TumbleStone Calendar 2026: July – Trace Fossils in Argillite

The Introduction to this Calendar can be found here. 

July’s stones in the 2026 TumbleStone Calendar are five Gemstone Beach argillite pebbles with trace fossils in them. Locally they are often called fossil worm cast stones. Argillite is a sedimentary rock, a hardened slightly recrystallised mudstone. Along with greywacke, it comprises the basement rock of New Zealand. On Gemstone Beach, argillite stones are mainly green in colour but there are grey and red ones as well, with some banded argillites being brown. Examples of Gemstone Beach argillite stones can be found here. Trace fossils can be found in sedimentary rocks – they represent the movement and burrowing of animals across or through the sediments. A “trace” fossil is not a fossil of the animal itself but of something it has left behind. In the case of July’s trace fossils, the traces are a different colour than the surrounding rock because the burrows or trails have been infilled by different coloured and often finer-grained sediment. Some of the trace fossils in argillite on Gemstone Beach can be plain, short and straight but others can be sinuous and long, with lines of chevron shapes – this diversity can be seen in “The Tattooed Rock, The Trace Fossils… Revisiting Gemstone Beach’s Trace Fossil Stones”. The most interesting, to me, are the traces that fit into the protovirgularia grouping – see “Ichnogenus Protovirgularia and the Permian Rocks of the Eglinton Valley”.

Stone One (top row, left, in the Calendar) contains a number of trace fossils, the main one being a good example of a long sinuous protovirgularia trace made up of a number of chevron shapes:

The smaller roundish “spot” traces in the stone are likely to be end-on views of long traces – the burrow or trail is at right angles to the longer traces we see. I found Stone One on 25 March 2025 – it is a grey-green colour, not uncommon for trace fossil stones.

Stone Two (top row, right, in the Calendar) has a straighter protovirgularia trace in between two plainer ones which also have a hint of chevrons:

Stone Two was found on 12 August 2025, and it is a lighter green than Stone One, again a common colour for Gemstone Beach’s trace fossil stones.

Stone Three (bottom row, left, in the Calendar) is a dark, almost black, argillite – the traces within it are a mix of black and light green, looking much like caterpillers crawling through it:

I found Stone Three on 3 August 2025, the day I saw an aircraft land on the beach between Gemstone Beach and Monkey Island.

Unlike Stones One, Two and Three, Stone Four (bottom row, centre, in the Calendar) has a couple of trace fossils that are darker in colour than the surrounding rock:

It looks like there are very tiny pieces of dark grit or fragments in the two central traces, giving them an unusual texture. I found a number of variations of trace fossil stones the day I found this one, on 21 August 2025.

Stone Five (bottom row, right, in the Calendar) is one of the more rare types, having red traces:

Again, the trace belongs to the protovirgularia family. I found Stone Five on a bitterly cold day, on 27 September 2024, another good day for finding a variety of trace fossil stones on Gemstone Beach.

For August’s set of yellow stones, see this Post.