Thanks for that Robert, can you please explain that.goldierocks said:I think that it is more likely a concretion
Thanks again Robert...a learned response as always. No need to google it, they could do no better.goldierocks said:The layering is both horizontal turning up to vertical, from what I can see in the photos. Growth lines in trees are all vertical - so that is the main clue.
Yet the layers are all concentric. whether or not they are vertical or horizontal. This suggests to me that these are not layers that reflect laying down of layers in a sedimentary rock (although I do suspect that the rock was sedimentary nevertheless) - that is clue 2.
The colour suggest to me that the rock is iron-rich - clue 3.
Therefore I suspect that the layering reflects chemical precipitation - this is most commonly iron oxide, silica (silicon dioxide) or carbonate minerals (such as calcite, dolomite, magnesite) - the colour suggests iron oxide.
Concretions are deposited layer upon layer, each layer precipitating on the previous layer. They can be deposited around something in the rock as these chemicals move through the rock dissolved in water. They might hit something like a tiny sphere such as a pebble - then they will precipitate around it to form a sphere. They might hit something linear like a tree root - then the layers will precipitate around it to form a cylinder. Or they might just precipitate parallel to the bedding in a sedimentary rock (bedding is usually laid down horizontally, by grains of sand and mud settling vertically in the sea, lake or river).
A shape like this could form in a sedimentary rock in which there was a vertical tree root, for example. Just a guess, I could only say for certain if it was in my hand. Imagine that you had a cake with a candle in it, and you were wrapping it in Glad Wrao. Each layer of Glad Wrap would be a similar shape to each layer in a concretion.
For more information, Google concretions.
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