ON THE ROCKS Newsletter of the Yorkshire Branch of the Open University Geological Society March 2019 Langdale Pikes from Side Pike on Day 1 of Blencathra 2018. Harrison’s Sickle is the highest peak in the centre, with the brooding crags of Pavey Ark on the right. (Peter Vallely took the photo and his report starts on page 3). CONTENTS Rick’s Musings 1. Rick’s Musings 3. Blencathra 2018 Dear Members 7. Editor’s piece 8. 2019 AGM report Firstly, as this is the first newsletter of the year, let me say to all of you a belated 10. Baildon Hill walk 11. Committee members Happy New Year!! 12. Field trips 12. Mapping course Most of my article for this newsletter is based around a plea for your input, ideas, and opinions on a subject of great importance, and I would like as many of you to respond as possible. This year at our AGM, Ann resigned as Branch Treasurer after a term of office probably spanning two decades. Personally, I would like to say thank you to Ann for all her hard work over the years, Ann has been a mentor to me and most certainly one of my closest friends. As a Branch we have been lucky to have Ann as treasurer for so long, and she was treasurer for at least three Branch Organisers before me. The Yorkshire Branch of the OUGS March 2019 As a Branch we have one major problem. While we have found a temporary replacement for Ann, Jean Sampson has kindly agreed to stand in as treasurer but is only prepared to take on the role for one year, at next year’s AGM we will have to find a replacement. In addition to needing a new treasurer, next year the Branch will also need a new Branch Organiser. At the AGM in January, after eight years’ service, I gave one year’s notice of my intention to resign the position of Yorkshire Branch Organiser. Hopefully, you will all know that for it to remain a branch within the Society each branch must have a Branch Organiser and Treasurer. There is at national level an ongoing discussion of amalgamating branches, the reason being that a reducing membership means the cost of having BO’s going to meetings is increasing per member. I was asked at the last national committee meeting in October whether or not I would support the possibility of branches amalgamating. Personally, this is an idea that I support, but most importantly one that I support with the caveat that no branch is forced to amalgamate, and that it occurs organically. I did raise the situation at our AGM in January and the initial response from those members in attendance did not seem in favour of the idea. However, due to the low turnout at the AGM and lack of the chance of a full discussion, the decision has been deferred until the AGM in 2020. I therefore invite everyone’s opinion on the subject in the meantime. There are two potential routes this could go down! The first is that a new Branch Organiser and treasurer are found by the Branch, at which point a vote will take place as to whether or not the Branch would like to amalgamate with another branch, which would most likely be Northumbria. The second option, if these positions were not filled, would be that the National Executive would take over and work with our existing committee. I asked the Chair of the national committee Sue Vernon for her advice and how it would work, and this was her response: “I would just like to clarify the situation as to the term forced amalgamation. There will be no forced amalgamation, the case will be if nobody comes forward next year to stand as BO then the EC will run the Branch for a year alongside the committee. During that year discussions would take place with the committee and members as to the state of the Branch. If nobody is willing to stand as BO after that time, then unfortunately we will have to amalgamate the Branch with those close by. The Yorkshire members would be consulted as to which branch, they would switch to. If they do not respond, then we would look at postcodes and reassign accordingly. This has been the case with Oxford Branch”. Sue Vernon, Chair. So folks, this really is down to your choice, it is up to you and you have the power to voice your opinion. Ultimately, we need you to get involved in your Branch. If you’d like to send me your opinions, please send me an email to [email protected]. Finally, my usual nag. Please keep your details up to date and if you have not paid your subs yet please send them as soon as possible. Best wishes Ricky Savage Yorkshire OUGS Branch Organiser (for 11 months, tick tock!) 2 The Yorkshire Branch of the OUGS March 2019 Blencathra Fieldtrip 2018 – Day One Borrowdale Volcanics in Langdale - Date: 30th October 2018 Introduction Today’s objective was to look at two contrasting environments within the Borrowdale Volcanic Group (BVG) in Langdale. The BVG forms the broad E-W band across the centre of the Lake District and contains its most dramatic scenery e.g. Pillar Rock, the Langdale Pikes and Helvellyn. The BVG was erupted from centres which collapsed to form calderas (notably the Scafell Caldera). These calderas were infilled mainly by volcaniclastic sediments, which we visited in the morning at Thrang Quarry. These sediments have been eroded in places to reveal the underlying pyroclastic rocks which we examined in the afternoon’s visit to Side Pike. Tectonic Background The BVG was formed in the late Ordovician period (Caradoc stage). At the time, England and Wales were part of thin continental fragment, called Avalonia, which was a bit like modern Java. During the Ordovician, the Iapetus Ocean to the north of Avalonia began to be subducted under its northern margin. The sinking plate disturbed the mantle and produced back-arc volcanism - again similar to modern Java (but with subduction to the north rather than the south). The volcanism only lasted about 5 million years probably because the spreading centre was overridden. The Iapetus Ocean eventually closed much later in the Acadian phase of the Caledonian Orogeny (Devonian) when the continents of Avalonia, Laurentia and Baltica were joined together (and England became joined to Scotland along the line of the Solway Firth/Hadrian’s Wall). Location 1. Thrang Quarry [NY 318055] (Seathwaite Sandstone Formation) From Elterwater, we walked through Chapel Stile, an odd mixture of slate tips and timeshare developments, to Thrang Quarry. Our leader, James, explained that the slates in the quarry were formed from volcanic material which was reworked and deposited in a lake after the Scafell caldera collapsed (Fig 1). Subsequently the rocks became buried and then compressed during the Acadian orogeny. This produced their strong cleavage and their low grade (chlorite) metamorphism which gives them their greenish tinge. The cleavage is roughly at right angles to the bedding, so when the rock breaks into slates along cleavage planes, we can see attractive sedimentary structures in cross-section such as cross-laminations, micro-faults and dewatering structures (Fig 2). Fig 1. Thrang Quarry. James explains the origin of the slaty cleavage 3 The Yorkshire Branch of the OUGS March 2019 Fig 2. Close up of flame structures indicating that the sediment has been loaded slightly while still soft. The flames (which show where water escaped) point upwards Location 2 Elterwater Quarry [NY323048] (Seathwaite Sandstone Formation) We continued to Elterwater Quarry (Figs 3 and 4) which has a convenient overlook. It is a large working quarry producing: roofing slates, ornamental tiles and aggregate. Again we saw many slates and blocks with interesting sedimentary structures. Fig 3. View of Elterwater Quarry Fig 4. Block at Elterwater Quarry showing flaser bedding. Most of the time dark mud was deposited under quiescent conditions, but sporadic higher energy conditions produced the lighter-coloured ripple marks . Location. 3 Side Pike [NY290052] (Side Pike Complex) We drove to the end of Langdale and then continued for another kilometre to a cattle grid at the base of Side Pike (a spur of Lingmoor Fell) where we had lunch. Side Pike provides easily accessible exposures of all three types of pyroclastic deposits i.e. ash fall, pyroclastic flow, and pyroclastic surge. The sequence we observed is detailed in the table below. 4 The Yorkshire Branch of the OUGS March 2019 Table 1. Generalised vertical section through Side Pike Complex (adapted from Branney (2007)) James led us up the flank of Side Pike to look at a thick exposure of ignimbrite (B). Fresh samples revealed a pink matrix with abundant black fiamme in parallel orientation (Fig 5). Ignimbrites are formed when a hot suspension of particles and gas moves swiftly down the side of a volcano as a density current – this is the type of eruption that overwhelmed the Roman city of Pompeii. Ignimbrites have a matrix of volcanic ash containing gravel-sized rock fragments called lapilli. If the flow is sufficiently hot when deposited, the lapilli may weld together and flatten to form elongate shapes known as fiamme. Fig 5. Ignimbrite showing fiamme (the dark elongated material which shows parallel alignment) 5 The Yorkshire Branch of the OUGS March 2019 We climbed further up the slope and found examples of the ignimbrite layer with brecciated blocks as inclusions. It seems that water penetrated this ignimbrite while it was still hot and caused explosions which shattered parts of it into the brecciated blocks (Fig 6).