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| A stream bank exposure of the Whinmoor Coal at Charlton Brook |
Arriving at the Bridge Inn, after my walk from Grenoside, I followed the path that runs east along the north side of Charlton Brook and soon came across Charlton Brook Dam, which was constructed by Newton Chambers and Co. in the 1870s, to supply water to the Thorncliffe Ironworks, but is now used as a fishing pond.
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| Charlton Brook Dam |
Continuing along the path for a short distance, I took the first opportunity to investigate the stream banks and immediately found an exposure of what is very probably the Penistone Flags, although the outcrop where I obtained my specimen is on the edge of its mapped outcrop and I didn’t have a GPS device to obtain a grid reference.
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| A geological map showing the position of the Penistone Flags (PF) |
The outcrop in the streambed and banks comprises irregularly jointed sandstone that occurs in thin beds of approximately 30-60 mm thick, which is very similar to the variation in thickness of the stone used in the dry stone boundary wall to Barnes Hall Farm on Bracken Hill, where the underlying bedrock is the Penistone Flags.
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| A streambed exposure of the Penistone Flags |
The sample that I obtained is grey/light brown in colour, with iron stained laminations giving it feint orange banding but no Liesegang rings were obvious in the bedrock. It has a very fine grained texture, with quartz being predominant and a small amount of mica scattered throughout the body of the sandstone.
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| A specimen of the Penistone Flags |
Exposures of disrupted flaggy sandstone in the banks and slabs in the streambed continued for some distance, after which it was succeeded by an extensive outcrop of sandstone, where the bedding plane was clearly exposed.
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| An exposed bedding plane in flaggy sandstone |
Standing on the edge of the brook to photograph this, I was very surprised to discover that I was standing over a small outcrop of well jointed coal, which was jutting out of the muddy stream bank just above the water level. Later looking on the online 1:50,000 map, its position coincides with an area that has been marked as a small infilled opencast coal site, which straddles Charlton Brook.
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| A small outcrop of the Whinmoor Coal |
The coal seam marked here is the Whinmoor Coal, which the geological memoir describes as a bright clean coal that is locally of value, and, when viewed from further downstream, the coal is seen to be overlain by yellow weathered mudstone.
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| An exposure of yellow weathered mudstone above the Whinmoor Coal |
Following the brook downstream, often walking along the streambed, there are further exposures of flaggy and very thinly bedded laminated sandstone, from the Pennine Lower Coal Measures Formation (PLCMF) higher in the stratigraphic column, from which I obtained another specimen.
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| An exposure of flaggy sandstone |
Although of a similar very fine grain size, it comes from a bed that is no more than 15 mm thick and has iron banding that has a more intense orange/red colour than the flaggy sandstone further upstream, with a scattering of unidentified black iron bearing minerals that have often broken down into oxides/hydroxides.
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| A specimen of flaggy PLCMF sandstone |
Continuing downstream, hopping from one rock to another with a decent pair of walking boots, I could easily imagine an undergraduate or postgraduate geology student - wearing Wellington boots - walking along the streambed to undertake a field mapping project along Charlton Brook.
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| An ochreous deposit |
In a couple of places I noticed ochreous deposits, which are associated with the microbial breakdown of the mineral pyrite found in coal seams, but I didn’t see any signs of the Black Band Coal that is marked on the geological map.
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| The streambed in Charlton Brook |
Further downstream, rock exposures become less frequent and the stream channel deepens and widens. Although small pieces of flaggy sandstone in the streambed are still abundant, the character of the deposits laid down in and adjacent to the stream channel changes noticeably and, by the time that I left Charlton Brook at Blackburn Drive, I could only see alluvial deposits.
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| Alluvial deposits at Charlton Brook |
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