Glamorgan Coast Geology 6 – Folds EarthCache
Glamorgan Coast Geology 6 – Folds
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One of a short series exploring the geology of the Glamorgan Coast, this one looks at features around Lavernock Point, including folds. Expect to walk 1-2 miles, mainly on a rocky/bouldery foreshore. Access is restricted to the low tide period.
A fold is a structure formed by the bending of rock layers. If it bends up in the middle (like an arch), it is an anticline: if it bends down, it is a syncline. Folding may be gentle or severe and the structures may be quite localised or span many miles. On either side of Lavernock point, folds span ½-1 mile. As well as looking at structures, there are fossils and minerals to be seen … and, possibly, collected on the foreshore.
There is access to the coast close to Lavernock Point and you will need to explore on both sides of this headland. There is limited parking near Lavernock Church (Waypoint 1P): it is worth looking at the Marconi plaque on the wall here. In the following, to aid location and identification, images and waypoints have been provided.
If you look west from the Point, across St Mary’s Well Bay, you will see that the rocks forming the cliff are in a gentle syncline. At the centre of the syncline, the cliff divides into three layers – at the bottom, the near vertical section is the St Mary’s Well Bay Formation: the middle band is in the softer Lavernock Shales which have eroded more readily into a distinct batter: at the top, the face is steeper again because the beds of this Porthkerry Formation are harder again. All of these beds are in the Blue Lias, which was formed at the beginning of the Jurassic Period, about 200-180 million years ago. Across the bay, at beach level, beds of the St Mary’s Well Bay Formation can be examined, with the youngest in the centre and the oldest close to Lavernock Point. Fossils to be seen include Ammonites and the lowest bed in which these appear (close to the Point) defines the start of the Jurassic Period. Lower beds at and to the north of the Point are of Triassic age. The Ammonite which defines the start of the Jurassic has a distinctive smooth shell, can be found in a shale bed and is called Psiloceras Planorbis (Image 1 and Waypoint 2R). Because of its smooth, low relief, this fossil can be particularly difficult to spot: its profile shows up more easily when the sun is low in the sky, heightening the effects of light and shade. In higher beds (i.e. as you move towards the centre of the bay), a species with strong ribs on its shell can be found. In one of the highest beds of the Triassic (again, close to the Point), the fossil oyster, Liostrea, can be found densely packed on the surface (Image 3 and Waypoint 3R).
Heading northwards from the Point, progressively older rocks become exposed – firstly the Lilstock Formation then, beyond the path down from the church, the Westbury Formation. The latter is notably darker than anything we have seen up to now. Look on the beach here for fossils that have been eroded from the cliffs (BUT DO NOT GO TOO CLOSE TO THESE UNSTABLE CLIFFS) – fossil bones which have been found here include the vertebrae of ichthyosaurs and plesiosaurs (but these are quite rare finds and, if you should be lucky, you should record its position and take it to the Welsh National Museum, in Cardiff). The in situ geology is next interrupted by some coastal defensive works – but don’t despair, the large boulders here are Carboniferous Limestone (brought from elsewhere in the Vale of Glamorgan and about 150 million years older than any of the adjacent native rocks) and contain different fossils, such as crinoids (Image 4 and Waypoint 4R) - if these do not show up clearly, try wetting the rocks! At the prominent headland just beyond this defensive work, the Blue Anchor Formation has now appeared, with the uppermost layer of Red Marl at the very base of the cliff (Waypoint 5R). Within the Blue Anchor Formation are thin bands containing nodules of gypsum, which look pinkish and translucent (Image 6 and Waypoint 5R). These would have formed when water evaporated from a salt-rich lake.
But you must be getting tired now, having walked through about 30 million years of geological time. THERE IS JUST ONE MORE THING TO DO BEFORE ENDING THIS EXPLORATION. WHEN YOU LOOK NORTHWARDS (TOWARDS PENARTH) BEYOND THIS HEADLAND, MORE FOLDS CAN BE SEEN. IS THE NEAREST ONE AN ANTICLINE OR A SYNCLINE? While many of the fossils around Lavernock Point are completely embedded, you may also find some in smaller, collectable, rocks on the foreshore to take home.
*** PLEASE HEED THE FOLLOWING WARNINGS ***
1. The features noted are below high tide level, so only attempt this cache when the tide is in the lower half of its cycle and start your excursion while the tide is still falling. You can check tide times at several web sites e.g.
(visit link)
… and get the prediction for Barry or Cardiff.
2. Take care on the foreshore: the rocks may be slippery, especially when wet. The cliffs in this area are very unstable, so stay well back from their base.
To claim this cache you must comply with all of the following three conditions:
1. Upload with your log an image of the Ammonite Psiloceras Planorbis (Waypoint 1F), with your GPS in the picture. As a result of a rule change effective from 01/01/11, this requirement is now optional. However, we hope that cachers will continue to upload photographs voluntarily to enhance the quality of their logs for the benefit of all who view this listing.
2. Upload with your log an image of another fossil (with its name), with your GPS in the picture. As a result of a rule change effective from 01/01/11, this requirement is now optional. However, we hope that cachers will continue to upload photographs voluntarily to enhance the quality of their logs for the benefit of all who view this listing.
3. Email to us the type of fold seen looking north towards Penarth.
Please do NOT include the answer to 3 above in your log.
If you are looking for more information on the geology of the Glamorgan Coast, “Walking the Rocks” (£7.95 from the National Museum of Wales) outlines six exploratory walks, within the overall context of the geological sequence and timescale.
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