East Anglian Branch Newsletter
October 2017 / /
Images from Wikipedia and Colin Mould
Branch Organiser’s Bit
Hi everyone
Autumn is here and I am ashamed to admit that I have done nothing geological this summer. Other things have happened including the arrival of my first grandchild. Just think I started as BO when my second child was a small baby.
I gather the trip to Sutton went well (I was unable to attend as it was the same day as Cara’s dedication) I was sorry to hear that not very many people attended and that some members who said they would be going failed to turn up with no explanation as to why they didn’t turn up
Only one person wanted to go to Walton on the Naze so I had to cancel the visit
As a committee we are asking whether members want field trips and if not what do you want in their place?
I hope that people are going on the Sedgwick visit and walk looking at building stones in Cambridge. Organising this trip is part of Emma’s OU course so please, please support her.
REMEMBER TO PUT THE AGM DATE IN YOUR DIARY/CALENDAR –NOW – FEBRUARY 10.
Does anyone know a possible speaker?
Gareth’s trip could well be Freshwater as he is writing a new book on the area. I hope to have the date soon but Gareth is away at the moment.
I am trying to arrange a visit to the Lapworth Museum in Birmingham probably March next year
On Saturday I will be attending the NE committee meeting at Keyworth (This will have happened by the time you are reading this)
I hope some of you have done some exciting trips this summer and that I will be reading about them in future newsletters. Today I went to Biddulph Grange (NT) which has a very interesting geological gallery but my mother was too tired at that point so we had to take her home (I did write about it in the newsletter a few years ago but as I am not at home I can’t look up when).
Wendy Hamilton
Field excursion to Charnwood Forest.
I was fortunate to be able to join this weekend excursion organised by OUGS Wessex branch and led by Keith Ambrose and Ian Williamson. Charnwood is justifiably famous for having some of the oldest exposed basement Precambrian rocks in England, and internationally important in yielding the first macro fossils of identifiable Precambrian age found anywhere in the world.
The area is well known for its rugged topography, a result of the very resistant Precambrian basement rocks protruding through less resistant Triassic and Quaternary cover, and our first stop on day one was to view Bardon Hill Quarry (figure 1); a good place to discuss the geological history of the whole area, with an impressive view of the paleo landscape spread out below us, illustrating the startling unconformity between the unbedded basement rocks and the Mercia Mudstone of the Triassic. The red mudstone is clearly draped over the much darker basement. Also in evidence is faulting associated with later tectonic movements, most notably movements related to the Caledonian Orogeny. Incidentally it is this faulting, and subsequent movements, that have kept the basement rocks here relatively close to the surface and contributed to their exposure.
Fig 1 Bardon Hill QuarryFig2 Peldar dacite wet sea floor
All this was after a steep climb up Bardon Hill, passing several quarried blocks deliberately placed to provide a close-up view of the Precambrian rock found in the quarry (figure 2). This is part of the Bardon Hill Volcanic Complex, mostly of grey green andesitic breccia with pinkish andesite inclusions.
The Bardon Breccia probably formed when andesitic magma rose through wet sediments of volcaniclastic sandstones that formed a sea floor, as evidenced by the glass rimmed andesite fragments seen in some samples. Elsewhere in the quarry are examples of a distinctive rock type called peperite where the magma and wet sediments mix more thoroughly. These are typical of a shallow sub-surface environment where magmas have risen to a high level but don’t quite break the surface. We were also able to examine the breccia forming the summit of Bardon Hill.
Fig 3 Mount St Bernard AbbeyFig 4 Porphorytic dacite
The next stop was at Mount St Bernard Abbey (figure 3), a 19th century building constructed of stones from Whitwick Quarry (owned by the Abbey), which is another volcanic centre close to Bardon Hill, and roofed with Swithland Slates. Here the building stones provide easily accessible examples of the volcanic breccia including Peldar Dacite with large phenocrysts enclosed by a dark grey fine-grained matrix (figure 4). The crags adjacent to the Abbey show tuffaceous sediments that may be the result of a more subdued volcanic episode.
A pleasant walk through the nearby Charnwood Lodge Nature Reserve led to the ‘Bomb Rocks’, a spectacular exposure of poorly sorted volcanic breccia, with clasts varying in size up to 2m (figure 5). Originally proposed as volcanic bombs, these are more likely the remains of volcanic breccia ejected as part of a debris flow. A little further on, the High Tor Farm exposure of Grimley Andesite appears as a homogenous fine-grained rock, but is identical to the bomb rocks in composition so may have been part of the feeders for the volcanoes and could have been ejected in a highly viscous fashion to form part of the debris flow.
Fig 5 “Bomb rocks”Fig 6 Middle Knoll turbidite
Our final stop on day one was the Warren Hills locality with its distinctive three Knolls. The Hills form a transition zone from the volcaniclastic breccias of the Charnwood Lodge Formation to the stratified sand and siltstones of the Bradgate Formation, and whereas the former are seen at North Knoll, only 20m or so further on we saw the coarse grained tuffaceous sandstones that are the base of the Bradgate Formation (figure 6).
Day two was spent nearby, within Bradgate Park, and our first stop was Old John Tower (figure 7) to examine nearby sequences of well bedded volcaniclastic strata (tuffs) of mudstone, siltstone and sandstone belonging to the Beacon Hill Formation. Within these we could see a range of sedimentary structures including graded bedding and microfaulting (probably earthquake induced associated with the volcanic activity), also soft sediment load casts and flame structures (figure 8).
Fig 7 Old John TowerFig 8 Load Structure
Moving on we came to the well laminated mudstones and siltstones sitting atop the Sliding Stone Slump Breccia. The fossiliferous bedding plane here exposed shows upwards of fifty fossil impressions including at least one example of Charnia masoni, the first macrofossil definitively dated to the Precambrian, and found by Roger Mason in 1957 whilst still a schoolboy, in the eastern part of Charnwood Forest. There is more to the story of course. The fossil had been found a year earlier by a schoolgirl, Tina Negus, but whereas Roger Mason’s contact, the geologist Trevor Ford, confirmed his discovery, Tina’s geography teacher had discounted the possibility of fossils in the Precambrian.
As we got our collective eye in, more and more fossils became evident, much to everyone’s delight. Discs were immediately apparent (figure 9), possibly impressions of holdfasts for frond-like structures such as Charnia masoni, then probably Charnia itself (figure 10), difficult to photograph but impressive in size, (this may have been Charnia grandis) and some specimens perhaps of Bradgatia linfordensis, a mass of frond like structures (figure 11). There are reputed to be upwards of fifty fossils on this one bedding surface, best seen in the rays of a low Sun around 6pm in September apparently, and here we were at midday in June……
Fig 9 Holdfasts in fossil bedFig 10 Charnia
Reluctantly we left for our next stop, the Sliding Stones Slump Breccia itself. These are bedded turbiditic tuffs, important as a marker bed, and correlated with the sediment rift breccia at the base of the Bradgate Formation seen the day before at Warren Hills.
Fig 11 Bradgatia linfordensisFig 12 Sliding Stones (Slump Breccia)
Spectacular sag structures, either water or gas escape structures, are in evidence (figure 13), along with distorted fragments of mudstone and siltstone (soft when ripped up, figure 12). One contorted raft of laminated mudstone is known locally as the Swiss Roll (figure 14).
Fig 13 Sag Structure - Sliding stonesFig 14 “The Swiss Roll”
On next to an exposure of conglomerate at Hanging Rocks (figure 15) which contains rounded as well as angular clasts, all poorly sorted, so perhaps representing a prolonged period of transport and reworking, probably in turbidity currents.
The next exposure, below the ruined Bradgate House (incidentally the home of Lady Jane Grey), revealed a completely different rock type (figure 16). This is South Charnwood Diorite, a coarse grained intrusive igneous rock, and the youngest (that is, the last) of the intrusions in the Charnian Supergroup, cutting through all the succession. We spotted haematite and epidote minerals in the joint planes, and slickensides indicating movement in response to later tectonic motion.
Fig 15 Hanging RocksFig 16 South Charnwood diorite
(slickensides)
Our next stop was the Stable Pit Quartzite from the Brand member. The well bedded exposure contains some cross bedding, perhaps from a current-agitated nearshore marine environment. We also found what may be current ripples on top of the exposure which would reinforce this possibility (figure 17). Also visible are two sets of quartz veins, one trending E-W, cutting a (therefore) earlier fainter set trending N-S. This led to some discussion about the later structural events that had shaped Charnwood.
Fig 17 Stable Pit quartziteFig 18 Brick pit – Mercia mudstone
Much younger Mercia Mudstone is exposed in a brick pit (figure 18) ‘across a fordable stream’, at least that’s what Keith Ambrose said. In the event it had obviously been raining. This is the Triassic Mercia Mudstone we had seen draped across the Precambrian basement rocks at Bardon Hill Quarry and here used for the bricks of Bradgate House. Those who did brave the water discovered within the horizontal red beds a green bed of dolomitic siltstone, and possibly some halite pseudomorphs; this the result of a high water table precipitating gypsum when the desert was laid down as wind-blown dust sticking to a damp surface, rather than as a sandy desert: It certainly stuck to us.
The last stop of the day was Swithland Wood to examine the Swithland Slate Quarry. It is unfortunate that the flooded quarry itself is quite hidden behind massive wood and metal fencing to prevent youthful and potentially fatal swimming exploits; fairly pointless because it had been broken through in several places, but it completely obliterates any view of the quarry. We were able to examine the Swithland Slates in situ nearby, however (figure 19), and could see the cleavage, and jointing perpendicular to this, so two episodes of deformation. The slates, much used locally for roofing and gravestones, were originally thought to be Precambrian, but detective work on the backs of gravestones revealed trace fossils, nudging the exposure into the Cambrian. The existence of fossils illustrates the weak nature of the metamorphism which is also the reason for the relative difficulty of cleaving the slates. This in turn led to the abandonment of the quarry as easier worked, and therefore cheaper, Welsh slate arrived by railway in the nineteenth century.
Fig 19 Swithland (Slate cleavage)
Putting all this together, over two days we had seen the massive unbedded andesite and dacite of the volcanism at Bardon Hill which,along with the nearby Whitwick Volcanic Complex, was the source for the volcaniclastic breccias, debris flows and then graded sedimentary sequences we observed as we moved away from the centres and up the sections.
An analogy for this is probably something like present day Montserrat in the Caribbean, an island arc above a subduction zone. The andesitic composition is similar to present day subduction zone volcanoes. The evidence of ejection into wet sediments and the debris flows point to fragmental material ejected and accumulated on a sea floor. The sort of slumping we saw at Warren Hills indicates deposition into water deeper than 50m., again indicative of an island arc.
This was a very rewarding weekend which was excellently led. There was so much to take in we felt we had only scratched the surface, literally, of the rocks in Charnwood. We hadn’t even begun to properly untangle the structural geology for example, and there are other easily accessible exposures and disused quarries we didn’t have time to explore. Another visit anyone?
A Simplified Stratigraphy of Charnwood Forest
Age / Lithostratigraphy / LithologyTriassic / Mercia Mudstone Group / Mudstones, dolomitic siltstones and sandstones
Cambrian / Brand Group
Swithland Slate Fm.
Brand Hills Fm.
Stable Pit Mbr. /
Purple to grey slaty meta-mudstones and greywackes with thin conglomeratic sandstones.
Coarse-grained greywackes interbedded with silty mudrocks
Quartz arenite
Pre-Cambrian / Maplewell Group
Hanging Rocks Fm.
Bradgate Fm.
Beacon Hill Fm.
Charnwood Lodge Volcanic Fm.
Blackbrook Group
Blackbrook Reservoir Fm.
Ives Head Fm.
Morley Lane Volcanic Fm.
Whitwick & Bardon Hill Complexes; South Charnwood Diorite /
Volcaniclastic sandstone with pebbly lenticles
Fine- to medium grained volcaniclastic sandstones in repetitive normally-graded units;
basal slump breccia
Tuffaceous mudstone, siltstone and sandstone.
Volcanic breccias
Green to grey, graded volcaniclastic mudstones, siltstones and sandstones with massive beds of
volcaniclastic sandstone and breccia.
Grey, volcaniclastic mudstones, siltstones and
sandstones with volcanic breccias; well-
bedded and graded
Porphyritic metadacites interbedded with
greenish-grey 'tuffs' that may in part represent
highly sheared fault-rocks
Diorite, dacite and andesite intrusions & breccias
Interpretation of Volcaniclastic & Sedimentary Units
Unit / InterpretationMercia Mudstone Group / Arid, desert conditions and the periodic infilling of wadi systems
Unconformity
Brand Group
Swithland Slate Fm. / Quiescent, off-shore marine
Brand Hills Fm.
Stable Pit Mbr. / Near-shore marine
Maplewell Group
Hanging Rocks Fm. / Initial, fluvial or shore environment, with eventual deposition in deep water as turbidity currents in submarine fan or fan-delta environments.
Bradgate Fm. / Distal turbidites and subaqueous debris-flow breccias; submarine landslides
Beacon Hill Fm. / Turbidites with some settling of ash through the water column
Charnwood Lodge Volcanic Fm. / Pyroclastic flow deposits and hyaloclastites
Blackbrook Group
Blackbrook Reservoir Fm. / Turbidites
Ives Head Fm. / Turbidites; breccia facies top
Morley Lane Volcanic Fm / Lavas
(tables courtesy of Ian Williamson)
Charnwood Forest is right by Junction 22 of the M1, near Leicester, and the British Geological Survey has produced an excellent Guide and geological map of the area. Also available, and providing much more information about processes, is an excellent DVD. These both cover more sites than those we visited, with the DVD going up into the Quaternary. These, and the more technical field guides on which our field trip was based, can be found by accessing the following on the BGS website:
Ambrose, K., Carney, J.N., Lott, G.K., Weightman, G. and McGrath, A., 2007.
Exploring the landscape of Charnwood Forest and Mountsorrel.
Keyworth, Nottingham: British Geological Survey. This is the published guide.
Carney, J N, 2010.
Guide to the geology of Mount St. Bernard, Charnwood Lodge, Warren Hills and Bardon Hill, Charnwood Forest.
British Geological Survey Open Report, OR/10/044.
Carney, J N, 2010.
Guide to the geology of Bradgate Park and Swithland Wood, Charnwood Forest. British Geological Survey Occasional Report, 0R/10/041.
The technical guides are part of the ‘open reports’ (search for the Open Report refs. given above) now all online at BGS ( and downloadable via NERC Open Research Archive. In addition, ‘research reports’ are readable online but not downloadable. These include, for example, all the memoirs and guides that go with the 1:50,000 geological maps, and much more: a goldmine.
Using these materials it is feasible to have a self-guided tour of Charnwood Forest.
Whilst on the subject of websites I notice that the Fault Analysis Group website of University College Dublin is very much alive. If anyone else remembers making the paper models of complicated faults and folds from the site’s printouts, to get a grip on their structural geology, it’s all still there and more besides, including superposed folding (all under educational material).
Phil Ridley
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Events Diary
Important Notice re OUGS events – Insurance. Even though the OUGS has Public Liability Insurance cover for field and indoor meetings it is the responsibility of each individual to arrange their own Personal Accident and Personal Liability Cover. None of the information in any of the advertisements for field trips in this newsletter constitutes a brochure under the Package Travel Regulations (1992)Saturday, October 28, 2017 – Sedgwick Museum