One of a short series exploring the geology of the Glamorgan Coast, this one looks at features on the foreshore around Nash Point (Image 1), including the fossil Gryphaea. Gryphaea, or Devil’s Toenail, is one of the characteristic fossils of the Blue Lias Limestone which was formed here up to about 200 million years ago, at the beginning of the Jurassic Period. Gryphaea was a bivalve, a relative of today’s oyster. In some locations, the fossil erodes cleanly and completely from the surrounding rock and is easily recognised – but not here! More commonly, along the Heritage Coast, the fossil and surrounding rock are of similar hardness, so erode at the same rate. The result is that most examples found are sections through the fossils at all sorts of angles, but not particularly distinctive in appearance. However, on the foreshore roughly below the more westerly of the two Nash Point lighthouses, there is an horizon where fossils have been silicified (i.e. their original calcium carbonate shells have been replaced by silica). This makes the fossils slightly more resistant to erosion than the surrounding rock, so that they can stand proud of the rock surface (Image 2). Surfaces crammed with shell fossils are not uncommon: Image 7 is an example that includes Gryphaea.
There are fossils and other structures worth looking for while you are on the foreshore. The waypoints for Locations A to D and the following notes will help: Image 3: the fossil Plagiostoma, a bivalve: examples up to 200mm across can be found. Image 4: another common bivalve in this area is Pina, often appearing in section as a diamond shape but, here, partially eroded out, with examples again up to 200mm across. Image 5: about 200m before the western lighthouse, there is a sharp corner in the cliff. Just to the east of this is a fault, which runs roughly north-south: across the foreshore, it shows as a gully, called Gwter Fawr. The beds on the west side of the fault have dropped about 15m relative to those on the east. In the cliff, the rocks on the east side are shattered, with many more joints and smaller faults, and dragged downwards close to the fault. Image 6: look out, too, for Ammonite fossils. Of course, it would have been more helpful to say which image was appropriate to each location – but this is part of your challenge: to match each image with its appropriate location.
*** PLEASE HEED THE FOLLOWING WARNINGS ***
. The features noted are below high tide level, so only attempt this cache when the tide is in the lower half of its cycle. You can check tide times at several web sites e.g. http://easytide.ukho.gov.uk … and get the prediction for Porthcawl.
2. Take care on the foreshore: the rocks may be slippery, especially when wet, even large boulders may be unstable, in places the rock platforms are deeply pitted by the action of seawater and the level differences may require a little scrambling. 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 a Gryphaea fossil that appears in relief on the surrounding rock, with you or 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. Email to us a list showing which of the 4 locations corresponds to each of the images 3 – 6.
3. Email to us the name of the geological formation which contains the fossils noted. Please do NOT include the answers to 2 and 3 above in your log.
There is a pay car park (£3 in October 2020) at the Waypoint Point given below although, if you pick the right time/season, you may enjoy a free stay. Alternatively, you can park freely in the villages of Marcross or St Donats (possibly combining this cache and the fourth in the series, Graben) and enjoy a longer walk.
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. Near the car park is the Glamorgan Heritage Coast Centre where, amongst other things, there is a small but relevant geological display. Unfortunately, the opening hours are limited to three days a week, plus times when there is a ranger resident. Available from the Centre is its own well-illustrated booklet “Glamorgan Heritage Coast – A Guide to its Geology”, covering the area from Ogmore to Llantwit Major.