Glamorgan Coast Geology 7 – Fault
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One of a short series exploring the geology of the Glamorgan Coast, this one looks at features around Sully Island, including a fault. Expect to walk about 1½ miles, mainly on a rocky/pebbly foreshore. Access is restricted to the low tide period.
A fault is a joint in a rock formation where there has been visible displacement. This displacement may be vertical, sloping, horizontal or a combination of these. Because of these movements, faults can be detected by the discontinuity of strata across them although, sometimes, they have resulted in a more dramatic illustration of their presence, for example a waterfall or a step in the landscape. The fault which passes through the western end of Sully Island continues across to the mainland as a step (Image 1) and, briefly on a fast rising tide, creates if not a waterfall, a cataract. The top of the step is frequently identified as a causeway (Waypoint 3F and Image 2), being the easiest route for crossing to the island at low tide. Despite this description, when the rocks are wet, they can be extremely slippery and care is essential.
The causeway was the site of a tragedy in 1949, when a party of scouts left it too late to return to the mainland. One of the scouts, 12-year old John Davies, had reached the safety of the mainland, but returned into the sea to help the last two of the group and their leader who were in difficulty. He supported one of the boys, a non-swimmer, until a rowing boat reached them, but was washed away just before rescue. All of the others were saved, but John’s body was only recovered days later. John’s heroism was acknowledged by the posthumous award of the Albert Medal and the Scout Award Cross. This is a cautionary tale: we do not want any geocachers to become tragic news!
The majority of the exposed rocks are red siltstones and yellow sandstones from the late Triassic Period, which would have been laid down in the margins of the lake, in the centre of which the red marls, which can be found just around Lavernock Point, were being deposited. The north side of the island boasts cliffs that are up to 15-20 metres high. These exhibit a range of marginal sediments of great interest to geologists, including clastic sediments, evaporates with nodular dolomite and carbonates with rippled and cryptalgal limestones, tepee structures and calcrete. The fault which forms the causeway can also be found here and, because it is fully exposed, it can be seen that the vertical displacement is about 3 metres.
This north cliff also shows permafrost structures in the form of gentle to sharp isoclinal folds which die out downwards within 3 metres of the surface. As their direction bears no relation to the regional folding, these folds are not considered to be tectonic in origin. Rather, they are interpreted as having formed under periglacial conditions, the gentle undulations resulting from differential squeezing caused by unequal freezing and the sharper folds resulting from the bursting through of ground water when the hydrostatic pressure increased from seasonal freezing. Their age is thought to be Pleistocene (less than 1.8 million years).
The rocks here have close similarities with those of nearby foreshore areas where bipedal and quadripedal dinosaur footprints have been found. As far as is known, no footprints have been found on Sully Island … but this may present an opportunity for a different kind of first to find! For a better chance of finding dinosaur footprints, explore the mainland foreshore for about 500 metres west of the causeway, but be warned that you may need an eye of faith to pick out indistinct prints as little as 50mm across.
Around the southeast corner of the island, an unconformity can be found (Waypoint 4F and Image 3). Above the unconformity, Triassic rocks dip west at a gentle 10o or so: below the rocks are tilted upwards at about 60o. WE ARE NOT TELLING YOU WHAT THESE ROCKS ARE: THIS IS PART OF YOUR CHALLENGE. But, from this southeast corner, you can look out across the Bristol Channel and pick out Flat Holm, Brean Down and Steep Holm (Image 4). The existence of these three and Sully Island has a common factor – these rocks. If you view Sully Island from the east (Lavernock Point or St Mary’s Well Bay, the unconformity can be seen as a near horizontal line, with clearly sloping strata above. (Image 5).
The eastern end of Sully Island is the site of an Iron Age hillfort and the remains of its ditches are evident about 150 metres from the eastern high point. On the north foreshore, can be found the remains of what some sources claim to be the wreck of the SY Scotia (Waypoint 5F), a ship used on the 1902 – 1904 Scottish National Antarctic Expedition, led by William Speirs Bruce. However, this ship is described elsewhere as “a three-masted wooden barque 42 metres long, with a beam of nine metres, draft of 4.5 metres and a freight-carrying capacity of 357 tons.” AS ANOTHER PART OF YOUR CHALLENGE, LOOK AT THE REMAINS TO DETERMINE IF OTHER REPORTS THAT IT FOUNDED ON A SANDBANK IN THE CHANNEL ARE MORE RELIABLE.
*** PLEASE HEED THE FOLLOWING WARNINGS ***
1. The features noted are generally 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.
http://easytide.ukho.gov.uk
… and get the prediction for Barry.
2. Take care on the foreshore: the rocks may be slippery, especially when wet. The southern cliffs are unstable in places, so stay well back from their base.
3. The approach to the unconformity is steeply sloping and there are deeply cut gulleys in the rocks, so exercise caution in this area.
To claim this cache you must comply with all of the following three conditions:
1. Upload with your log an image of one of the geological features mentioned, with you or your GPS in the picture, making sure that your caption properly identifies it. If you should find and use an image of dinosaur footprints, please also include a waypoint. 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 the name of the rock strata below the unconformity (it would help to have done some of the other caches in this series first, to help with this question).
3. Email to us your view on whether the ship remains are of the SY Scotia and why you have come to this conclusion.
Please do NOT include the answer to 2 and 3 above in your log.
In the winter, there is limited roadside parking at the end of the lane from St Mary’s Well Bay (Waypoint 1P). Otherwise, there is parking at Swanbridge at the Captain’s Wife (Waypoint 2P) where there is a parking fee, but this is refundable if you give this hostelry your custom and sometimes is not collected outside of peak periods. Free street parking is also available in Sully, offering the opportunity to approach across the area where dinosaur footprints have been found.
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. It is understood that a second volume of these walks is due to be published shortly and will include this locality.
Additional Hints
(No hints available.)