September 2024 Stone Collecting Trip –Part 24, Fossil Rhodoliths and Farewell to Kakanui’s Seadown Beach, Tuesday 1 October

My third and last visit to Seadown Beach, in this stop on my way home to Whanganui, was two hours long. I arrived at 9.15am on a cloudy morning with a cool breeze. It was low tide, and and the overnight high tide had moved some sand around, revealed a few more stones than were on the beach yesterday. There was still very few at the south end but noticeably more further along the beach.

Towards the end of my fossick, as I was returning to the south end of the beach where my car was parked, I met up with a rockhound who is a fellow member of the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. We have often commented on each other’s Posts but this was the first time I had met Christchurch-based “Kween Bee” face-to-face. She was accompanied by her husband and two boys, and had been looking for interesting minerals in the rocks. I showed her some of my finds, and also some polished stones I had with me, including the fossil coral I found on Gemstone Beach in 2020. When I showed Kween Bee the following find from today, what I usually called “fossilised sea-floor” and which is likely to be a kind of limestone, she pointed out that the round features within it are fossil rhodoliths:

I have often seen these round things in stones I have collected from this beach (including one yesterday!) so it was good to finally identify them (thanks, Kween Bee!). According to Wikipedia, rhodoliths are marine nodules of algae, often mauve in colour, made up mainly of calcium carbonate that can grow to resemble beds of coral (see photos below) – “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) – see the first photo below. The Waitaki Museum in Oamaru contains specimens of fossil rhodoliths (see other photos below).

Among my other finds today was this colourful quartzite:

This one looks like quartz with clouds of a mineral and a few different veins:

A light coloured quartzite with veins and some brecciation:

Three of the yellow quartzites I picked up this morning, a regular favourite from this beach:

An unusual very dark brown stone. I could just detect some interesting patterns within it so I put it in the bag:

There’s some intense colour in this find – it has been suggested by another member of the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils” that this is a plasma jasper (I have not yet looked into this further):

One of my smaller finds:

The final stone is nicely shaped and subtly patterned:

After leaving Seadown Beach, I drove north, stopping overnight at Woodend, just past of Christchurch, before travelling to Ward in Marlborough the next day.

Part 25 describes my final fossick for this trip, at Ward Beach in Marlborough. The Index to the Posts in this Series can be found here.