Tuesday, 7 September 2021

The Crawshaw Sandstone in Crookes

Cross-bedding in the Crawshaw Sandstone
When visiting Crookes, although I had made plans to look at the work of the Sheffield architect, WJ Hale, I had also read an article by Peter Kennett on the Sheffield Area Geology Trust (SAGT) website, where the use of stone from Bole Hill and Walkley Bank to build some of the Sheffield Board Schools has been highlighted.
Cocked Hat Cottages
I was particularly interested in the photo of the outcrop of the Crawshaw Sandstone at Bolehills, which showed a form of cross-bedding that I had not seen before during many surveys of the Upper Carboniferous sandstones in and around South Yorkshire.

A view from Cocked Hat Cottages
Having walked up Springvale Road, after looking at the former Crookes Congregational Church, and continuing along Bole Hill Lane, I briefly stopped at Cocked Hat Cottages to admire the fantastic view of the area to the west of Stannington.
An outcrop of Crawshaw Sandstone at Bolehills

Despite the ‘rules’ that were in force during week 50 of the COVID-19 Pandemic, I wasn’t at all bothered about the number of people there – especially since we all wanted to appreciate the best view in Sheffield - but I was concerned that the rock exposures were ‘occupied’.
Looking for a place to sample the Crawshaw Sandstone
Armed with my new Estwing 20 oz brick hammer, I wanted to collect a sample of the Crawshaw Sandstone but, although there were a few loose blocks lying around, these can often be deeply weathered - as every geologist will know.
Exposed ripples on a bedding plane
I spent a few minutes wandering around, to look at some of the ripples exposed on the bedding planes, before I finally found a spot where I could collect a sample. At Treeton Wood, I easily hacked out a piece of sandstone from the lower subsoil, but here I had to give a corner of rock a few good whacks with the blunt end before a surprisingly large lump came away.
A fresh surface of the Crawshaw Sandstone
The samples of sandstone that I obtained are fine grained, with predominantly angular quartz, a high proportion of weathered feldspar and a significant amount of mica – especially where the bedding planes are exposed – and the body of the stone is speckled with the rust coloured hydrated iron oxide limonite.
A hand specimen and a view with a digital microscope

Feeling a touch of guilt that I had knocked off a lump of a RIGS (Regionally Important Geological Site), having followed a general principle of geological conservation that no hammering should be undertaken for many years, I continued with my investigations.
Well defined cross-bedding in the Crawshaw Sandstone

I didn’t want to disturb the girls that were eating their lunch on the outcrop of rock that I had set out to photograph and - after signalling my intentions to them - I had a good look at the well defined beds, which are split into beds that are often not much more than an inch (24 mm) thick.
Cross-bedding in the Crawshaw Sandstone

I had seen various flaggy sandstones in Canklow Wood just recently, which I had interpreted as being formed in the upper flow regime, but these usually passed upwards without a break in sedimentation from a very massive sandstone with large-scale cross-bedding.

The Crawshaw Sandstone at Bolehills

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