Sunday, 20 February 2022

Grenoside Sandstone at the Octagon

Silty beds in the Grenoside Sandstone

On a day that I had set out to explore the Sheffield Board Schools at Netherthorpe, Crookesmoor and Broomhill, I also encountered several hills of various steepness that reflected the strike and dip of the underlying Pennine Lower Coal Measures Formation sandstones, along with the intervening vales forming on the mudstones.
A vale between the Loxley Edge Rock and the Greenmoor Rock
When preparing my route, as I had done for my walk around Attercliffe and Darnall a month earlier, I had referred to the list of publicly accessible Local Geological Sites on the Sheffield Area Geology Trust (SAGT) website – one of which was at the Octagon Centre at Sheffield University – and I decided to go and investigate it on the walk back to Sheffield city centre.
Local Geological Sites accessible to the public in Sheffield

Having spent several months working as a geologist at the nearby Weston Park Museum and knowing the curator of geology well, through my involvement with the South Yorkshire RIGS Group, I was very surprised that I hadn’t known about this site before.
The outcrop of Grenoside Sandstone at the Octagon Centre

According to the 1957 Geological Survey of Greet Britain Memoir, the Grenoside Sandstone is, as a rule “remarkably micaceous and fissile, in places splitting into thin flat plates bounded by bedding planes which are covered in large plates of mica”.
A description of the Grenoside Sandstone in the geological memoir

The accompanying British Geological Survey map shows the actual outcrop of the steeply dipping Grenoside Sandstone to be slightly to the north-east of the Octagon Centre, as recorded by SAGT, but the sandstone seen here certainly has these characteristics.
A geological map of the area around the Octagon Centre

The exposure is no more than 15 metres long and 2 metres high, but it displays a section of predominantly thin beds of fine micaceous sandstone, which is pervaded by persistent thinly spaced joints that breaks the rock into small irregular lumps.
Joints in the Grenoside Sandstone

Although I had my Estwing hammer with me, I had no need to use it when collecting a sample and, in places, the sandstone passes into much finer grained siltstone beds, which are highly laminated and can easily be pulled apart with the fingers.
Fine sandstone and siltstone at the Octagon Centre

In places like Grenoside and Norton, where it occurs in thicker beds, it has been used locally for building stone but the stone found here, which has been disrupted by the folding that formed the Don Monocline, is unsuitable for this. The sample that I collected is fine grained in texture, grey/light brown in colour, with iron banding and very micaceous on the bedding planes.
A sample of Grenoside Sandstone

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