The Open University Geological Society Wessex Branch Newsletter Website http://ougs.org/wessex October 2014 Branch Organiser’s Letter CONTENTS Dear All Branch Organiser’s Letter Page 1 I hope you have had a lovely summer and are Barton-on-Sea fossils, May 2014 Pages 2-5 enjoying your geology. Wessex AGM 2015 notice Page 5 Northumberland Part 1, 2 June 2014 Pages 6-8 We have a couple of trips in October: West Bay Dinosaurs in the news Page 8 and Burton Bradstock on the Dorset Coast with Wessex Branch committee Page 8 Prof Chris Wilson on 12th October; and then the following week 19th October to Watership Down Minerals guide no. 11 – Sodalite Page 9 (Kingsclere Anticline) on the Hampshire/ Other organisations’ events Page 10 Berkshire borders with Dr Bernard Skillerne de Forthcoming Wessex Branch events Page 11 Bristow. Contact Jeremy on OUGS events listing Page 12 [email protected] Wessex Branch committee Page 12 We had a tremendous time in Northumberland In July 2016 it’s our turn to be involved in co- in June. Many thanks to Mark Barrett for ordinating the symposium. We have looked in organising the week and to Lesley Dunlop our detail at what the universities have to offer and leader. See the reports on day one on pages 6 have chosen Exeter over the weekend of 8th to to 8 of this edition; reports on the remainder of 10th July 2016. Participants can also book to the week will appear in subsequent newsletters. stay longer to make it a longer break. At the end of July I helped to organise a week in Kindrogan for NW and Wessex branches with The Wessex AGM and Lecture Day in Wool, near an expert in the Grampian geology Dr John Wareham in Dorset, is all arranged for Saturday th Mendum, retired from BGS Edinburgh. This 24 January 2015. The theme is: was hugely enjoyable. He did a tremendous “Hydrocarbons. How do they do it.“ As we have amount of preparation and explained an had such a successful year we have decided this extremely complex area very effectively. Look will be free of charge as a thank you. (See the out for reports in the National newsletter. AGM notice on page 5). th Do look at the events list as Jeremy has The National OUGS AGM is in Bristol on 25 arranged more day trips for next year. Also, April 2015 so we have plenty going on within Mark Barrett [email protected] has reach of us! arranged a Mendips weekend in April 2015 and Do let Hilary Barton know if you would like to is finalising details for Anglesey in September receive Footnotes by email rather than post so 2015. All in addition to Linda and Colin Morley’s you have the advantage of a full colour version trips to Arizona. and of course to cut costs. Her email address is [email protected] I thoroughly enjoyed the Canterbury Symposium and was pleased to see so many Also check the branch website for updates Wessex members there including some for the http://ougs.org/events/index.php?branchcode= first time. In 2015 the Symposium is in wsx Newcastle - see http://ougs.org/symposium for details. It will be held at Northumbria I look forward to seeing you on future events th th University from 17 to 19 July 2015 with the Sheila Alderman, Branch Organiser Wessex title “PANGAEA: LIFE & TIMES ON A xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx SUPERCONTINENT - A celebration of Britain’s Tel. (evenings only) unique marine Permian strata”. E-mail: [email protected] Wessex Footnotes October 2014 Page 1 WESSEX OUGS FIELD TRIP TO BARTON-ON-SEA, 11 MAY 2014 Leader: Ray Chapman Report by Giles Watts On a mainly sunny but quite chilly Sunday in May, 22 geologists met on the cliff top at Barton-on-Sea to take a look at the famous Eocene type section, the impressive but somewhat ineffectual sea defences and some very stormy seas left over from a series of strong south-westerly storms. The trip was led by Ray Chapman, who is a local expert on the geology of the area. We met near the Cliff House Hotel on Marine Drive West, which stands only about 25m from the edge of the cliff and commands imposing views across Christchurch Bay with Cliff House the Isle of Wight in the west Hotel and Old Harry Rocks in the Pebble Beach Restaurant east. In front of the hotel is Beachcomber Cafe an expanse of ocean that stretches 4000km to the coast Fig. 1 Map of Barton-on-Sea area http://www.openstreetmap.org/copyright of South America. Although storm waves are attenuated as they enter the shallow coastal waters, the high wave-energy pounds the coastline removing fallen material and destabilising the weak Tertiary cliffs. Erosion rates at the top of the cliff are between 0.5-1.5m per year and will soon be threatening the hotel. Further to the East, in an even more precarious position, stands the Pebble Beach Café and Beachcomber Café (Fig. 1) that lie right against the present-day cliff edge. Here the old café and adjacent Manor Lodge were demolished in 1975. Sadly, it cannot be long before the remaining buildings will also have to be abandoned. The trip began with a walk along the windswept cliff and an overview of the geomorphology. Ray set the scene with a description of the ancient East-West Solent river and how it captured a succession of local rivers such as the Frome, the Piddle and the Stour (Fig. 2), which now flow directly into the sea having left cliff cuts or “chines “ at their exit. We discussed the rising sea levels since the last ice age and the eventual break of the chalk ridge between the Isle of Wight and Old Harry Rocks. It is now thought that the first breach occurred during the Late Devensian some 12,000 years ago when the Frome and Piddle broke through the chalk from what is now Poole Harbour. One theory is that, as the temperature rose, a giant Fig 2. The Solent River of Early to Middle lake in Canada full of glacial meltwater broke its banks Pleistocene joining the Channel River sending a tidal wave across the Atlantic that breached Ian West ©2013 http://www.southampton.ac.uk/~imw/Solent- Introduction.htm the chalk ridge in several places. The group headed for the beach via Hoskin’s Gap where Pleistocene terrace gravels are well exposed along with the top of the Becton (Barton) Sands (Fig. 3). The stratification of the pebble and sand sediment was noted and the cemented iron staining. We walked down to the sea to look at the extensive landslips and slides, and the many attempts at protection that have been attempted over the last 80 years. The first coastal protection scheme, initiated in the 1930s, used timber groynes. These fell into disrepair during the war and in the 1960s a timber and rock revetment was constructed along with a series of timber and rock groynes along the shingle beach. At the same time, water flow through the cliff was Fig. 3 Hoskin’s Gap with Pleistocene stopped with a sheet pile cut-off wall and collected into a drain Gravels Photo Giles Watts before being piped down the cliff via rock drains. Wessex Footnotes October 2014 Page 2 These works subsequently failed in the 1970s but the remains of the sheet pile wall is still visible today (Fig. 4) with more in the lower cliff that have been overturned. In the 1990s the old drainage pipes were fully replaced and the timber revetment was upgraded with a rock one using 90,000 tonnes of Carboniferous limestone from the Mendips (Fig. 5). Finally, rock groynes or “strongpoints” were built in the foreshore. The idea Fig. 4 Remains of sheet pile was that “pocket” wall Photo Giles Watts beaches would form between the groynes helping to prevent erosion. Unfortunately the combination of storms and longshore drift is too strong and the pocket beaches are minimal. Nevertheless the extensive rock dumping has been efficient in dissipating wave energy and in preventing erosion of the cliff toe. However, cliff top erosion has not Fig. 5. Cross section showing stone revetment, been halted by the defences. and landslides now drainage and typical slip mechanism (from Barton on Sea Coastal Protection information Leaflet) cover the undercliff path, preventing access and obliterating features of geological interest. The main process of cliff erosion is through percolation of water through the Pleistocene gravels and Barton sands until the impermeable Barton Clay is reached. As this becomes lubricated, the bedding becomes a horizontal shear surface allowing large blocks to move laterally (without much rotation) to form elongated ridges with a graben (ditches) behind. There is a strong correlation between rainfall and cliff failure due to the build up of pore water pressures. Ground investigation suggests that pore pressures are recovering which increases the risk of further cliff failures. Recent very wet winters in 2013 and 2014 will not have helped. In 2013 the District Council instigated a £300,000 scheme to monitor groundwater movement and identify options for the future management of the cliffs. 18 boreholes were drilled to a depth of 20-35m and fitted with inclinometers (to detect movement), piezometers (to measure groundwater levels) and pressure sensors (for pore pressures). A separate subsurface geological survey was also conducted Fig. 6 Map showing borehole locations for 2013 monitoring along the cliff top. Data is being programme Photo by Giles Watts of District Council sign at Barton-on-Sea collected for around 18 months.