Glamorgan Coast Geology 4 – Graben EarthCache
Glamorgan Coast Geology 4 – Graben
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One of a short series exploring the geology of the Glamorgan Coast, this one looks at a graben at St Donat’s Bay. Expect to walk about 2 miles, half of this on a foreshore of boulders or rock platforms. Access is restricted to the low tide period.
A graben is a downfaulted block, between two parallel faults. The one at Stradling Well, in St Donat’s Bay, is very modest in size when compared with the East African Rift Valley (about 4000 km long) or even the Midland Valley (Central Lowlands) in Scotland, which is the largest example in the British Isles. Grabens are most easily explained as the result of the release of tension when the strata are being stretched. At Stradling Well, the beds at the edge of the downfaulted block dip towards the centre, as a result of drag. There are limited access points to the foreshore in this area and one of these is steps which descend in the middle of the graben.
The west side of the graben is at Waypoint 2R and shown in Image A. The east side of the graben – THIS IS PART OF THE CHALLENGE! YOU NEED TO FIND IT, PHOTOGRAPH IT AND DEDUCE THE WIDTH OF THE GRABEN.
There is much more of geological interest to find here and it is proposed to explore the foreshore eastwards to Tresilian Bay. The uploaded images and waypoints below should help in finding and identifying features and fossils. All of the rocks on the foreshore here are Blue Lias Limestones which were formed up to about 200 million years ago, at the beginning of the Jurassic Period. On the east side of the graben, heavy water seepage from the cliff face has resulted in the formation of tufa, the deposition of calcium carbonate which the water has picked up in solution during its passage through the limestone rocks. You may be more familiar with this process where it forms stalactites and stalagmites in caves. Red staining of the tufa is indicative of traces of other minerals, particularly iron. The fresh water seeping from the cliffs also encourages the growth of lichens and moss which, in time, can become petrified within the tufa (Image B). Heading east over the rock platforms, look out for some ENE – WSW faults which initially are almost parallel to the cliffs (Images C and D and Waypoint 4R). One of these faults cuts the cliff where this turns sharply south-east. You can find fossils of gryphaea, pina, coral (for example, Image F at Waypoint 6R) and crinoid. Fossil worm burrows can also be found – these worms were much like today’s lugworms: although the excreted sand would have looked very similar to the undisturbed deposit, small chemical, grading and constituent variations means that they show up today because of colour and/or hardness differences (Image E and Waypoint 5R). FINDING AND PHOTOGRAPHING FOSSIL WORM BURROWS IS THE FINAL PART OF THE CHALLENGE. As you enter Tresilian Bay, you pass Reynard’s Cave (Image G and Waypoint 7R), about 12m wide, 12m high and 44m long. Tresilian Bay is a convenient place to re-join the coastal path.
*** PLEASE HEED THE FOLLOWING WARNINGS ***
1. Most of 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.
(visit link)
… 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. The cliffs in this area are very unstable, so stay well back from their base and do not enter Reynard’s Cave.
3. The lowest section of steps at Stradling Well is steep, narrow and incomplete, but usable with reasonable care.
To claim this cache you must comply with all of the following three conditions:
1. Upload with your log an image of the east side of the graben, 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. Upload with your log an image of fossil worm burrows 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 estimated width of the graben.
Please do NOT include the answers to 3 above in your log.
There is limited roadside parking about 300m to the east of the entrance to Atlantic College, St Donat’s (Waypoint 0P). Alternatively, the area can be reached from Nash Point or Col-huw beach, Llantwit Major, via the coastal path or across the foreshore. This cache and the third in this series, Gryphaea, could be combined in a 5-7 mile 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. A couple of miles to the east, at Dunraven Bay, 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.
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