The Fossilised Worm Cast Stones of Gemstone Beach and Riverton – Part One: Initial Identification

Large fossil worm cast stone, near McCracken’s Rest, Orepuki. It was too large to tumble polish so I did not take it home with me.

Other side of large fossil worm cast stone, near McCracken’s Rest, Orepuki

Fossil worm cast stone on Gemstone Beach

Fossil worm cast stones found on Gemstone Beach, May 2019

The tourist information on Gemstone Beach near Orepuki, 29 kilometres west of Riverton, sometimes mentions that fossil worm cast stones can be found there. For example: “A few hours beachcombing could easily yield gems such as hydrogrossular, jasper, fossil worm casts and the elusive sapphire” (Heritage Trail website). The Heritage Trail sign that used to stand at Gemstone Beach also mentions fossil worm casts.

Source: https://talltales.me/2013/02/24/the-south-of-the-south/photo-2-01-13-10-30-21-am

Over the past couple of years, I have believed that the sort of stones being referred to here would have within them, or on them, the fossilised remains left by worms at the bottom of the sea, these remains being the castings excreted by them as they fed. This series of Posts reports on the research I have done to check out this belief.

The photos below show what I have come to think of as fossil worm cast stones. They have all been collected by myself mostly from Gemstone Beach but some also from Riverton beaches:

Polished fossil worm cast stones from Gemstone Beach.

Fossil worm cast stone 1

Fossil worm cast stone 2

Fossil worm cast stone 3

Fossil worm cast stone 4

Fossil worm cast stone 5

Fossil worm cast stone 6

Fossil worm cast stone 7

Fossil worm cast stone 8

Fossil worm cast stone 9

In a small entry on types of fossils written by Hamish Campbell in 2006 in Te Ara The Encyclopedia of New Zealand, the example of a fossilised worm burrow is very similar to the stone in the photo centre right above. Campbell writes: “The worm-like impression is called a trace fossil, as it shows traces left by a worm burrowing through mud.” 

Top left is a rock with a fossilised worm burrow. Source: https://teara.govt.nz/en/photograph/9020/three-types-of-fossil

Note that Campbell mentions “burrows”, not “casts”. Maybe the distinction is important – we will return to this in a later Post.

Furthermore, the Riverton Museum Te Hikoi includes two “worm trace fossil” stones in one of it rock drawers (see photo below). They look similar to some of the ones I have collected on Gemstone Beach and Riverton.

Riverton Museum Te Hikoi display. Photo taken with permission.

Jocelyn Thornton’s (1985) book, “Gemstones”, has a page on “Orepuki Beach Pebbles” (page 36). She refers to a group of stones (number “5” in her diagram) as follows: “Those at the top of this group clearly show layers of mud, worm trails and wave ripples…” (see photos below). She uses the term “trail” and not “cast”. None of these stones really look like the fossil worm cast stones I have found.

Page 36 of Jocelyn Thornton’s (1985) “Gemstones”. Source: https://www.onnaturesedge.co.nz/downloads/nzgemstones.pdf

Detail of page 36 of Jocelyn Thornton’s (1985) “Gemstones”. Source: https://www.onnaturesedge.co.nz/downloads/nzgemstones.pdf

The next Post in this series will look more closely at what a “trace” fossil is.