That’s a textbook septarian concretion, sometimes called a “turtle rock” or “dragon stone.” Nice find for the Greensburg area!
The woods around Greensburg sit on top of Pennsylvanian-age shale and sandstone formations from ∼300 million years ago. These concretions are super common in that geology.
Why I’m calling it septarian:
The cracking pattern: You’ve got that distinct blocky, turtle-shell look. Those cracks are called “septa.” They form when a mudstone/siltstone concretion shrinks and cracks internally as it dries.
The infill: The cracks got filled later with darker minerals, usually calcite or siderite. You can see that dark brown/black crystalline material between the lighter tan/gray blocks. That’s the telltale sign.
The shape: That rounded lobe on the side is classic. Concretions grow outward from a nucleus and often have odd knobs.
Are these fossils?
Not the rock itself. Septarian concretions are geologic, not biologic. But they sometimes form around a fossil nucleus like a shell or plant bit. If you split it, you might find something inside. Most are just mud though.
People cut and polish these because the contrast between the blocks and cracks looks amazing. Western PA has some great ones due to all the old coal swamps and shallow seas we used to have.
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