SAGT News 2024

Geological Ramble No. 3

Our Geological Ramble was held as part of the Nether Edge Festival and Sheffield Heritage Open Day. It took place on Saturday 14th September 2024 walking down the Porter Valley in Sheffield. It was led by Peter Kennett and John Cripps, assisted by myself, all retired geologists. We attracted an audience of 29 people (our very maximum due to the nature of the sites we were to visit), 3 dogs (keen amateur geologists) and several children of various sizes in prams, all of whom we need to attract into our industry.

Our Ramble commenced just below Forge Dam Cafe with an introduction to the geology of the Porter Valley with maps, rock and fossil specimens and a scale model. We discussed what we hoped that we would see. We were in luck with the weather; dry and bright - perfect weather for our visit.

We first walked downstream looking at down the valley floor spotting the sandstone unit known as ‘Rough Rock’ that is exposed below the dam and in the valley sides. There are the remains of a large quarry directly above the valley on the moorland top. The stone was used for many types of construction in the City’s past and was a valuable resource.

We then walked off the path crossing the valley to view a monocline, an arch like structure ‘pointing downwards’ and a syncline, with one limb nearly vertical, in the Porter Brook bank sides and floor, created over millions of years by intense pressure squeezing together the rock, shortening the space it occupies.

After a further clamber over undergrowth we saw in the valley side what were quite large fossil tree roots from the Carboniferous Period in two distinct places. From the Carboniferous Period we noted the junction of Millstone Grit Series and Coal Measures rocks, which all date from around 320 million years ago. Thin coal seams are present in the valley sides, although their presence is now only marked by the red ochre-coloured water issuing into the valley from the previously worked coal seams which have now collapsed or been filled in. Coal is present in other parts of the local area such as under Abbeydale Woods and off Ecclesall Road.

We left the valley to walk up a short but steep section of Highcliffe Road, past the site of a small drift mine which closed in 1929, using maps and photos for the evidence of this type of mining.

We then took the path below High Storrs field to see line of some coal extraction “bell pits” and some possibly not! In addition to the coal, many of these small local mines worked the underlying ganister – a very hard pure silica sandstone much prized in the refractory industry.

We returned to Forge Dam via the old allotments and across the former playing field of King Edward’s School, noting the importance of the Loxley Edge Rock in forming the scarp edge at Bents Green above. This year’s Ramble ended in the cafe for coffee and cake. Will there be a 4th Ramble next year? Watch this space!

Ian Prior