Issue 7 The Silurian January 2021 1 Issue 7 The Silurian January 2021 Contents As always many thanks to those who have written a contribution for this edition, and also as always this is a plea for more articles Coastal Landslips South for a future edition. of Aberystwyth. 3 With the possibility of an end to the covid Chris Simpson. pandemic in the reasonably near future and the end of lockdowns now may be the time Geodes. to get writing, before field trips and other 6 activities take up all of your time again. Bill Bagley. Michele Becker Evolution of the Eye 9 Tony Thorp. Geological Gems of 13 Jersey. Submissions Chris Simpson. Please read this before sending in an article. Please send articles for the magazine 16 Book Reviews digitally as either plain text (.txt) or generic Word format (.doc), and keep formatting to a minimum. Do not include photographs or illustrations in the document. These should be The Magazine of the Mid Wales sent as separate files saved as Geology Club maximum quality JPEG files and sized www.midwalesgeology.org.uk to a minimum size of 1200 pixels on the long side. List captions for the Cover Photo: View from the top of Gouray photographs at the end of the text, or Castle Jersey, looking west over the in a separate file. harbour. ©Chris Simpson 'Members Photographs' and cover All photographs and other illustrations are by the photos are also wanted. Cover photos author unless otherwise stated. need to be in 'portrait' format and a All rights reserved. No reproduction without minimum of 3000X2000 pixels. permission. 2 Issue 7 The Silurian January 2021 Background Geology Coastal Landslips The rocks forming this part of the coast are part of the Aberystwyth Grits Formation – South of Aberystwyth alternating sandstones and mudstones from the Silurian period around 440 Ma. This article focuses on landslips at an easily The Landslips accessible part of the Wales Coastal Path, and the underlying cause of this phenomenon. Even from a distance, as you approach Allt Wen from the north, the irregular nature of the The Location cliff face is apparent. Closer inspection reveals The landslips are visible along a two mile extensive landslips of varying scale. They also stretch of the coast path south of Aberystwyth vary in age, with the older landslips now from Allt Wen to Morfa Bychan, (SN577796 to showing complete regrowth of the surface SN566778). The closest car parking is at the vegetation, while the younger ones still show Tanybwlch car park, (SN579807). extensive bare rock. At the bottom of the slope there are blocks of rock several metres across Warning: the ascent up Allt Wen is steep, and containing up to 20 strata. Down on the is slippery in wet weather. foreshore in places, you can see large pieces of flat rock which are clearly broken fragments of single intact rock strata that have fallen down the slope in a landslip, but were strong enough to retain their shape without being shattered into small pieces. Fig. 1 The view of Allt Wen from Tanybwlch beach. Fig. 3 Fallen blocks of rock at shore level, Much of the hillside is bare rock due to landslips of haphazardly arranged with differing varying size and age. Near the top, there is a V- orientation. Each block consists of multiple shaped indentation in the grass which is an old, strata. vegetated landslip. Fig. 2 Near the top of Allt Wen. The ridge along the right hand side of the picture is material which has Fig. 4 Individual slabs of fallen rock visible at low slipped a short way down the slope leaving the bare tide. Each piece is clearly identifiable as a large, rock along the left. It happened many years ago as it flat piece of a single layer of the Aberystwyth has stabilised and vegetated. Grits. 3 Issue 7 The Silurian January 2021 Fig. 5 The area at the top of the picture has slipped Fig. 6 Enlarged section of the slipped partially down the slope. The bare area at the area at the top of Fig. 5 with individual bottom of the picture is where rock has previously strata visible on the surface rotated from slipped. around 40° to near vertical. Fig. 7 View looking South towards Morfa Bychan caravan park. Fig. 8 Enlarged section of the There are three separate lines of exposed rock running down the view in Fig. 7 c l e a r l y slope: one at top left, a second above that crossing the whole demonstrating the general trend picture and a third (difficult to see) in the distance beyond that. of the rock strata on this hillside. These rock exposures demonstrate the general trend of the rock strata – dipping at around 40° to the west. Figs. 9 (L) & 10 The hillside from the shore. The downward slope of the rocks at about 40° is clear, as is the almost complete lack of vegetation cover. Near the top of Fig. 10 the edges of individual strata are clearly seen. 4 Issue 7 The Silurian January 2021 Why is this area subject to landslips? The main reason is the orientation of the strata along this area of the coast. When the rocks are dipping towards the sea, as they do south of Aberystwyth, any weakness in a particular rock stratum will result in the overlying rocks sliding down towards the sea. Rainwater on the top of the hill will percolate down through the rocks. If any layer is more permeable that the others, then water will preferentially track along that Fig. 11 The rocks at the northern end of the North layer producing weakness and subsequent Beach at Aberystwyth at the base of Constitution Hill. landslip. A superficial landslip can involve just The orientation is dipping to the east, away from the one stratum; and small-scale slippage on sea. bare areas merges into ordinary rock erosion as can be seen anywhere. Bigger landslips produce the large blocks as seen above. What happens north of Aberystwyth? The predominant orientation of the Aberystwyth Grits north of Aberystwyth is dipping towards the east – the opposite orientation of the rocks south of Aberystwyth – as seen in Constitution Hill (Fig. 11). These rocks are therefore immune from the type of slumping seen south of Aberystwyth. There are, however, occasional small areas of the coast north of Aberystwyth where the rocks are dipping to the west, and these do show Fig. 12 Shows a localised area of rapid slumping the same slumping as seen south of erosion, the photo was taken from the small headland Aberystwyth, albeit on a smaller scale. Figs. jutting out into the sea in Fig. 13 12 & 13 are from the coast north of Clarach at grid reference SN585845. Finally If you time your descent of Allt Wen for late afternoon or early evening, you will be rewarded with splendid views of Aberystwyth and the Welsh coastline to the north. If you can also time your descent to coincide with low tide, you will have fantastic views of the Aberystwyth Grits beds exposed on the shoreline with their intricate patterns caused by compression and reorientation due to large-scale movements of the earth’s crust in the 440 million years since they were first laid down. Chris Simpson Fig. 13 View of the headland at the southern end of the area of rapid slumping erosion. It is clear that the triangular area of rock in the foreground is sloping to the west at about 45°. In contrast, the rocks of the headland slope at around 45° to the east, which is why they are resistant to erosion and project out into the sea. 5 Issue 7 The Silurian January 2021 sometimes even describing a nodule as a type of concretion, but that is another story, so to Geodes continue on the subject of geodes. Most geodes have a silica or quartz content, I have always been fascinated by the beauty of for instance amethyst geodes are the most geodes, and by writing this article I have been striking example. and can often be seen in prompted to learn much more about them. retail shops advertised as Cathedral geodes. Geodes are highly complicated, so a (Fig. 4) These are probably recovered from comprehensive description of all the different basaltic lava tubes and voids. By its very types and how they form would take up a nature a lava flow has the ability to cool quite tremendous amount of space. Therefore this quickly entrapping pockets of gas, and leaving a r t i c l e i s a b a s i c behind lava tubes. Over description of the how time the silica content and where geodes occur of the lava migrates to together with a few the pockets and tubes illustrations. depositing crystals on the walls, The The first thing to do is to specimen illustrated explain the definition of a has been levelled at its geode. base to give it stability There are three major when displayed. types of inclusions in One specimen, from sedimentary, and my own collection (Fig. v o l c a n i c r o c k s . 5) poses the question, Concretions, Nodules, Is it a nodule or is it a and Geodes. geode? I think it is a In sedimentary rocks, Fig. 1 geode, because it has concretions (Fig. 1) are a hollow interior. The w h a t t h e i r n a m e walls are chalcedony, suggests, that is smaller and the interior deposit sedimentary particles are i s b o t r y o i d a l cemented together by a chalcedony.