Recently the Facebook Group I belong to, “New Zealand Lapidary, Rocks, Minerals, Fossils”, invited its members to submit photos each week of a stone, mineral or fossil in their collection that starts with a letter of the alphabet. My Posts for the first two letters can be found here. The following are my next two Posts in the series.
“C” is for “(Fossil) Coral” – I found this fossil coral stone on Gemstone Beach (Orepuki, Southland) on 10 June 2020. My sister was showing me a stone she had found when I looked down and saw this stone with faint unusual markings. Not only the top of the corals can be seen but also the view from the side. I have not seen anything like it before on Gemstone Beach, nor since.
Of the fossil corals, there appear to be two types (families) which this stone could belong to – rugose and scleractinian. They both have “septa”, the radiating segments seen from the top. But the symmetry in rugose is bilateral, meaning that two identical halves can be created, while scleractinians have radial symmetry (reflecting two different types of growth processes). See Palaeo Post. The well-known Petoskey stone, the official stone of the US State of Michigan, is a rugose coral – the photo of a polished Petoskey stone below is from http://www.geologyin.com/…/what-is-petoskey-stone-and…
I have also posted below two diagrams from Wikipedia, showing the rugose (“tetracorallia”) and scleractinian (“hexacoralla”) corals from a 1904 book.
To my eyes, my stone looks closer to the rugose family but I am far from certain, given the great diversity within each family type. The book “A Photographic Guide to Fossils of NZ” has an entry (p.29) for a rugose coral (see above) (one of a total of six entries for coral fossils in the book). The stone I found on Gemstone Beach has tumble-polished well, with just one small shallow hole to interrupt its smoothness.
I failed to anticipate how much attention this Facebook Post would receive from the members of the “New Zealand Lapidary, Rocks, Minerals, Fossils” Group. The week after the Post, it was given 53 “Wow”, 78 “Love” and 176 “Like” along with more than 50 Comments about its beauty and interest. The Administrators of the Group decided to make it the Group banner for a while.
I was not happy with the photos I had used for the Facebook Post, the close-ups in particular looked too yellowish. So I took some more photos and tried to produce better close-ups. I wrote: It is a difficult stone to photograph as it is curved, and focus plus lighting are difficult (I use my camera’s automatic settings). I have tried again but don’t know if these are much of an improvement. Creating the close-ups seems to accentuate the yellow tinge, and I have tried to adjust for this, without much success. However, interesting aspects of the coral structure can still be seen.
A few days later, I was searching for information on the geology of Slope Point, to provide some context for the rhyolite stones I found there. In one book I consulted, “The Natural History of Southern New Zealand”, I came across a reference to fossil coral at Te Waewae Bay. Gemstone Beach is located towards the eastern end of Te Waewae Bay. The following is stated on pages 62 and 63 of the book, in Ewan Fordyce’s outstanding chapter on “Fossils and the History of Life”: “Te Waewae Bay rocks yield the only specimens of a bizarre pseudo-colonial form of sceleractinian coral apparently related to Flabellum… [It sits in the] siltstone of the Te Waewae Formation; probably upper Kapitean, latest Miocene; western Te Waewae Bay.”
Note: “The Natural History of Southern New Zealand” is edited by John Darby, R. Ewan Fordyce, Alan Mark, Keith Probert and Colin Townsend, published in 2003 by University of Otago Press. Currently available from University of Otago Press for $80.
Coral of the Flabellum family are fan-shaped, usually solitary, and seem quite different from the fossil coral I found – see NIWA Critter of the Week., Mindat Fig. 4 (esp. A & B) and Mindat Fig 5. I have not yet come across any other reference to fossil coral found along the south coast.
To return to the alphabetical series of Posts in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils” – I posted the following on 25 May 2021:
“D” is for “Dendrites” – I found this stone in March 2021, on a beach not far from my motel on the northern side of Kaikoura town, while I was looking for stones with trace fossils (zoophycos). The stone is probably a limestone(?) or mudstone(?), too soft for tumble polishing. The word “dendrite” is from the Greek word for “tree”, referring to its branching. In geology, dendrites are thin, branching crystals, often of some variety of manganese oxide (or iron), that grow over a surface in a rock or mineral. They are found in cracks or along bedding planes.
See Mindat for some good photos of dendrites https://www.mindat.org/min-26645.html. See Sandatlas for a technical explanation of their growth https://www.sandatlas.org/dendritic-growth-in-crystals. Jocelyn Thornton provides some NZ examples of dendrites in agates in her book “Gemstones” – see page 15 here (see photo below). Dendrites are often given as examples of “pseudofossils”, natural objects that may be mistaken for fossils (of a plant leaf, for example) – see https://www.dmp.wa.gov.au/Pseudofossils-1663.aspx
For “E” and “F”, see this Post. The Series Index is here.
18 thoughts on ““C” is for “(Fossil) Coral” and “D” is for “Dendrites””