Yesterday was another cold wet one so I stayed inside. This morning, I had itchy feet so I visited Gemstone Beach at a time when the forecast suggested the weather would be best. I arrived there just after 9am, with high tide due around noon. I realised it wouldn’t be safe to venture over the Taunoa Stream, with the waves coming progressively higher up the beach, so I knew I would be spending my time on the 200 to 300 metres of beach in front of the carpark. During the 80 minutes I was there, it was 10 degrees with a chilly though light wind, and with only a sprinkling or two of rain. I was able to wield my camera today, unlike my last fossick here.
There is a small stream just to the east of the carpark – it is not named on any map I have seen so I have christened it The Stream With No Name. A large area of stones has been moved by the sea to the east of that stream. These stones weren’t there two days ago, and they proved worthy of a good fossick before I moved westwards to the Taunoa Stream.
I spent 80 minutes on the beach, collecting 60 stones, 14 of which are featured below. I saw only two passing motorists briefly venturing down towards the end of that time. They were chased up The Stream With No Name by an incoming wave and likely decided it was safer and warmer in their vehicle.
I spotted this small dark stone right next to the Taunoa Stream. I could see there was something going on in it, but I needed to see the close-up photos to work out its brecciated character.
In “breccia”, the fragments (or “clasts”) that make up the stone have not travelled far before being cemented together and so are angular and sharp. In a “conglomerate”, the fragments have undergone some rounding from the travel they have experienced, usually from water, prior to being cemented together. For some information on brecciated stones and Gemstone Beach, see “January 2022, Stone of the Day #2 – Breccia and the Giant Landslides of Fiordland”. ThoughtCo has a good article on breccia. Sometimes, a brecciated stone is caused when massive stresses on a rock tears fragments from it which are then cemented in a matrix – this is often known as “tectonic” breccia. Other times, “sedimentary” breccia occurs, when fragments from different stones end up cemented together in a fine-grained matrix. A third main type is “igneous” breccia. A breccia where the fragments represent more than one rock type is called “polymictic” while one where the fragments come from just one rock type is termed “monomictic” (University of Auckland Geology). I would guess that the small dark stone I found is a monomictic sedimentary breccia of maybe mudstone clasts. There seems to be a little quartz and green epidote in it as well.
I suspect the following two stones I also found this morning are “monomictic sedimentary breccia of mudstone clasts” as well. The second appears to be a “dirtier” version of the first:
Three of my other interesting finds this morning are an amazing mossy jasper, a pastel blue/green quartzite with intriguing depths, and a small pink stone with large areas of black:
The best areas to find hydrogrossular garnets are a few hundred metres west of the Taunoa Stream but it is also possible to find a specimen or two in front of the carpark. I found this one just three minutes into my fossick:
Trace fossil argillite stones are more common right along the beach, and can also be found as far as Riverton Aparima over 30 kms to the east (and far beyond). This morning, I found a handful of them during the last ten minutes of my fossick, while walking up The Stream With No Name. Here are four of them, including a larger stone which is destined for the rock garden at home.
Circles and dots in a stone always attract my eye. But in the case of the next find, I actually spotted the linear squiggles side on the beach and only noticed the dots when turning the stone over:
The circular features of spotted argillite are always interesting to me:
Gemstone Beach’s spotted argillites come in a wide range of colours. Sometimes the spots are darker than the rest of the stone, sometimes lighter. Sometimes the spots are circular, sometimes more “deformed”. I provide 12 varieties of spotted argillite in this Post – see Stones Gn20 to Gn31. There I suggest that such spots occur when argillite comes under increased pressure and heat, more than is usual for a sedimentary rock. Certain minerals then become concentrated in specific areas.
The final stone I picked up this morning, lying in The Stream With No Name, is this igneous one. Ten centimetres long and a fine specimen of a type of stone that can be found on Gemstone Beach, it is destined for the rock garden,
Part 14 is another Gemstone Beach fossick. The Index to the Posts in this Series can be found here.
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