Many of the stones on Slope Point beaches are different from those I have found on other South Island beaches. There is a preponderance of volcanic stones, especially rhyolites (see Thornton’s “Gemstones”, page 35 here) (see photos below). Many of the rhyolites are colourful and contain interesting patterns, such as flow-bands and orbs. One kind of rhyolite is spherulitic, also known as “chrysanthemum rhyolite”. “Flower garden” is the local name for a rhyolite with “white and grey circles and centres” (Thornton, page 35 here) (second row below, far right).
Rhyolite has a very high silica content and is the volcanic equivalent of granite. It is formed from magma that is extruded from a volcanic vent to cool quickly on the surface rather than slowly below ground. Generally fine-grained in texture, rhyolite may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matrix. Depending on the nature of its emergence, rhyolite can also take the form of obsidian and pumice. According to John Farndon’s (2015) “Illustrated Guide to Rocks & Minerals”, spherulitic rhyolite originates in “gas pockets of volatile vapours” which develop into “balls of radiating needle-like crystals of quartz and feldspar” (page 64) . Useful online sources on rhyolite are Geology.com and Alex Strekeisen. Mindat has photos of a variety of rhyolites, including spherulitic ones.
The petrified wood found on Slope Point beaches is often black but it can also be brown or grey (see photos below, top row). It is likely to have been caused by burial from volcanic-induced land slides. Fossilised tree fern stems (punga) can also be found here (see below, bottom row), though they are not common. Some of the specimens below were found by Oliver Simpson and I tumble polished them for him. As Thornton puts it in her “Gemstones” booklet (1985), “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 or seal it from decay” (page 21). Interesting online material about petrified wood can be found at Rockseeker.com and Wikipedia.
Another Slope Point stone is called fossil forest floor – Thornton refers to it as material preserved in silica: “stems, roots, leaves and so on” (see page 24 here). One specimen is shown immediately above, far right of second row.
Oliver once found a fossil coral stone at Slope Point (photo below, top left), maybe agatised (see this Post about a Gemstone Beach find for some information on fossil coral). Beach agates are rare on these beaches, though agate or chalcedony veins or infilled crevices can be seen in some stones. One unusual type of agate, a joon agate, can be found here but it too is rare. The final stone below, second row far right, is full of tiny unidentified fossils of some type.
There are a lot of breccia stones amongst my Slope Point finds. I have also found two poppy jaspers (one quite tiny) and one stone similar to Kakanui’s fossil bryozoa. A range of other more common stones occur at Slope Point as well, such as jasper, granite and quartzite.
The following are the Posts on TumbleStone Blog significantly featuring these Slope Point stones:
- “Christmas Stones: Part One – Slope Point” – These polished stones include petrified wood, spherulitic rhyolite (chrysanthemum rhyolite), flower garden rhyolite, flow-banded rhyolite, and breccia. December 2022.
- “Some More Recently Polished Stones from Slope Point and Kakanui” – The seven Slope Point stones include a fossil tree fern, a chrysanthemum rhyolite, a flower garden rhyolite, two flow-banded rhyolites, a volcanic breccia, and a black petrified wood. December 2022.
- “Early 2024 Fossicking Trip – Part 24, Second Slope Point Fossick, Wednesday 13 March” – Features my best fossil tree fern find along with 17 other stones of various types. March 2024.
- “Slope Point Diversity, 25 Recently Polished Slope Point Stones: Part One – Stones 1 to 8” – Includes mainly a variety of rhyolites. December 2022.
- “Slope Point Diversity, 25 Recently Polished Slope Point Stones: Part Two – Stones 9 to 16” – Three flower garden rhyolites, other rhyolites, and a petrified wood stone. January 2023.
- “Slope Point Diversity, 25 Recently Polished Slope Point Stones: Part Three – Stones 17 to 25” – Six rhyolites. January 2023.
- “Well that was hard! 36 Milestones for a Completed Thesis – Part Three: Stones 23 to 36 (Slope Point)” – The 14 polished stones are a range of Slope Point finds, including rhyolites and one brown petrified wood with agate veins. August 2022.
- “Southern Sojourn 2023(50): An Orbicular Rhyolite and A Large Piece of Petrified Wood, A Return to Slope Point, Saturday 22 April” – The large petrified wood rock is 2.5 kilograms in weight. Ten other interesting stones are featured. April 2023.
- “September 2024 Stone Collecting Trip –Part 15, Fossick at a Slope Point Beach, Friday 20 September” – Features an intriguing fossil stone, as yet unidentified. Photos of 20 other finds are in the Post, including a yellow quartzite. September 2024.
- “Twenty-Four Slope Point Stones Polished for Oliver: Part 1, Stones 1 to 10” – Nine excellent petrified wood stones and one fossil tree fern, found by Oliver, polished by me. September 2021.
- “Twenty-Four Slope Point Stones Polished for Oliver: Part 2, Stones 11 to 24” – Twelve great petrified wood stones and two fossil tree ferns, found by Oliver, polished by me. September 2021.
- “February-March 2022 Fossicking Trip: Stone of the Day #9, Small Brecciated(?) Slope Point Stone” – Includes photos of ten of my finds, mostly rhyolite, and a fossil tree fern find by Oliver. February 2022.
- “March 2025 Stone Collecting Trip – Part 11, ‘Subtle Colours and Patterns’, 90 Finds from Slope Point, Monday 10 March” – Includes a stone similar to the bryozoan fossil stones from Kakanui, a gorgeous small jasper, and 16 other finds. March 2025.
- “February-March 2022 Fossicking Trip: Stone of the Day #12, A Slope Point Rhyolite” – Selected finds included rhyolites, one orbicular/spherical, and a quartzite. February 2022.
- “Twenty-Five Slope Point Stones Polished for Oliver, July 2022: Part 1, Stones 1 to 8” – Includes three orbicular/spherical rhyolites (Note: 2nd stone in Post is not a Slope Point one). July 2022.
- “Twenty-Five Slope Point Stones Polished for Oliver, July 2022: Part 2, Stones 9 to 15 (Petrified Wood)” – Nine are petrified wood, the tenth is probably fossil tree fern. July 2022.
- “Twenty-Five Slope Point Stones Polished for Oliver, July 2022: Part 3, Stones 16 to 25 (Dots, Orbs, Breccia)” – Includes four “flower garden” rhyolites. July 2022.
- “January 2022, Stone of the Day #10 – Light-Coloured Petrified Wood from Slope Point” – A gorgeous small brown petrified wood stone, and some photos of black petrified wood as well. January 2022.
- “Oliver’s Fossil Coral Stone” – Found by Oliver, polished by me. September 2021.
- “S is for Slope Point Stone” – Probably a spherulitic rhyolite, though initially I thought this was a fossil tree fern. September 2021.
- “January 2022, Stone of the Day #12 – A Slope Point Rhyolite?” – Good example of a colourful rhyolite from Slope Point, despite my stated uncertainty at the time. January 2022.
- “National Lockdown Number Two: Stone 14” – A good specimen of a small Slope Point breccia. September 2021.
- “Southern Sojourn 2023(45): Fine Autumn Fossick, Black Point Beach, Slope Point, Saturday 15 April” – The 17 finds featured here include petrified wood, a fossil tree fern, a fossil forest floor stone, a joon agate, and orbicular rhyolite. April 2023.
- “March 2025 Stone Collecting Trip – Part 21, Visiting Slope Point Again, Monday 24 March, Post 1 of 2” – The third stone in this Post is an unusual agate. Twenty-one other finds are featured, including breccia. March 2025.
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