Manchester Building Stones
The North West Geologist
|A Building Stones Guide to Central Manchester|
Third Edition (2014)
Four self-guided walks through the city centre
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Newsletter - December 2017
The full, illustrated newsletter is available as a pdf for download.
Text extracts are given below.
Joint MGA/GeoLancashire field excursion to the Shap area on 1 June 2017
A group of about twelve people met at Shap pink quarry to which access had kindly been granted by Armstrong Ltd.
The quarry has recently been reopened and loose blocks provided plenty of new surfaces for inspection. For safety reasons the main face was not approached. From a distance at least two suites of cross-cutting joints were observed.
Examples of dark and light granite facies were identified. Basic enclaves with and without the pink potassium feldspars were also seen. These are thought to represent an early primitive Shap magma stage formed during mixing of basaltic magma and the granite host.
A small number of potassium feldspars were surrounded by pale halos. Similar halos are seen in Rapakivi granite. However, here the feldspars retain their original crystal form and cleavage.
No-one present was able to provide an explanation of this phenomenon. The author has since tried to find peer reviewed papers on the subject, and failed. Can anyone help?
A hand specimen of the granite margin was found, which showed the temperature of the intrusion was clearly not high enough to alter the texture of the country rock. On the other side of the specimen the long axes of feldspars, close to the contact, are aligned parallel with it.
The next stop was Shap blue quarry. We are most grateful to Paul Jennings of CEMEX who led a fascinating tour of this quarry. The quarry, which is within the metamorphic aureole of the granite, is famous for the occurrence of garnets in mineralised joints.
The quarry is worked at a number of levels, which was fortunate because during the torrential rain of Storm Desmond in December 2015 about 12 inches fell locally in 24 hours. The lowest part of the quarry became a lake more than 20 m deep and took several days to pump out.
With permission from CEMEX an exposure in a stream between the blue and pink quarries was visited. Here could be seen aplitic veins of granite, several tens of millimetres thick extending into the country rock.Some contained potassium feldspar phenocrysts.
The group took lunch by the stream at Shap Wells hotel, serenaded by a noisy cuckoo. We expressed thanks to the hotel staff for permitting us to park cars in the hotel carpark. The basal Carboniferous unconformity was examined. Feldspar phenocrysts were seen in the conglomerate. Fortunately the water level in the stream was sufficiently low for the braver members of the group to cross, albeit at a cost of some wet feet.
The group then moved on to Orton. A track near Gamelands stone circle leads onto the local typical stepped limestone topography. Gamelands consists of a circle of Shap granite erratics nearly a cubic metre size. The circle is thought to date between Late Neolithic and Middle Bronze Age. It is a scheduled Ancient Monument - see Historic England List Entry Number: 1011138.
The erratics are deeply weathered and typical of many that can be found on nearby hills and fields. The track ends at a lime kiln beyond which is a small quarry in the Ashfell Limestone of Holkerian age. Examples of the tabulate coral Syringpora and the calcareous sponge Chaetetes were prominent.
Pushed for time the group moved on to have a brief look at the Silurian turbidites that are exposed in road cuttings near the M6. Cleavage bedding relationships were discussed and how these relate to adjacent synclines and anticlines. Flute casts were observed and textures, which were probably the result of beddingc leavage interaction could, be seen on some bedding parallel surfaces.
The leaders were thanked and members made their various ways back home.
Peter del Strother
Fred Broadhurst Memorial Field Trip Saturday 5 August 2017
This annual commemorative field trip to Lud's Church and the Roaches was led by Jane Michael and attended by ten members of the MGA and the OUGS. The route started from the car park in Gradbach and, after a visit to the tors above the Black Brook, we reached the impressive rift of Lud's church. From here a short, if steep, climb to Roach End and then onward (and upwards) to the trig point on Roaches Ridge. Here we broke for a well-earned lunch before walking back (thankfully downhill) through Forest Wood to the Black Brook and the café at Gradbach Mill.
At the carpark Jane gave us an overview of the area and what we could expect to see. Then we moved off to the side of the River Dane where we could see, on the far bank, signs of hummocky soil from landslips further up the hill. Unfortunately the view was obscured by the trees but we could just see the slip effects.
With the river behind us we followed the road towards Gradbach Mill stopping on the way to examine a small exposure of probably Chatsworth Grit that had been exposed during the construction of the road. It was overgrown but the bedding planes and dips were just visible.
From the exposure we walked along the road to Gradbach Mill. The mill, built in the 1780s, was originally used for the possessing of flax for jute production; later it was converted into a saw mill. Recently it has been developed as a conference centre. Jane explained that, as a water mill, it had a wheel with a diameter of 38 feet and a gearing ratio of 1: 2500! (Some speed for a water mill).
After a short walk through Forest Wood we reached the Scout camp and, looking across the flat ground, we could see how the river Dane had incised into the valley floor. On the far bank we could see an exposure of the Roaches Grit. It was noted that it appeared to dip eastwards. Regrettably we were unable to get closer to examine it in detail.
Moving on we reached the confluence of the river Dane and the Black Brook. The beds of both rivers contained a number of large boulders. Jane indicated that they were deposited by the river when the water flow was much higher than today; possibly quite recently. Some may have washed down from the sides when the river incised the bank.
On the banks of the river Dane there is evidence of river terraces; signs of the past river levels.
Climbing up through Forest Wood we reached Castle Rocks; the tors that stand above the woods. We paused here to give everyone a chance to examine the formations. Formed of Roaches Grit they exhibited good examples of cross bedding and extensive jointing. A wide range of grain sizes is indicative of changes in the dispositional environment. Large fallen blocks were scattered around the site. The land-slippage is post glacial and there is evidence that the movement is still occurring.
Leaving the tors we climbed up to Lud's Church. This unique feature in the Roaches Grit provides an atmospheric environment that has a long history of religious persecution and ancient folk law. As we approached Jane explain some of the history and a general description of the site.
The rift runs approximately SE to NW with a landslip on the NE side; it is thought to be post glacial. It is about 100m long and 15m wide at the top, narrowing as it reaches the path at the base. As it is a popular visitors spot some effort has been made to provide a safe path though the gorge.
The cause of the landslip is not known, but is possibly due to erosion at the base by the river (Black Brook) as happened in Edale. Gravity and a possible slip plane, maybe a layer of mudstone/shales, allowed the land to move and tilt on mass.
As we walked through the gorge we noted an example of fossil wood in the path. It was surmised that this would have been carried by the water that laid down the Roaches Grit.
Having passed through Lud's Church we continued our climb through the woods to Roach End; the end of the Roaches themselves. At the crest we had a spectacular view east across the Cheshire Plain towards Macclesfield and north the peak of Shutlingsloe.
Slightly tired and wind swept the party reached the trig point on the Roaches. It afforded us a fantastic view across the Cheshire Plain and beyond including a very good view of the Goyt Syncline.
Battling against the wind Jane indicated the shape of the syncline, the dip and scarp slopes and some of the drainage basin that fed the Black Brook. Ramshaw rocks scarp slope is not visible as it dips away from our location.
Seeking shelter off the trig point, and with one eye on the ominous clouds gathering in the distance, we all enjoyed our lunch.
Using the shelter from the wind Jane took the opportunity to explain and point out the features of the syncline. She indicated the changes in vegetation that showed the changes in rock types from the Rough Rock to the Woodhead Hill Rock. The exposure of the differentially eroded rocks of the tor provided good 3D exposures; a lively debate ensued about the cross bedding, deposition and processes involved.
Following the path back down from the tor (and the impending rain) we returned to Forest Wood and followed the course of the Black Brook until we reached a point where the rock on the far bank was exposed. Here we could see the bedding planes, the jointing and possibly a small fault. It was obvious from the bank sides that the Black Brook had incised deeply into thev alley bed and from the size of the bed material the flow rate, even in normal conditions flow was quite high.
Unfortunately by this time the rain had started and we made our way back to the café at the Mill where we posed for a group photo and enjoyed a relaxing drink.
It had proved to be an interesting and most enjoyable day and all thanks goes to Jane for the excellent commentary and organisation.