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A view of the first tors encountered at the Ox Stones |
When exploring the Rough Rock in Ringinglow, I was unable to find the old quarry faces at the Brown Edge Quarries which, I have subsequently learned from friends in the Sheffield U3A Geology Group, are not easy to find. On a field trip with Peter Kennett, in whose publication the Geology of the Porter Valley I learned about this site, they approached from a completely different route to the one I attempted.
After retuning to the public footpath back to Ringinglow Road, I got my first glimpse of the Ox Stones, a group of tors to the west of Lady Canning’s Plantation, which stand proud of the surrounding heather and blanket bog covered moorland formed on the Chatsworth Grit bedrock.
Crossing over Ringinglow Road and following the well maintained public byway for a short distance, the path to the Ox Stones is less well maintained and subject to erosion by water run off and, to mitigate this in places where it presumably gets quite boggy, it is paved with riven flagstones and on one of these I noted some spectacular ripple marks.
Arriving at the first tor, the first thing that I noticed was that the thinly bedded Chatsworth Grit here displays steep cross-bedding in its lower part, which I would consider to be quite typical of most coarse to medium grained Upper Carboniferous sandstones that I have seen, but the upper part has extremely shallow cross-bedding.
Looking at the relationship between the two parts on my high resolution photos, instead of a planar surface, which typically separates beds of sediment, the upper surface of the lower part seems to be rippled, with these original sedimentary structures being filled with later sediment instead of them being eroded away.
Moving on to the second tor, this difference between the lower and upper part is seen again, with the latter beds being essentially horizontal and the cross-bedding seen is again at a very shallow angle. Differential weathering has highlighted the individual beds in the tors, but it is also noticeable that the lowest exposures of the gritstone are very massive.
I have seen many outcrops of the Chatsworth Grit and have noted considerable variation in bed thickness, grain size and the scale of cross-bedding which, to my understanding, reflect variations of the flow regime – with large underwater dunes at the base and thinner planar beds at the top reflecting a faster flow of the river.
Looking around me, I could see several large blocks of gritstone scattered around the moorland, which are large enough to be marked on the Ordnance Survey map. I didn’t spend any time investigating these, but instead headed towards another group of less well developed tors that lie a short distance further to the west.
Although the principal rock formation that forms the spectacular gritstone edges, the Chatsworth Grit, is much harder and more durable than the mudstones and shales of the Marsden Formation that occur above and below, it contains a high proportion of feldspar and clay minerals.
When broken down into much smaller pieces than seen on the escarpments and tors, the weathering of these minerals leaves the Chatsworth Grit very friable and its rapid erosion accounts for the thick layers of coarse sandy soil that I had seen exposed on paths when walking along Curbar Edge and exploring Blacka Moor Nature Reserve and here at the Ox Stones.
I have not seen much information about how these and other tors – especially those seen by hardened walkers on the high moors of Kinderscout and Derwent Edge - were formed, but an article by Professor Matthew Blackett suggests that they most likely formed when the region had a tropical climate and the gritstone was subjected to deep weathering processes and with widening of joints in the bedrock, which have since become exposed as the weathered material above has been eroded away.
An alternative theory is that they were produced in periglacial environments with freeze-thaw activity shattering the exposed bedrock material and erosive processes removing the material; however, the only other tors that I have seen are the Eagle Stone on Baslow Edge, the Cork Stone at Stanton Moor and a prominent example on Higger Tor, none of which I have studied closely and I am therefore unable to comment further.
I only spent 15 minutes exploring the Ox Stones, but there are a wide variety of interesting sedimentary structures here, including what I think must be trough cross-bedding, and I would be very interested to visit the site again with an experienced sedimentary petrologist.
Of paerticular interest were some very fine examples of very coarse grained beds, which contain rounded pebbles of white quartz, which are up to 15 mm in diameter. They were formed at the base of the large subaqueous dunes formed in a large river that flowed into a vast Namurian delta, with the source being granite in the Caledonian Mountains that existed far to the north-east.
The collecting of rock specimens from spectacular rock formations as The Ox Stones is considered by the geological fraternity to be completely unacceptable; however, numerous small pieces of Chatsworth Grit were lying around the eroded paths. Although many of these, as I have previously discovered, can be quite ‘rotten’ and easily crumble, I did manage to obtain a specimen of unweathered rock to add to my growing rock collection.