Glamorgan Coast Geology 2 – Unconformity EarthCache
Glamorgan Coast Geology 2 – Unconformity
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The feature can be viewed from the inter-tidal foreshore and you should expect to walk about half a mile, mainly on the foreshore which varies from sand to cobbles to boulders to rock platforms. Access is restricted to the low tide period.
One of a series of Earthcaches exploring the geology of the Glamorgan Coast, this one looks at features on the foreshore at Dunraven Bay, including an unconformity. An unconformity is a time gap in the rock record. At Trwyn y Witch, the headland that forms the south-east side of Dunraven Bay, Sutton Stone rests upon Carboniferous Limestone (Image 1). The latter dates from the first phase of the Carboniferous Period, about 350 million years ago, while the former was laid down at the beginning of the Jurassic Period some 200 million years ago. The 150 million year time gap means that rocks like Millstone Grit and the Coal Measures of the later Carboniferous Period, that can be found not many miles to the north, are missing. The whole of the Permian and Triassic Periods are also absent including, for example, the sandstones, shales and red marls found between Lavernock Point and Penarth.
In general, the gap in the rock sequence may be a result of the material never having been deposited in the first place, because the appropriate conditions never existed in this location, or the rocks were formed but then eroded away. In this case, it is probably a combination of the two. At the end of the Carboniferous Period, about 300 million years ago, continental collision caused considerable squeezing and folding and the Vale of Glamorgan was uplifted into a series of east-west trending folds. These were deeply eroded to form a chain of craggy hills. Sea levels rose to surround this upland area, with Triassic rocks being deposited around the margins, and then followed a major inundation during the Jurassic, when the Blue Lias was formed. By this time, the higher hills of the Vale of Glamorgan were just islands in the sea. At Trwyn y Witch, the initial stages of this rise in sea level would have seen the small limestone hill being planed off on the shore. Following complete inundation, deposition occurred over it – firstly the fairly massive bed of creamy white Sutton Stone, then the Southerndown Beds followed by the Blue Lias. If you look directly at the cliffs from the reference coordinates for this listing and use Image 1 you should have no difficulty picking out the unconformity.
There are fossils and other structures worth looking for while you are on the beach. The waypoints for Locations A to E and the following notes will help:
Image 2: in places, the Carboniferous Limestone of the lower cliff is heavily folded.
Image 3: the Blue Lias beds (to the left) are severely distorted at a complex fault where they abut the Sutton Stone (on the right), which here is draped over the Carboniferous Limestone.
Image 4: look out for cross-sections of the bivalve Pina, with its characteristic diamond shape.
Image 5: a fault forms a dogleg in the cliff and then runs roughly westwards through the rock platforms and under the beach. As you cross this last fault on the rock platform, it may look like any other joint crack. But if you look carefully, it can be seen that the beds in the two faces do not match. Also, in the joint itself, fragments of rock have been ground at the interface and ‘welded’ onto the sides.
Image 6: fossils are plentiful in the rocks including, as here, bigger Ammonites up to 200-300mm across.
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 ***
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. 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. The cliffs in this area are very unstable, so stay well back from their base.
3. The foreshore immediately below the car park consists of cobbles and boulders – using the slipway down to the sand is advised.
There is a pay car park adjacent to the beach (see Waypoint below) although, if you pick the right time/season, you may enjoy a free stay. Alternatively, you can park freely in the villages of Southerndown, St Brides Major or Ogmore and enjoy a longer walk.
To claim this cache you must comply with all of the following three conditions:
1. Upload with your log an image of the unconformity, 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 5 locations corresponds to each of the images 2 – 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.
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.
Additional Hints
(No hints available.)