The second revised edition of the photobook “The Trace Fossils of Gemstone Beach” includes pages of photos of stones that I have collected from South Island beaches other than Gemstone Beach. One of those beaches is Birdlings Flat, which is located about 50 kilometres south of the city of Christchurch. This beach is at the far eastern end of a large gravel spit called Kaitorete Spit. The spit has been built up from stones carried to the coast by a number of rivers flowing from the Southern Alps.
Birdlings Flat has been the most well-known South Island rockhound beach over the past 60 years or so. Much sought after are the beach agates often found there. See this Post for what a number of books say about Birdlings Flat. I first visited it in May 2016. This Post sets out seven types of stones I collected that year. I have not fossicked there much over the last five years and a few more visits are overdue.
Page 26 contains the stones from Birdlings Flat – note that the stone numbers don’t appear in the book:
#1 – This is a classic banded beach agate, also known as a waterline agate. It is thought that these form inside gas cavities within volcanic rock, the banding resulting from deposits precipitated out of silica-rich water. The bands are flat, horizontal layers rather than concentric rings as can occur in other situations. I found this stone in February 2020. I tumble polished it in May 2021, as shown in the entry for Monday 17 May 2021 in this Post. There are lots of tiny concussion markings in it, maybe from travelling down a glacier in the far past, or from bumping into other stones in the sea (see close-up below, after Stone #3). The photo in the book is of the polished stone.
#2 – This is a quartzite stone. Likely collected in 2020, I tumble polished it in June 2021. Birdlings Flat has many quartzites, often plainer in marking, many of then originating in Central Otago. The circular rusty-brown shapes, likely to be iron oxide, attract the eye.
#3 – Another that I tumble polished in June 2021, this stone looks like a jasper but is more likely a quartzite. Viewing it up-close (see below, right), it contains tiny bits of quartz which you don’t find in jaspers.
#4 – Another colourful quartzite. This one featured in the entry of Thursday 13 May 2021 in this Post. “Just out of the tumbler this morning”, I wrote. As with Stone #2, it has some circular features though the basic stone is grey rather than creamy-yellow.
#5 – I have vacillated over whether this stone is mahogany obsidian or another form of rhyolite or even an andesite. It is volcanic and hard and shiny, and at least some of its vesicles (gas holes) seem to be filled with chalcedony. I found it on 24 August 2020 and it features in the entry for Saturday 15 May 2021 in this Post in a Series of Facebook Group Posts. Facebook comments at the time implied this type of stone is well-known in Canterbury. The fact that it didn’t polish well seems to me to argue against obsidian.
#6 – This can perhaps be called a moss agate. I photographed it after tumble polishing it in June 2021. It doesn’t have the bands present in Stone #1. Dark clumps of a mineral are embedded in the clear chalcedony. Holding it up to the light reveals what’s inside.
#7 – Petrified wood, found in May 2020 (see “Day Two, Cheviot to Birdlings Flat” in this Post). See below, after Stone #8, for a close-up. A good description of the process of wood petrification can be found in Jocelyn Thornton’s “Gemstones” booklet – see page 21 of the online version.
#8 – Stones of chalcedony, like this one, are usually called agates in New Zealand while overseas the term “agate” is reserved for banded stones like #1 above. I found Stone #8 on Birdlings Flat on 24 August 2020. Holding it up to the light (photo below, right), you can see it is transparent with some opaque inclusions.
#9 – A grey quartzite with black veins, also picked up on 24 August 2020. This is a different kind of quartzite to Stones #2, #3, #4 and #10 – it does not contain tiny clear fragments of quartz like they do.
#10 – Another quartzite, this one is much lighter in colour. Quartzites are hard stones which tumble polish well. The photo was taken in June 2021.
#11 – I have found a few stones like this on Birdlings Flat and occasionally elsewhere. They have a light brown colour and usually contain a range of interesting small crystals or amygdales. This one comes from an early 2020 fossick. I suspect it could be the volcanic rock andesite but am unsure.
The next Post in this Series looks at the stones on Page 27, collected from Timaru South.
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