediments with intertidal and shallow subtidal molluscan faunas, and cross-bedded, pebbly shell gravels” – it is likely that the stones I collected and polished came from this kind of layer. The second part of each sequence is typically “a shellbed, which contains in situ offshore molluscs in a matrix of muddy fine sandstone or fine sandy siltstone”, with the third part being “siltstone, either bedded and barren of fossils, or bioturbated and with a sparsely scattered in situ fauna”. In other words, there are lots of layers here, many of which contain fossil shells and a scattering of stones.

The beaches along this coastline are often littered with rocks and stones that contain many fossil shellfish. A recent visit to the wild and secluded Ototoka Beach, not far from Kai-Iwi Beach, resulted in many fossil shell finds.

The way down to Ototoka Beach from the carpark – Petra is on the bridge.
Below the waterfall is a cliff full of fossil shells.
Ototoka Beach on a grey day.
Cliff along Ototoka Beach.
A spill of rocks from the cliffs – large boulders full of fossil shells. Petra is in the distance.
This boulder is about a metre long.
Layers in the rocks.
Fossil shell.
Some of the fossil shells I collected.

See here for a two minute YouTube video on Kai-Iwi Beach – the first part is on the more popular part of the beach a few hundred metres to the southwest of the Kai-Iwi Stream mouth. At 1 minute 15 seconds, the Kai-Iwi Stream is shown, then the beach past the stream mouth. 

This YouTube video features drone footage of Kai-Iwi Beach. Most of the first part of the video is of the beach at Mowhanau Village but at 2 minutes 4 seconds it goes up the beach past Kai-Iwi Stream, the area in which I found most of these stones, and it shows more detail of the stream itself from 2 minutes 25 seconds.

A YouTube video of drone footage of Ototoka Beach.