These look to me like regular intrusive iron rich veins. Boulder is about 1m across, and sandstone/mudstone, area is volcanic.
I am not a geologist.. any ideas?
It looks like mud cracks/desiccation structures to me.
this post was submitted on 01 Apr 2024
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Yeah, looks like a septarian nodule to me. I'm not sure what makes something a septarian nodule versus some other name, but basically, mud dries out leaving cracks that later fill with something else. Sometimes that "something else" is stronger than the underlying mudstone, and sometimes it's weaker.
It certainly looks stronger! I was suspecting it's acting somewhat like rebar in reinforced concrete.