The second revised edition of the photobook “The Trace Fossils of Gemstone Beach” includes two pages of photos of stones that I have collected from a stretch of beach just north of Kakanui. I first visited here in June 2020. I bestowed upon it the name of Seadown Beach after the name of the road that connects with Thousand Acre Road at the car parking area. To the south, at the mouth of the Kakanui River is Kakanui Beach while further south lies All Day Bay. These areas, and other nearby stretches of beach, are all worthwhile when looking for interesting stones but my favourite is Seadown Beach, the source of all the stones on Pages 28 and 29.
The stone numbers on the scan of page 28 below don’t appear in the book but have been added so I can refer to each of them in this Post:
#1 – This stone is perhaps my most interesting find from Seadown Beach, picked up among the rocks at the south end in September 2024. I had never seen anything like it on a New Zealand beach. It turned out to be agatised bone:
I have since seen references to other people finding similar stones in this area. I am not sure what animal the bone originally came from.
#2 – This is not a stone that will tumble polish as it is too soft and will break into bits. It is Kakanui Mineral Breccia, found on 23 March 2024 (see the end of this Post). It’s famous in this part of the world (and even beyond) so I sometimes collect the odd specimen. This breccia (rock made up of angular fragments) formed from the contact of magma with water about 30 million years ago. The resulting violent eruption totally blew the magma components apart and they formed into a mineral-rich fragmental rock. The Kakanui School Centennial History (1969) includes a short chapter “Geology of the Kakanui District” which notes on page 8: “The Breccia contains single crystals of garnet (deep red), feldspar (pale cream), hornblende and augite (black), chrome diopside (bright green) and other minerals. Many of the mineral grains attain large size but all are shattered and fractured… This deep-seated material, brought to the surface by volcanic eruption, is unique in the New Zealand region.” NZ Petroleum and Mineral’s “Mineral Commodity Report 17 – Rare Earths and Related Elements” lists the rare earth elements in Kakanui Mineral Breccia (page 10 of pdf). It was partly because of the rare earth minerals that the Breccia attracted a lot of geological attention in the late 1960s and early 1970s (see Mineralogical Magazine, 1974), with publications on it continuing to the present day. The stone I found will not necessarily contain any or all of the various minerals. Close-up photo below (after Stone #4).
#3 – This is a piece of petrified wood, found on 29 March 2026. It is one of maybe half a dozen of such stones that I have found on Seadown Beach.
#4 – This stone contains tiny bryozoan fossils in chalcedony. I found it on 5 September 2024. I discovered my first bryozoan fossil stone in mid-2022. This Post provides some background on them. Below (right) is a close-up view.
#5 – This quartzite was found on 30 March 2026. These are shiny smooth stones that tumble polish very nicely.
#6 – This banded agate has an attractive pattern which has been constructed as hot water has laid down layers of chalcedony in a hole in a rock. I found it on 1 March 2025. I have found other beach agates on Seadown Beach, though not very often. Further to the south, All Day Bay has been known to be a good source – see the photos on the Otago Rock and Mineral Club site.
#7 – I don’t know what this stone is, found on 21 July 2026. Its light purple colour is unusual.
#8 – When I found this stone on 6 September 2024, I wrote: “Today’s most unusual find is a kind of orbed stone I have not seen here before. The circular features are part of the stone, not algal growths or surface stains. I don’t know what they are.” There are intriguing linear marks inside the lighter coloured circles. See close-up below, after Stone #9.
#9 – The circular features in this limestone are fossil rhodoliths (see photo, below right). I found this stone on 30 September 2024. After I first identified them, the next day, with the help of another rockhound, I did some research and wrote: “Rhodoliths are marine nodules of algae, made up mainly of calcium carbonate, that can grow to resemble beds of coral. Rhodoliths do not attach themselves to the rocky seabed. Rather, they roll like tumbleweeds along the seafloor until they become too large in size to be mobilised by the prevailing wave and current regime. Rhodoliths have been around for about 55 million years, hence their presence as fossils. Page 6 of the 2012 publication “Rhodolith Beds in Northern New Zealand” notes that North Otago is one of the places in the South Island that rhodolith beds have been reported. A field trip guide on Coastal Otago, written by Daphne Lee for the Geosciences Conference in 2009, describes the fossil rhodolith beds in the Kakanui area (pages 11-12).”
#10 – I found this beautiful little stone the same day that I found Stone #9, on 30 September 2024. I have not seen anything like it before or since, and I don’t know what it is. The details of its pattern can be seen up-close in the photo after Stone 311 below.
#11 – This limestone pebble is fossiliferous. It contains lots of tiny fossils, especially shells (see photo below, right). There are quite a few limestone pebbles on this beach but not all contain fossils like this. A soft stone, it will wear away quickly if tumbled – I tumble such stones briefly in a fine grit to clarify the surface. I found this specimen on 22 July 2026.
The next Post in this Series (not yet available) describes the stones on Page 29, also collected from Kakanui.
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