The Open University Geological Society Wessex Branch Newsletter Website http://ougs.org/wessex March 2017 Branch Organiser’s Letter CONTENTS Dear Members Branch Organiser’s Letter Page 1 If you were at our AGM then you will already be Corfe Castle RIGS up-dated Pages 2-3 aware that Sheila Alderman stood down as Bowleaze Cove, 13 November 2016 Pages 4-6 branch organiser and that I was voted into the Wessex Branch committee Page 6 post. Thank you all – I think. I must admit A Moroccan Geo-Safari Pages 7-8 that I have a good idea of what the job is as, Minerals guide no. 23 – Augite Page 8 although I joined in 1994, I have been working 2017 AGM minutes & 2016 accounts Pages 9-11 for the branch in various ways since 2001 when Other organisations’ events Page 12 I helped Alf Tingey with the newsletter although Forthcoming Wessex Branch events Page 13 it wasn’t known as ‘Footnotes’ in those days. In addition to various other jobs, I helped with OUGS events listing Page 14 both the 2004 Symposium and the one in 2016 theme is continued with the Corfe Castle report in Exeter; so I only have myself to blame for in this newsletter. being persuaded to stand. I’m pleased to say that the collection at the AGM So, what will this mean for you the members? for Cancer Research UK, in memory of our long- Not a lot I hope. Sheila has done a fantastic standing member Janet Griffiths, raised £220. job as our branch organiser and I hope to just continue her good work and keep our branch In the coming year we are looking forward to the active, helpful and useful branch that it has exciting trips to Charnwood and Jersey, as well always been. We still have the same core of as our usual fully packed day trips programme. enthusiastic committee members working for us I would like to emphasise that as an OUGS and Sheila is staying on the committee so that member you do not have to restrict yourself to we do not lose any ‘corporate memory’. To just Wessex trips, you can also attend the trips those who decided not to stand for the organised by other branches including Mainland committee again, I would like to say a big Europe. It is the same procedure as for Wessex, ‘thank you' for their work on behalf of the just contact the organiser. We also have an branch. I also extend a welcome to those who affiliation with the Dorset group of the GA stood for the first time. The draft minutes of (DGAG) and they too would be delighted to have the AGM and the 2016 accounts are in this you join their trips. newsletter (pages 9-11) and the new Good luck to everyone who is busy with courses committee for 2017 is on page 6. This at the moment, but I still look forward to seeing information is also available in the membership you on the field trips. services area of our website. Best Wishes If you didn’t attend the AGM, you missed an outstanding day of camaraderie and displays, Colin Morley, Branch Organiser Wessex as well as three top quality speakers with the E-mail: [email protected] theme of the ‘Cretaceous world’. On top of this was the wonderful lunch, probably better known as ‘the banquet’. I must also say thank you so THANK YOU FROM SHEILA much to all the members who worked together Thank you to Wessex members for the lovely to produce an exceptional meal for us all. We flowers, polished ammonite and warm wishes watched the number of attendees swell from when I retired as Branch Organiser at the AGM the 60s to the 70s to the 80s and ended up on 21st January. (See photo on page 10. Ed.) with 90 and still they coped. The Cretaceous Sheila Alderman Wessex Footnotes March 2017 Page 1 CORFE CASTLE RIGS RE-VISITED, OCTOBER 2016 Report by Sheila Alderman with contributions from John Chaffey and Prof. Rory Mortimore Corfe Castle West Hill Chalk Pit On 4 October 2015 we had a conservation day at West Hill Chalk Pit where Alan Holiday (Chairman of both the Dorset Geological Association Group and the Dorset Important Geological Sites group) was joined by 11 members of Wessex OUGS, DGAG and the DIGS group to clear the scrub, small trees, brambles and grass obscuring the Pit. Corfe Castle Regionally Important Geomorphological Site SY959823 Reproduced by kind permission of John Chaffey, President of DGAG. http://www.dorsetrigs.org.uk/ Corfe Castle is the site of a double water gap in the It is not easy to explain why the other members of this Chalk ridge of the Purbeck Hills. Two streams, the superimposed series of streams abandoned their Corfe River and the Byle Brook, both of which rise in courses across the Purbeck ridge at such a relatively the vale cut along the outcrop of the Wealden Beds to early stage. Perhaps their upper courses to the south the south of Corfe Castle, have eroded deep parallel were disrupted by subsequent development in the gaps to isolate the hill on which the castle is built. Wealden Clay vale. Similar development to the south The two streams unite immediately to the north of the of Corfe Castle may have increased the erosive power castle. of the Corfe streams and enabled them to continue to incise their courses across the Chalk ridge at Corfe. The site is remarkable because of these small streams The western of the two streams has cut a fine incised cut directly across the structural grain of the Purbeck meander through the ridge, leaving a well-developed Hills, after flowing for most of their courses parallel to meander scar on the eastern slopes of West Hill and the strike of the Wealden Beds, where they may be the eastern stream has cut an equally impressive regarded as subsequent streams. It is likely that they gorge between the Castle Hill and East Hill. have been superimposed from the emerged Pliocene sea floor, which would have cut across the various Twin water gaps in the Chalk ridges in southern beds now exposed in an east-west direction in the Isle England are relatively uncommon. Two twin gaps do of Purbeck. exist in the Isle of Wight, at Carisbrooke and at Brading, but in both cases one of the gaps has now The Corfe streams would have been members of a become dry. The gaps at Corfe Castle, each still series of streams initiated on the uplifted Pliocene sea occupied by a stream that has deeply incised its floor, including those whose past courses are marked course across the Chalk ridge, constitute a site of by the wind gaps at Cocknowle, Lutton and Tyneham much geomorphological interest and worthy of to the west, and the gap at Ulwell to the east, recognition as a RIGS. although the latter does appear to have had a more complicated recent history. John Chaffey. 12.7.1999 I revisited the site in October 2016 with Prof. Rory Mortimore. He was able to give me up-to-date information about the Cretaceous geology. The site is important as it shows the boundary between the Cenomanian Zig Zag Chalk Formation which is to the left of the pit when you face it (west) and the Turonian Plenus Marls delineating the Cenomanian-Turonian boundary in the gully at the centre of the pit. To the right (east) of the pit the Turonian Melbourne Rock and the Holywell Grit Beds in the Holywell Nodular Chalk Formation are exposed. The beds are all very steeply dipping (nearly vertical) 80° north. Prof Mortimore showed me that the Cenomanian Zig Zag Formation (named after the steep hill south of Shaftesbury) is part of the Grey Chalk (in what was known as Lower Chalk). We found no C-T boundary at West Hill Chalk Pit Photo and annotations by Prof. Rory Mortimore flints or fossils. The BGS Lexicon of Named Rock Units states that the upper surface of the Zig Zag Chalk Formation “is redefined as the bedding plane beneath the lowest of the marls in the Plenus Marls Member in the overlying Holywell Nodular Chalk Formation. (Note that the Plenus Marls Member is now considered as part of the overlying formation, thus providing a consistent datum throughout the Group of England and the North Sea.)” (http://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=ZZCH) Wessex Footnotes March 2017 Page 2 The Plenus Marls are mainly covered in vegetation and are much softer than the beds above and below so were more easily extracted for use in lime kilns. Above the Plenus Marls the chalk is greeny-coloured due to chlorite, again with no fossils. This is the Melbourne Rock member of the Turonian Holywell Nodular Chalk Formation. Above this are the hard nodular chalks with distinctive thin marl seams that can be used as markers throughout Southern Britain. In the north face we found a typical Mytiloides fossil - bivalves indicative of the basal Holywell Grit Beds at the base of the Turonian. Mytiloides cf mytiloides Photo by Prof. Rory Mortimore Rollington Farm Quarry SY970824 After lunch on 15 October 2015 we walked up to the top of the Chalk Ridge above the Corfe Castle to Swanage railway, enjoying views of steam trains en route. Alan Holiday then took us along the footpath by Rollington Quarry which is also a RIGS site, and where the chalk appears to be very different.