Month: July 2025

August 2025 Stone Collecting Trip – Part 2, A Stop on My Way: Lots More Sand on Kakanui’s Seadown Beach, Wednesday 30 July

The rocks at the south of the beach.

I arrived at Kakanui’s Seadown Beach, North Otago, at 1pm, just before low tide, and spent 90 minutes there. The afternoon was grey and cold. I had been told that the beach had less stones on it than usual. Looking north from the carpark area, I could see only a few scattered stones above the high tide mark, the rest being smooth sand.

Seadown Beach this afternoon – you can just see a person standing by the rocks at the far right.
Looking north, a long stretch of bare sand near the waves’ edge, a few scattered stones above the high tide level.
The rocks at the south of the beach.

But the biggest surprise was when I walked to the nearby south end of the beach – I couldn’t recognise the rocks there. I eventually worked out that a great quantity of sand had been dumped there by the sea, burying over a metre of the rocks that were normally exposed. Comparison with a photo I took in May 2023 (see below) shows how much sand has been added recently.

The May 2023 photo is on the top row, with a large rock outcrop in the centre foreground (the roadside carpark in the distance). A photo from today is on the second row, showing the same rock but from a greater distance. And the same rock is viewed from in front in the third row. Circled in black on the May 2023 photo (top row) is the part of the rock that is now buried. The green line shows the part of the rock still exposed today (compare with rows two and three). The red line on the May 2023 photo is the top of a bank of sand – the beach suddenly dropped away below that maybe two metres. Today’s photo in the second row has a black line running along the same bank, except the whole beach is now at that level.

A photo of the area in May 2023.
Circled in black is a rock that is now buried by sand. Red line is the top of a 2 metre high bank of sand. The green part of the rock is what is now left exposed.
Looking at the same rock today.
The black line is the former 2 metre bank, now no longer existing.
Looking at the same rock from in front.
Nearly 2 metres depth of sand has been dumped here.

The area just past this rock usually has a great patch of stones. Not today. But a bit further south was a good patch, and I spent most of my time there.

The best patch of stones was just beyond the buried rock.
The patch of stones is circled in orange.
Seadown Beach stones today.
Seadown Beach stones today.
Seadown Beach stones today.

I collected about 50 stones today. Ten of them are featured below.

The yellow flashes in this dark stone caught my eye on the beach. The close-up photos showed the interesting structure of the flashes:

A dark stone with flashes of yellow.
Other side of the stone.

Another stone that the close-ups revealed as very interesting. I almost didn’t put it in the bag as it’s kind of plain and has lots to tiny indentations in it, not good for tumble polishing. The photos reveal an intriguing grey material in it that I suspect could be chalcedony:

Maybe tumble polishing can remove some of the indentations and clarify the constituent material.

The best of the limestone pebbles I found, with many tiny fossils in it:

Limestone with tiny fossils.
Other side of stone.

Limestone is quite soft and I tumble polish it only in fine grit.

I found a couple of caramel-hued quartz stones with interesting veins, patches and patterns. Here is one:

Caramel quartz #1.
Other side of stone.

Here is the other:

Caramel quartz #2.
Other side of stone.

Seadown Beach has some gorgeous quartzites. One of my best finds was this small mainly red quartzite:

Small red/yellow quartzite.

Three other quartzites I collected, illustrating some of the yellow hues common here:

Quartzite.
Quartzite.
Quartzite.

Finally a multi-veined seven-shades-of-grey quartzite:

Grey quartzite with many veins.

Today also saw an 8.8 magnitude earthquake near Russia. This triggered tsunami waves across the Pacific, and New Zealand issued a Civil Defence alert for people to stay off beaches for at least a day. I had planned to visit Seadown Beach again the next morning but cancelled in accordance with the Civil Defence advice.

There wasn’t enough sunlight to take photos of the day’s finds on Wednesday. On Thursday, when I was passing through Milton on the drive south, the sun came out. So I parked down a back street and got my camera out, and took the above photos.

So I continue on my way south to Gemstone Beach, through Milton and onto Balclutha for Thursday night. Friday sees the last stage of the trip to the Te Waewae Bay coast. Cue the latest addition to my Spotify playlist, added because I heard it on a recent episode of the New Zealand-made TV programme “The Brokenwood Mysteries”. The song is “On My Way to You” released in 2015 by New Zealander Don McGlashan [“you” being Gemstone Beach, in my current case]. It’s a good song to drive to, with a travelling beat:

Speedo blurs, and the white lines rolling
Through the clouds and the telegraph poles
A little farmhouse that’s up in the hills
Dairy factories and timber mills
Every valley shall be lifted high
Every mountain will fall, too

′Cos I’m on my way to you
I’m on my way to you

Woke up this morning, expecting rain
Kept my foot down, and it never came
Petrol, road dust, on my tongue
Every song that I′ve ever sung
Pouring out into the sky
Sounding beautiful and new

′Cos I’m on my way to you
I’m on my way to you

And just because something’s impossible
That doesn′t mean it isn’t true
So I′m on my way to you
I’m on my way to you…

My next fossick will be at Gemstone Beach. An Index to this Series is here.

B is for Banded Argillite

This is my second contribution to the current Alphabetical Series in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. In the 2021 version of this Series I posted “B is for Botryoidal Chalcedony”. My first 2025 entry was “A is for Agatised Fossil Bone”.

“B is for Banded Argillite”

Large banded argillite, Gemstone Beach.

I found this stone on Gemstone Beach, Southland, in September 2024 – see the end of the Post “Sunday 8 September 2024: Fifteen Finds from a First Gemstone Beach Fossick”. It is a handful, nine centimetres wide. Argillite is a sedimentary rock that sometimes shows a banding, reflecting layers of different coloured sediments. On Gemstone Beach, banded argillites can be found in hues of green, grey, black and brown. Sometimes I think that this particular stone is brown, sometimes I wonder if it is grey – colour perception is subjective. Many of Gemstone Beach’s banded argillites appear to have been subject to more heat during formation than other argillites, their bands looking as if they melt into each other, like pottery glaze. They tumble polish well. This one is too big for me to tumble polish – it sits outside in the garden at my home in Whanganui and looks brilliant after rain.

For more details about Gemstone Beach’s banded argillites, see Stones Gn8 to Gn14 in “Gemstone Beach and its Stones: An Introduction for the Passing Motorist – Part Seven-A, Green Argillite Stones”.

The next entry is “C is for Calcite (Calcium Carbonate) in a Ward Beach Fossil Oyster Stone”. An Index for this Alphabetical Series is here.

August 2025 Stone Collecting Trip – Part 1, Outrunning the Storm to Ward Beach, Tuesday 29 July

My car and two widely-spaced camper vans.

My August 2025 trip actually begins at the end of July. I left home in Whanganui yesterday, 28 July, to catch a Bluebridge Cook Strait ferry in Wellington. On this trip, I will be visiting my favourite South Island beaches again, as well as consulting with Chrissy about the book we are preparing on the stones of Gemstone Beach. Chrissy lives just to the east of Gemstone Beach and writes the Tumble and Polish Blog. After reaching Picton in the South Island just before sunset yesterday, I drove for an hour to Ward in Marlborough, checking into Ian and Ronnie’s Flaxbourne Motels for the night – they now know me well from my regular visits. This morning, I stopped at Ward Beach before continuing on to Woodend near Christchurch. A storm is currently approaching New Zealand from the northwest, bringing high winds and heavy rain. There were some stong gusts of wind at Ward Beach, along with a little scattered rain, but it looks like I will be keeping ahead of the storm as I drive south.

On the way to Ward Beach, I passed a major construction site in a paddock, what appeared to be frames being put up for a large solar energy farm. It turns out that Marlborough Lines are locating 18,000 solar panels 0n two sites right next to Ward – see “South Marlborough solar farm to generate 3% of region’s power”. The last couple of kilometres of the road to the beach are gravel and halfway along it a recent water flow had gouged out a small but deepish channel – I had to drive carefully across the least deep part. At this time of year, in mid-winter, there were only two campervans in the camping area behind the carpark. No-one else was on the beach for the 90 minutes while I was there.

Solar energy farm under construction near Ward.
Looking out to sea from the carpark.
Looked south from in front of the carpark.
Looking towards the carpark from the river mouth, Ward Beach.
My car and two widely-spaced camper vans.
View from in front of the carpark, towards the river mouth. “X” marks the spot from where previous 2 photos were taken.

In front of the carpark, down near the waves, a lot of stones could be found today, along more than 100 metres of the beach. This is much more than during my previous visits over the last few years. I walked back and forth across that area. Then I went behind the beach to the Flaxbourne River, walking downstream. From that viewpoint, a bank of stones up to about eight metres high can be seen (see photo below, third row), indicating the significant depth of stones on the beach. More about this area, the “abandoned estuary” of the Flaxbourne River, a result of the 2016 earthquake, can be found on Geotrips.

Strong waves breaking at Ward Beach.
Ward Beach stones.
Ward Beach stones.
Buried in the stones.
Really interesting colours. Probably limestone and flint??
Flaxbourne River, as it winds behind the beach.
The bank of stones at the back of the beach. Up to 8 metres high.
Stones near the mouth of the Flaxbourne River.
Across the other side of the river, a lone black cattle beast on a steep slope.
I saw a few dotterels on the beach today.

During my 90 minutes at Ward Beach, I collected about 40 stones. Following are nine of them. The first three, of three different rock material, had interesting white veins:

I found some trace fossil stones – here’s one such stone with a “wrinkled” composition, and it’s my guess that the second one also has a kind of trace fossil in it:

Traces in a “wrinkled” stone.
Trace fossils?

A nice small jasper with some quartz in it – or is it a quartz stone with a lot of iron oxide in it?

Small jasper with quartz?

In this find, the white quartz dominates the red iron oxide:

The blue in this stone caught my eye – it could be limestone:

Finally, what could be a breccia (a stone made up of rock fragments in a matrix) or maybe a stone containing pieces of shell (or fossilised shell):

Next beach visit, as I continued south, was at Kakanui, near Oamaru in North Otago – see Part 2. An Index to the Series is here.

Unidentified Fossil Stone from Slope Point: A Rayleigh-Taylor Instability Hypothesis

“Sally” made a comment on my recent Post “Unidentified Fossil Stone from Slope Point Polished” that is very intriguing. I am grateful for her suggestion. There are some tiny shapes in this stone that have been very difficult to identify. Some of them look like long-stemmed mushrooms (as circled below).

The unpolished stone, wet.
The stone after polishing.

One suggestion is that the stone is fossilised forest floor, something that occurs in the Slope Point area. I had wondered at one point if they might be fossil sponge spicules. Sally commented as follows:

The tiny long-stemmed fungi resemble Rayleigh-Taylor instability. This concept is used to describe various fluid dynamical systems, including when a heavier fluid is suspended above a lighter fluid within a gravitational field. I wonder if it can approximate cooling lava as it forms solid rock. See this link for images of Rayleigh-Taylor simulations: https://computing.llnl.gov/projects/blast/rayleigh-taylor-instability. I’m going to guess this pebble is/was orbicular rhyolite, an extrusive igneous rock formed by silica-rich volcanic ash. I’d love to hear from a geologist about it. Very beautiful!

Wikipedia notes: The Rayleigh–Taylor instability… is an instability of an interface between two fluids of different densities which occurs when the lighter fluid is pushing the heavier fluid. Examples include the behaviour of water suspended above oil while under the influence of Earth’s gravity, and mushroom clouds like those from volcanic eruptions and atmospheric nuclear explosions. One publication illustrates the way the two fluids interact, over time and at a certain temperature, as follows:

Source = http://www.mdpi.com/2227-9717/13/2/491

The fourth and fifth stages here produce a shape very similar to the tiny “fossils” observed in my Slope Point find.

Sally suggested the stone could have started off as orbicular rhyolite – the very similar sphericular rhyolite is often found at Slope Point, though it is relatively rare.

Sally’s suggestion that the tiny long-stemmed fungi shapes could be due to Rayleigh-Taylor instability may or may not be correct, and it will be difficult to verify it. However, it is an intriguing hypothesis that is not out of the bounds of possibility.

A is for Agatised Fossil Bone

I am a member of the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. In 2021, the Administrators invited members to submit photos each week of a stone, mineral or fossil in their collection that started with a letter of the alphabet. I posted my entries on TumbleStone Blog – see here for the Index to them. Recently, some members of the Group thought it would be a good idea to do it again. Here is my first entry (slightly revised and extended) for the 2025 Series. (In 2021, I posted “A is for Amygdaloidal”.)

“A is for Agatised Fossil Bone”

I found this stone on a beach near Kakanui, North Otago, in September 2024. I had not seen anything like it before, nor since. It has been suggested that it is agatised fossil bone. You can view similar material if you google “dinosaur gembone”. The process that produces it is called “permineralisation”, where the empty spaces within a buried organism, spaces filled with liquid or gas during life, become filled with mineral-rich groundwater. Minerals precipitate from the groundwater, occupying the empty spaces. This process is well described in Geologyin.com (though sometimes the ads can be annoying on this website). I thought about whether to tumble polish the stone for a while and eventually decided to do so.

After tumble polishing.
After tumble polishing.
After tumble polishing.

I tried some gentle tumble polishing: a couple of short tumbles in 400 grit, then a 600 grit stage followed by a polish powder tumble. I did not have spectacular success (see photos above) – it looks like the material varies significantly in hardness, parts of it resisting taking a shine.

NOTE: A couple of members of the group, from the local area, mentioned they had similar though non-agatised stones, identified as fossil whale bone.

My next entry in this Series is “B is for Banded Argillite”. An Index for this Alphabetical Series is here.

A Review of Peter Ballance’s (2017) “New Zealand Geology: An Illustrated Guide”

I wrote this Review for “StoneSpeak”, the Newsletter of the Southland Geological and Lapidary Club, and it appeared there in June 2025. The following is a slightly fuller version. The Club’s website is here and its Facebook Page is here.

June 2025 Edition of Newsletter of Southland Geological and Lapidary Club.
My Review in the June 2025 Newsletter of Southland Geological and Lapidary Club.
Cover of Ballance’s book, Available free online at https://gsnz.org.nz/assets/Uploads/Shop/Products/MP148_Ballance_NZ_Geology.pdf
Contents of Ballance’s book.

Peter Ballance’s excellent “New Zealand Geology: An Illustrated Guide” was published as an e-book in 2017. It is available for free download from the Geoscience Society of New Zealand here. It is not a book to help you identify a stone – rather, it is a consideration of New Zealand’s geology within a tectonic plate framework. It can be patchy in its coverage, due to how it came to be published (after the author’s death, before he had completed it), but it contains numerous small sections and diagrams which provide insights into a range of geological matters. These include the weathering of greywacke to clay (page 57), the formation of pillow lava (pages 70-71), rhyolite volcanism and ignimbrite (pages 127-129), the Moeraki boulders (pages 263-264), Curio Bay (page 275), the tectonic zone from Te Waewae Bay to Te Anau (page 286), and Nelson’s Boulder Bank (page 377).

Page 57 of Peter Ballance’s “New Zealand Geology: An Illustrated Guide”.
Page 128 of Peter Ballance’s “New Zealand Geology: An Illustrated Guide”.
Page 275 of Peter Ballance’s “New Zealand Geology: An Illustrated Guide”.
Page 286 of Peter Ballance’s “New Zealand Geology: An Illustrated Guide”.

Peter Ballance was an Englishman who came to teach at New Zealand’s University of Auckland in 1961. He taught geology there until his retirement in 1998. He died in 2009. A 2004 article on Ballance in the “New Zealand Journal of Geology and Geophysics” states: “In New Zealand geological circles, Peter is well regarded for his high standards, the personification of the perfect gentleman who has always shared his data and knowledge with anyone who is interested, and being scrupulously fair in his appraisal or criticism of the work of others.”

“New Zealand Geology: An Illustrated Guide” was Ballance’s retirement project. However, the final draft was not quite complete before his death and other people compiled, formatted and edited his writings and updated his illustrations and maps. The book describes aspects of the geology of New Zealand in some depth, with efforts being made to communicate clearly to the general reader. The text is accompanied by over 100 colour photographs and a comparable number of colour maps and illustrations. A review in the Geoscience Society Newsletter makes the following observations: “It is written with very clear explanations, using non-technical language and supplemented with many excellent diagrams, maps, and photos, making the content accessible to interested non-specialists. At the same time, with nearly 400 pages of densely packed information, this is not a light read, but something to invest considerable time and effort in. Anyone who does so will be richly rewarded with fascinating insights into a myriad of processes and events that have created our multifaceted landscape.” I wouldn’t say all the language is non-technical – some effort is required at times to follow the use of geological terms.

Peter Ballance. Source: http://www.researchgate.net/profile/Bruce-Hayward/publication/232993016_Peter_Ballance_the_geologist/links/0046352f3400ca3976000000/Peter-Ballance-the-geologist.pdf
Peter Ballance (right), in the field. Source: http://www.facebook.com/photo.php?fbid=859527596222754&id=100064965635442&set=a.468638545311663&locale=is_IS
Page 7 of Peter Ballance’s “New Zealand Geology: An Illustrated Guide”.

The book is in three parts. The first part is an introductory section dealing with the dating of rocks, plate tectonic processes and the New Zealand plate boundary, followed by an overview of the development of the continent of Zealandia. Part two describes eight geological regions of New Zealand’s North Island with Part Three consisting of nine chapters dealing with the South Island’s regional geology. Chapter 16 is on Southland and Stewart Island/Rakiura but is perhaps the most disappointing chapter in the book – it feels underdone to me, perhaps incomplete. But this is the kind of book that I go to from time to time to clarify some of the context to the stones I find on local beaches. Unfortunately, there is no index provided, so it’s a matter of hunting through the pages for what might be relevant. But every now and then, I strike gold there. And I find myself reading more, purely out of interest.

The next Review in this Series is of the New Zealand “Geotrips” website.