The Stones of Ossett

granite sett

arms of Ossett

Like most towns, Ossett has developed in part because of the underlying rocks. The winding gear of a coal mine appears on the town’s coat of arms, reproduced on the paving stones of the precinct behind the town hall.

But the granite setts alongside it have come from further afield. The igneous/metamorphic base layer that forms the foundation for the British Isles is buried too far beneath Ossett to be accessible, even from the deepest coal mine.

quartz vein

batholithIt’s hard to believe that something so solid was once molten but in some of the slabs you can see streaking that suggests that there were currents flowing through the magma as the granite started to cool and crystallise. Quartz veins indicate that mineral-rich fluid was once able to flow through what is now impermeable rock.

Cross Bedding

cross-beddingcross-beddingYou can see some of the local rock in the old walls along New Street (which despite the name, is Victorian) to the south-east of the precinct. This cross-bedding in 300 million year old coal measures sandstone reminds me of the gritstone edges of the Pennines, and the weathered formations at Brimham Rocks.

deltaThese layers were deposited by a river, or a river delta, as underwater sandbanks. Coarser sediment was deposited at the start of each pulse of the current, finer sediment as the current started to ebb.

Ironstone

iron clawconcretionThis ironstone ‘claw’ attracted my attention. As I understand it, iron tends to precipitate out of solution when the freshwater of a river meets the brackish water of the delta.

Sometimes it is obvious that a concretion of iron must have formed within an underwater sandbank because the ring of iron cuts across sedimentary structures in the sandstone.

iron rollBut in other places, it looks to me as if a layer of iron has formed on the bed of the river and that this has since been rolled and ruckled, while still pliable.

Weathering

weatheringThis pitted surface looks like the result of weathering picking weaker parts of the rock.

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