Rocks in the canal cutting at Old Royston


Click on pictures to enlarge with captions

The Barnsley Canal at Old Royston lies in a cutting about a kilometre long, north and south of High Bridge.  Along the cutting can be seen some interestingSmall cliff of fine-grained sandstone with closely spaced bedding planes rock exposures which are of Upper Carboniferous age and about 300 million years old. 

The rocks were laid down in a swampy delta which would have been similar to the present Amazon Basin.  300 million years ago the climate was hot and rainy and the land was covered by tall forests unlike any of today’s forests.  Giant ferns grew to 30 metres tall: there were no birds, only large flying insects with a wing span of 80 cm; no flowers or blossoms; no monkeys or other mammals. 

In the muddy lagoons, there were carnivorous amphibians. The muds in the lagoons and swamps under the trees slowly accumulated and were compressed to form black or dark grey, laminated rocks called mudstones.   Dead plants decomposed to peat in stagnant waters.   Coal seams formed when peat was deeply buried by other sediments. 

About 500m north of High Bridge some boulders have fallen down onto the canal towpath from some thick sandstone beds at the top of the cutting.  The sands were deposited in the channels of the rivers that meandered across the delta, close to sea-level.  Eventually, the sands were buried by hundreds of metres of sediment and compressed.  As water was squeezed out under pressure, it carried minerals which cemented the sand and mud grains together to make sandstones and mudstones. 

Sandstone, with fine bedding and ripple marksIf you look closely at the sandstone boulders on the towpath, you can see layers or beds of sand grains, which show which way the sand was washed along the bottom of the river bed.  This feature is called cross-bedding.   In places there are traces of ripple marks where gentle currents moved the sand in shallow water.  The sandstones would have been good quality for building stones and they may have been exploited for building bridges and walls locally.

The canal runs through the Yorkshire coalfield and boats would have carried plenty of coal, so it is fitting that two coal seams run through the cutting.  The Houghton Thin Coal, as its name suggests, was only about 1ft 2in (35 cm) thick in this area and was recorded as being thin and variable in quality.  However, the Sharlston Yard Coal was up to 36in (nearly a metre) thick and was worked widely in the Shafton and Crofton areas.

Beneath each coal seam lies a layer of the soil (or seatearth) in which the trees grew.  Clay soils were leached of iron by the organic acids produced by tree roots and so the seatearths are white or yellow.  In this area, the seatearths are clay rich and are named fireclays, because they could be fired to make refractory bricks for furnaces and domestic hearths. 

On the east side of the canal, to the north of High Bridge, are two overgrown quarries, which were probably excavated to remove both the coals and fireclays found on the canal sides.

The reason why all these interesting rocks can be seen in the canal cutting is because the continent was pushed up above sea-level and tilted to the south by plate tectonic movements, a few million years after they were deposited.   This area of Yorkshire was weathered and eroded for millions of years, so that any rocks laid down later, were removed, leaving the navvies of the late eighteenth century to expose these interesting and useful coals, fireclays and sandstones as they excavated the canal cutting.

Alison Tymon
West Yorkshire Geology Trust
November 2012

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