Very early in April I had the great pleasure of leading a group of 8-9 staff members of UWC Atlantic College (based at St. Donats) to a number of locations on the coast of the Vale of Glamorgan in order for them to understand in greater depth the landscape of the area in which the college is based. Although it being early Spring the weather was glorious and perfect for field work, albeit after an initial cold start!
Atlantic College is just one in a number of colleges within the United World Colleges organisation. There are currently 17 different schools and colleges on four continents that teach the International Baccalaureate. The ethos of the UWC is to select students based not on their academic achievement rather, on their personal and aspirational potential.
The leadership team of the Outdoor Activities department made contact with South Wales GeoTours late in 2018 when they became aware of some of my promotional material. The Outdoor Activities department at Atlantic College are responsible for leading students on activities such as rock climbing, caving and kayaking amongst others both on the Heritage Coastline as well as the Brecon Beacons and Gower Peninsula of South Wales. I was contacted to explore if I could provide understanding of the geology of the local coastline to allow staff (with little or no geological background) to communicate the primary geological elements of what their students were climbing on or kayaking past.
The emphasis of the day was to be on the geology at the site of the college itself plus one or two other coastal sites where students go rock climbing. To that end I arranged for them to see the Carboniferous Limestone at Ogmore-by-Sea because they go caving in the limestone caves of the Brecon Beacons, the Lower Jurassic rocks on which Atlantic College is built and where students climb & kayak, and the Triassic rocks at Ogmore-by-Sea and near Barry because they are simply spectacular – to a geologist!
We gathered at Ogmore-by-Sea for an introductory discussion on the basic principles of geology (Fig 1) before introducing the geological history of the Vale of Glamorgan. Throughout the group session I tried to put the rocks and fossils into context with the Plate Tectonic and Continental configuration at the time as they are the fundamental driver on what type of rocks are deposited where. Unfortunately, the weather wasn’t too helpful when I was talking as it was windy and cold and often my presentation material was blowing around.
Fig 1. Introductory talk to the Atlantic College group
The first rocks we looked at were the Early Carboniferous limestones of the High Tor Limestone Formation that were formed between 346-330 million years ago. At that time South Wales was located just south of the Equator and as a result experienced a tropical climate with warm, shallow seas and occasional large reefs forming barrier islands (Fig 2).
Fig 2.Schematic paleo-geography of Wales during the Lower Carboniferous
Limestone is a sedimentary rock that is comprised of crystals of Calcite and Aragonite the differing forms of calcium carbonate. The hot, tropical seas led to high evaporation rates causing the precipitation of calcium carbonate.
A feature of limestone is that it is soluble in the presence of fresh water. As that flowing water runs along natural partings in the limestone it dissolves the fabric of the limestone enlarging the partings to form wide cracks eventually leading to caves. The Limestones exposed at Ogmore-by-Sea are well-bedded but with evidence of Karstic features (Fig 3).
Fig 3. Carboniferous limestone exposed at Ogmore-by-Sea
The limestone exposed at Ogmore-by-Sea has a lot of fossils incorporated within it (Fig 4). We saw many Crinoids (Sea Ferns) that are related to modern day brittle stars & sea urchins plus a number of specimens of a coiled Gastropod (much like modern-day Whelks) and Corals (polyps living within a structure made of calcium carbonate). These organisms were living on and growing up from the seabed or reef surface probably in shallow water and within a diverse and vibrant ecosystem.
Fig 4. Fossil of a coiled Gastropod (Straparollus?) within Carboniferous Limestone
Ogmore-by-Sea is also a great place to see some great exposures of Triassic rocks. Britain during the Triassic Period was situated to the north of the Equator in the interior of the vast supercontinent Pangaea that had previously formed during the late Carboniferous/early Permian Periods (Fig 5). The climate during the Triassic was hot and arid with intense storms and winds leading to the formation of deserts and inland seasonal lakes.
Fig 5. Plate reconstruction during the Permian & Triassic Periods (Britain highlighted in Red)
The Triassic rocks at Ogmore-by-Sea comprise jumbled-up, grey-coloured pebbles and cobbles without any horizontal bedding and are what geologists refer to as conglomerates and breccias (Fig 6). Because the pebbles in a conglomerate are rounded geologist infer that the pebbles have moved a long way from the place where they were eroded as abrasion as the rock fragment flows knocks-off the sharp edges. Whereas a breccia with its sharp-sided rock fragments indicates that it has not travelled very far before being re-deposited.
Fig 6. Triassic Conglomerate exposed at Ogmore-by-Sea containing pebbles of Carboniferous limestone set in a red sandy matrix
The pebbles are formed primarily of eroded fragments of Carboniferous limestone and are set within a red-coloured matrix or cement which is characteristic of the arid Triassic period. The conglomerates and breccias are filling a desert river that has eroded/cut-through the already hard Carboniferous limestones during the Triassic (Fig 7). These desert river systems are known as Wadis in the Middle East today. I have seen countless modern Wadis in Oman and they are always steep-sided and full of a jumble of boulder, pebbles and sand. Most of the time they are dry and only ever have water flowing through them when it rains heavily in the mountains. But when it does rain it does so very hard and leads to water rushing through at great speed and with a lot of power (flash-floods). It is suggested that the Triassic conglomerates and breccias at Ogmore-by-Sea were formed in such a way.
Fig 7. Triassic Wadi (to the right) cutting through Carboniferous limestone (to the left)
We finished our time at Ogmore-by-Sea by moving to the far eastern end of the beach and we saw more Limestone. However, this limestone is different to the Carboniferous limestone in that they are more uniformly bedded and browner in colour (Fig 8).
Fig 8. Early Jurassic Limestones at Ogmore-by-Sea
These rocks have been dated as Early Jurassic in age (circa 199 million years old) implying that there is a time gap of about 130 million years between the two units. They rest with slightly different angle relative to the Carboniferous limestone. Academic geologists have interpreted these Early Jurassic limestones as being shallow sea limestones deposited onto the sides of islands formed of Carboniferous limestone.
You will recall that earlier-on I mentioned about South Wales being located within the supercontinent Pangaea during the Permian and Triassic Periods meaning that almost all rocks deposited during those times were deposited on land. During the latest Triassic and earliest Jurassic Pangaea began to break-up with the separate pieces moving apart from each other to form the continents that ultimately, we are familiar with today. The process of break-up or, “rifting” as geologists’ term it, allowed the seas to start to flood into the former land areas. This is the process that allowed the Early Jurassic limestones that we see at Ogmore-by-Sea to “onlap” onto the islands of Carboniferous limestone.
A really interesting feature of the Early Jurassic limestones is the huge amounts of fossils to be found within them (Fig 9). We found Corals, Crinoids and worm burrows some of which are almost 0.5m in diameter. A fossil collectors dream!
Fig 9. Crinoid stems and coral colonies within Early Jurassic Limestone
All the rocks at Ogmore-by-Sea have been cut by mineral veins. Mostly these mineral veins are coloured white, but some have a pink tinge to them. The crystals shapes vary from plates to much smaller indistinguishable shapes. The majority of the minerals are Calcite, but some are of Baryte which looks very much like Calcite but is heavier. All minerals veins seem to be exploiting (filling-in) fracturs or cracks in the rocks.
After we left Ogmore-by-Sea we drove to Southerndown Bay for a brief look at the spectacular faulting in the high cliffs of Trwyn-y-Witch (Witches Nose in Welsh). Here a large fault has pushed-up older rocks on one side of the fault over younger rocks (Fig 10). The pressures that existed on the fault plane were intense because the rocks immediately adjacent to the fault plane have been squeezed (deformed) into tight folds.
Fig 10. Interpretation of the structure at Trwyn y Witch, Southerndown
It is thought that Trwyn-y-Witch is the result of two different tectonic events. Firstly, during the earliest stages in the formation of Pangaea the Carboniferous limestones were folded and faulted. About 130-140 million years later the Jurassic rocks were deposited on top of the Carboniferous limestone. Sometime after deposition of the Jurassic rocks a second phase of earth movement re-activated the same fault pushing the earliest Jurassic rocks to the right of the fault up over younger Jurassic rocks on the left-hand side of the fault.
Following departure from Southerndown we drove to Atlantic College for lunch. What a beautiful group of buildings form the heart of the college. Effectively it is a castle built in the 13thCentury and known as St. Donats Castle. In the late 19th& earliest 20thcenturies it was owned by the American newspaper publisher Randolph Hearst as a holiday retreat before becoming Atlantic College in 1962.
Straight after lunch we walked down to the foreshore of Atlantic College to take a look at the Early Jurassic Porthkerry formation that is beautifully exposed in the cliffs and the wave cut platform. The Porthkerry formation extends from Southerndown to St. Donats and eastwards to near Rhoose which is a distance of about 12 miles in total. Cliff heights range between 5m to 50m.
A feature of the Porthkerry formation is the regularly alternating limestones and mudstones (deposited in the sea) with individual layers (beds) extending laterally along the coast as far as the eye can see (Fig 11). This means that the geological conditions that produced the rocks was continuous over a large area and in a stable environment. So, what geological process could have caused the repetitive and continuous layering?
Fig 11. Regularly alternating limestone and mudstone beds, of the Porthkerry Formation, St. Donats
The most likely explanation is that deposition of the rocks was controlled by regular alternations in the rising and falling of sea-level. Geologists believe that the primary control on sea-level is the amount of ice at the poles which in turn is controlled by climate. When the climate is cold there is more ice at the poles and sea levels are lower. Conversely, when the climate is warmer there was less ice at the poles and sea-level was higher. Geologists theorise that when the sea-level is lower the limestones were deposited whereas when sea levels rose the mudstones were deposited as the precise location in 3D space was in deeper water.
Other interesting features seen in the cliffs at St. Donats are the clear faults cutting through and offsetting the Porthkerry formation rocks either side of the faults. One small bay is bounded by two faults with the cliff moving downwards between these faults to form what geologists term a “graben.” The entire area of the Vale of Glamorgan is also affected by at least two sets of regional jointing. Jointing forms regular cracks in the rocks with some of the cracks having very small offsets of a centimeter or two to no offset at all. Sometimes the joints become filled with minerals, mainly white calcite. Jointing is likely a late stage flexing of the Earth’s crust as it relaxes after a tectonic period.
One final feature of note is that the formation has abundant fossils. All fossils are marine (sea) animals (recall that during the late Triassic to early Jurassic Pangaea began to rift-apart, allowing the sea to flood over what was desert). Fossils include Ammonites and bi-valve shells (Fig 12) such as Plagiostoma (a clam) and Lingula (a mussel). Plesiosaur and Ichthyosaur bones have been found in the Porthkerry formation, although not by us on the day!
Fig 12. Fossils found at St. Donats: Clams (left) and Ammonites (right)
The final location we visited was a little further east near Barry. At this locality Triassic rocks are exposed but they are very different to the conglomerates and breccias we saw at Ogmore-by-Sea. At Barry the Triassic rocks are very much finer-grained. The rocks are formed of mudstones and fine-grained sandstones and are thought to have been deposited around a lake.
The location is famous for the abundant fossil dinosaur footprints (Fig 13) and tracks seen there. Footprints include the rounded footprints of large plant-eating dinosaurs and smaller 3-toed predatory meat-eating dinosaurs. It is thought that primitive Triassic-aged dinosaurs were living around the lake in a generally arid, desert-like environment and came down to the lake to drink with the small carnivorous dinosaurs hunting the herbivores.
Fig 13. Dinosaur tracks exposed in Triassic rocks, near Barry
We finished the day with the dinosaur tracks at about 3:30pm. The UWC Atlantic College staff were exhilarated if a little tired, after the long day out trying to absorb so much new and often technical concepts & information. The fantastic sunshine also helped everyone enjoy the day and most if not, all left for home knowing more about the geology of their area than they did at the start of the day.
Since the trip I have received feedback from UWC Atlantic College. The leadership team said that everyone really enjoyed the day and that they learned a lot and now have a better appreciation of the landscape when out with their students. Constructive criticism of the day was that some of the group were a little lost at times with some of the geological concepts I introduced and to help them re-enforce the concepts it would be a good idea to provide hand-outs summarizing the key points. This I will prepare for future trips.