This stone is a light coloured argillite with trace fossil markings on it. It is another stone from the 22 December 2021 batch that Stones #19, #20 and #23 came from. Trace fossil markings can sometimes be difficult to see in photos so I have provided darker and lighter versions of some of the close-ups:
These traces were likely left in ocean floor sediment about 250 million years ago, probably by a small burrowing animal, some kind of worm or shellfish. Some of the traces look like they may have started as ripples in the surface sediment left by a fish swimming close by. Sometimes the traces can be seen on only one side of the stone, and if appearing on both sides (if the stone is relatively thin and flat) they can be quite different.
Argillite tends not to take a high polish and, in order to preserve the traces, I tend to start with only a 400 grit tumble before polishing.
For more information on TumbleStone Blog about these trace fossils, see Photo-Book “The Trace Fossil Stones of Gemstone Beach”; the Series of Posts that begin with The Fossilised Worm Cast Stones of Gemstone Beach and Riverton – Part One: Initial Identification; “I” is for “Ichnogenus Protovirgularia”; and “U” is for “Unusual Variations of Trace Fossil Stones” and “V” is for “The Chevron Shape of Trace Fossils”.
The Index to the January 2022 Stone of the Day Series is here.
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