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
Now available to purchase
Newsletter - December 2016
The full, illustrated newsletter is available as a pdf for download.
Text extracts are given below.
MGA Field Trip to Mam Tor
20 July 2016
Leader: Cathy Hollis
On a hot and sunny day a group of ten MGA members and guests visited key outcrops near Castleton. The late Visean to Namurian succession of carbonates and clastics, together with the regional setting, offered an excellent opportunity to examine an exhumed hydrocarbon system, which is analogous to the East Midlands hydrocarbon province in Nottinghamshire and Lincolnshire. The excursion provided an opportunity to examine all elements of the hydrocarbon system. We discussed the source and migration of the metalliferous fluids responsible for the emplacement of Pb, Zn, F, Ba; Mississippi Valley-type (MVT) mineralisation on the Derbyshire Platform. The trip allowed examination of the source rocks for the fluids, reservoir and host rocks within the Lower Carboniferous limestones also examples of hydrocarbon and MVT mineralisation.
Mam Tor outcrop allowed examination of the Edale Shale and the Mam Tor Beds. The Edale Shales are early Namurian in age. The dark grey, organic rich and often highly fissile shales represent the basinal mudstone unit that precedes the first influx of coarse clastic material into the basin - the MamTor beds. The Shales have a high TOC up to 5-10% in places and are analogous to the source rocks of the East Midlands oil fields and broadly equivalent to the Bowland Shale; potential Shale Gas reservoir in the NW. This makes the Edale Shale a source rock. Shell rich bands record death assemblages that show the link to open marine conditions to the South.
The Mam Tor beds are a series of stacked, basin-plain turbidites that record the first influx of coarse clastics into the basin. A variety of gravity flow related features are observabed; from classic Bouma-type fining upwards beds to debrites. There has been considerable debate whether there are any stacking patterns in the succession that might indicate that they are part of a more ordered submarine fan system.
The Mam Tor outcrops afford the opportunity to examine the recent land slip that affect the old A625 road, which was closed in 1979. The highway surface provides a magnificent and durable 'time-surface' recording slip events over the past 30 years. The landslip as a whole has features within it that provide a good opportunity to examine a range of modern features which offer a visual analogue to subsurface extensional settings (listric faulting) and rollover antiform formation. Slippage tends to occur in the winter months when groundwater levels are high; owing to lack of plant evapotranspiration.
Windy Knoll (Grid reference: SK124 829)
This former quarry comprises an exposure of Dinantian Limestone, with a palaeokarstic surface, which is overlain with poorly exposed boulder beds. Bitumen is seen within fissures associated with the karst surface and in the limestone breccias; providing proof of a once-active petroleum system. At the top of the outcrop, less viscous, complex oil (including sticky 'elaterite') is seen seeping from within the soil. The bitumen is biodegraded, resulting in removal of n-alkanes, reduced proportions of aliphatic and aromatic hydrocarbons and enrichment in nitrogen, sulphur and oxygen compounds (Ewbank et al., 1995). The hydrocarbons are consistent with derivation from type II (marine planktonic) organic matter and high thermal maturity, most likely within the Edale Basin (Ewbank et al., 1995), where the Edale Shales reached a maximum burial depth of ~3km in the Late Carboniferous.
Winnats Pass (Grid reference: SK135 826)
Winnats Pass offers the opportunity to traverse through the margins of an exhumed carbonate platform margin. The road follows the path of a palaeo-marine channel that dissected the platform margin and was further exploited by glacial melt-water in the Pleistocene. The architecture of the margin can be clearly seen with horizontal beds passing into basin-ward dipping beds that define the slope of the former platform. Small reef build- ups (mounds) can also be seen along the upper slope/platform edge. The group walked down the section and discussed the composition of the build-ups, the nature of the platform margin and its transition into the Edale Basin. The limestones are highly fossiliferous with abundant crinoids, green algae, corals (Lithostrotion, Dibunophyllum), brachiopods (Productus, Pugnax, Schizophoria) and occasional goniatites (Beyrichoceras), trilobites and bryozoa.
Odin's Rake (Grid reference: SK134 824)
Much of the mineralisation on the Derbyshire Platform is hosted within the faults and fractures that dissect the Lower Carboniferous (Dinantian) limestone. Odin's Rake (Fig. 5) is an E-W trending fault that hosts fluorite, calcite, galena, barite and quartz deposits; the group examined both the mineralisation in the walls of the fault and small samples.
Across the road is the crushing circle and mounds of waste material. Here the group had a very interesting discussion about the specialist plants that can survive in such environments.
Our thanks go to Cathy Hollis for an extremely interesting and thought provoking day and most of this report.
Edale Valley Field Trip Report
8 October 2016
Leader: Jane Michael
The group gathered on a cool but pleasant morning in the Edale car park, accompanied by a Wobble (the collective name for a group of cyclists) of cyclists and a multitude of walkers. When everyone had arrived Jane gave a short safety briefing, described the purpose of the day's trip and the route we were to take. She explained we would be looking at the form of the valley, the part the River Noe had to play in forming the valley, and the underlying geology.
The 8km Edale valley is asymmetric, with the river Noe flowing on the south side. The geology of the area is Namurian (320-
315Ma, a division of the Carboniferous period) and comprises alternating layers of weak shale and stronger sandstones on the upper slopes. To the north the upper levels are capped with Kinder Scout grit, which is displayed in the prominent tors that dot the skyline. Shale grit is represented by the bench features on the spurs that stand between valleys that incise the valley walls.
Jane explained that, despite the rounded shape, the valley was not formed by glaciers because the ice of the last maxima never reached here; the shape was due to deposition of periglacial deposits from higher up the slope sides. These deposits are termed 'head', a collection of fragmented bedrock caused by freeze/thaw effects at the edge of the glaciated area. Some of the rocks show a more shale-like consistency having come from the shale grit to the south. The valley floor comprises Edale Shale, a dark fissile mudstone.
The route took us up Jacob's Ladder; a laid pathway on route to Kinder Scout. From there we could see sphagnum peat up to 3m thick that was deposited 8000/6500 years BP. We could also see the tors on the ridges above us and, with the aid of binoculars, see the cross bedding.
With the briefing complete we set off following well defined footpaths and innumerable gates. At the first stop, where the path crosses the railway, Jane pointed out the difference between the two sides of the valley. Towards Mam Tor the valley side was incised by many short valleys up to 15m deep, whereas on the other side there were fewer but much larger valleys. She also pointed out the way the river followed a course closer to the Southern side of the valley. We also saw the lumpy topography due to numerous landslips.
We continued to follow the footpath and saw how the river had incised into the soft Edale Shale. The banks were much undercut making it too dangerous to get close. Where the underlying rocks were visible we noted the purple/grey colouration compared to the rocks of the valley sides. From here we could see the outcrops and sides of landslips on the northern valley side.
Having passed the information hut we proceeded to the foot of Jacob's Ladder and began the climb up to the cairn. From here we had a spectacular view down the valley that revealed the smooth sides and signs of landslips; a predominant feature of the landscape. From here we also had a good view on the tors on the skyline above us.
Having recovered from the climb we returned to the foot of Jacob's Ladder for a well-earned lunch. During lunch we discussed the landslip features and their possible causes. A section of hillside close to the bridge displayed a good example of the a slip and how the vegetation, in this case plants that thrive in wet conditions, could give a clue to the conditions beneath the surface.
With lunch over we returned along the same path as before, climbing the north side of the valley to Broadlee Bank Tor, where we examined an extensive landslip. This is a large slip and indicates the extent of the land movement in the valley. Smaller slips, obvious by the exposed earth, showed how dynamic this landscape still is.
On our way back to Edale we examined the river that runs through the village. It was clear how it had undercut the soft Edale Shale. On the banks we could see the orange colouration from the iron in the slate; the cobbles in the river bed having come from upstream.
As we walked back through the village to the car park Jane explained how some of the houses had sandstone tiles and others had slate. The latter only appeared when the railway was established so that they could be transported here.
At the end of the walk we all thanked Jane for her presentation and posed for a group picture.