The Iron-Stained Stones of Kai Iwi Beach

I moved to Whanganui from Cambridge just over six months ago. I now live quite close to the seaside village of Mowhanau situated on Kai Iwi Beach. I have walked on the beach a few times, especially near the Kai Iwi Stream mouth. I have not been particularly impressed with the few stones to be found amidst the scattering of stones and shells and other bits and pieces in the sand. They seemed to originate in a layer in the cliffs, and some were maybe being brought down by the stream.

Then I took a closer look at the small brown (iron-stained) stones lying here and there on the beach and in the stream. On this closer inspection, they looked like they might repay tumbling as there were hints of patterns within them and they appeared to be made up mainly of quartz, which polishes well. It took three weeks to tumble the first batch. The results surprised me. Here are four of the 60 polished stones.

To take a closer look at two of these stones which have very different colours and patterns:

Here are three stones which are practically orange-coloured:

Three Kai-Iwi Beach polished stones of complex construction:

Some of the darker-coloured stones:

And, to finish off this part of the Post, three of the lighter-coloured stones:

What is the source of these stones? The following is a brief report on my initial investigations. Kai Iwi Beach is geologically part of the Whanganui Basin. This Basin has been described in “Landscape and Quaternary Environmental Change in New Zealand” (2016) as “a unique global archive”, unique because of the way it reveals “a shallow marine basinal sequence, exposed on land, which spans the entire Quaternary” (the last 2.6 million years). The cliffs and marine terraces along the coast from the mouth of the Whanganui River to the mouth of the Patea River expose a series of “sequences” of geological layering.

There are three parts to each sequence, described in a technical manner in a 1999 article in the New Zealand Journal of Geology and Geophysics as follows: The first part is “a basal suite of shore face and inner shelf sediments with intertidal and shallow subtidal molluscan faunas, and cross-bedded, pebbly shell gravels” – it is likely that the stones I collected and polished came from this kind of layer. The second part of each sequence is typically “a shellbed, which contains in situ offshore molluscs in a matrix of muddy fine sandstone or fine sandy siltstone”, with the third part being “siltstone, either bedded and barren of fossils, or bioturbated and with a sparsely scattered in situ fauna”. In other words, there are lots of layers here, many of which contain fossil shells and a scattering of stones.

The beaches along this coastline are often littered with rocks and stones that contain many fossil shellfish. A recent visit to the wild and secluded Ototoka Beach, not far from Kai Iwi Beach, resulted in many fossil shell finds.

See here for a two minute YouTube video on Kai Iwi Beach – the first part is on the more popular part of the beach a few hundred metres to the southwest of the Kai Iwi Stream mouth. At 1 minute 15 seconds, the Kai Iwi Stream is shown, then the beach past the stream mouth. 

This YouTube video features drone footage of Kai Iwi Beach. Most of the first part of the video is of the beach at Mowhanau Village but at 2 minutes 4 seconds it goes up the beach past Kai Iwi Stream, the area in which I found most of these stones, and it shows more detail of the stream itself from 2 minutes 25 seconds.

A YouTube video of drone footage of Ototoka Beach.