Tag: Concretion

Ironstone Concretions

ironstone concretion

Natural sandblasting has hollowed out the softer sandstone and left the resistant bands of ironstone standing to give a Swiss cheese effect to this block in an old sandstone wall.

Iron-rich deposits can be precipitated as a scummy layer where a river meets salt water in an estuary. Perhaps that happened 300 million years ago when this sandstone was laid down.

Without knowing exactly what happened, you can still sometimes deduce the sequence of events. Sometimes a rolled pebble – a mini-Swiss roll of ironstone – suggests that a layer of iron had begun to form on a river bed but that it was rolled away by the current before it got covered by the next pulse of sediment.

In other cases, target-shaped nodules conspicuously cut across multiple layers of sandstone, suggesting that the layers came first and that the iron precipitated out as mineral-rich solutions percolated through the sediment.

Ironstone Concretion

Lens of white sandstone in Coxley Quarryironstone noduleLike a ripple on a pond or a blot on a page ironstone concretions spread out across the layers of sandstone in Coxley Quarry so I guess that they must have formed after the sandbanks were laid down, perhaps during the process of solidification.

There’s a sausage-shaped patch of pure white sand a couple of feet across which is encased in a rusty crust. It looks as if the iron has been leached out by a mineral-rich solution and I guess that this then bubbled upwards through the sand because above the lens of white there’s a knobbly network of weathered-out rusty chambers.

rolled ironstone noduleThere are also rolled pebbles of ironstone. What seems to have happened here is that a rubbery crust of iron-rich gunge has formed on the bed of the prehistoric river then a strong current has dislodged it and trundled it into an ironstone Swiss roll. Iron is deposited when river water, rich in iron salts, meets brackish water.

deltacoal forestYorkshire was on the equator at the time this rock was laid down 300 million years ago, in a low lying area of lagoons, river deltas and the tropical forests which would form coal.

The surface of the Earth would be lacking in colour if it wasn’t for iron-rich minerals which range through ochre yellows, rusty reds and mineral greens to the fool’s gold of iron pyrites.

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.