The Caerbwdy Sandstone is present on a fault surrounded by St Non’s Sandstone and Caerfai Shale. There are multiple planes of faulting and it is quite difficult to make out the bedding. The massive bedding planes that are visible are parallel and planar, which measure to approximately 1-3m in width and multiple joints run slightly perpendicular to them. Finally, slickensides are present due to the fault causing friction on the sandstone.
Menevian and Solva Group (Middle-Cambrian)
The Middle Cambrian rocks were studied at St Davids peninsula. These sandstones have clear bedding planes, which are approximately 10cm thick. The beds also have a dip of 80 degrees out towards sea, which makes them very steep and vertical. There is also a large dolerite intrusion of igneous rock (approx. 5m) that is in-between two sections of the steep-bedded sandstone. This is either a most definite sill or maybe due to lava flow. There is also a wave-cut platform that is the result of erosion from hydraulic power and corrasion. The sandstone has a medium sized grain and contains feldspar and low quartz content. The igneous dolerite was previously pyroxene and it contains iron magnesium silicate (olivine).
At the Middle Solva beds and Menevian Group many marine trace fossils are present in the sandstone. The grain size of the sandstone is medium to fine and the rock beds are 5cm in width. Thick thalassinades and chondrites burrows as well as gastropods are preserved as casts in the sandstone. The area is known for its storm sequences, which ultimately alternates the energy. Low energy produces mudstone from slow movement and high energy produces sandstone from rapid movement. Therefore the mud settles from suspension in calm conditions over time. Ripples are also present on the sandstone showing that it was a shallow marine condition. The ripples are a few millimeters in height and there is approximately 5cm between the wave crests. The fossils in these sandstones show that there was a huge amount of life in the Cambrian.
Lingula Flags Formation (Upper-Cambrian)
The Lingula Flags Formation is part of the Merioneth Series and was studied at White Sands Bay, Pembrokeshire. It is a shallow sedimentation of mudstone and sandstone formed on a shelf margin. The grain size is fine/mud, which makes up the thin beds of 1-3cm in width. These beds are not to be confused with the strong cleavage and cross-bedding that is present. Joints can also be recognized being relatively perpendicular to the bedding. There is also many weathered surfaces and areas of erosion show some iron oxidized on eroded surfaces giving a reddish colour to the surface of the rock. Mudstone is the predominant rock on this formation but there is some sandstone also. Slight folding can be seen on the bedding planes showing that over time, pressure was exerted to give the rock a bending look. There are dark and light patches showing different types of sediment. The dark patches are clays and muds, whereas the light patches are coarse fine sands. These add to the finely alternating beds. Calcite mineral veins are visible and there is some quartz present. Coarse sediment lies on top of the finer sediments. This is due to the environmental deposition, tidal regimes and there is a variation in energy (finer/coarser deposited). There is a regressive sequence. The environment becomes shallower towards the north and finally there is more sand to the north.
Geological History of Pembrokeshire
During the Precambrian, volcanic eruptions and intrusions of magma meant the area became largely predominant of lavas and ash. St David’s head is an example of where the Precambrian rocks can be found. Towards the end of the period there was folding due to uplift and erosion. Ultimately, intrusions were exposed at the surface. By the time of the Cambrian, the sea flooded all volcanic landforms and landscapes. The depth was mostly shallow apart from in the winter where it then got deeper. The sediment was given a regressive sequence due to the settling of mudstones last due to the increasing depth of the water. The fauna and biota was extremely diverse and many organisms’ fossils can be found in the sediments. During the Ordovician period, the landscape was again flooded even deeper after an uplift and erosion. The area was characterized by the accumulation of fine muds. Underwater volcanic activity meant that pillow lavas and igneous rocks (rhyolite) were formed. The Silurian brought warm, tropical seas to the shallow shelf area. Brachiopods and corals are easily seen during this period. Fold and faults were created by the time of the Late Silurian and Devonian. Old Red Sandstone was especially frequent showing the product of deposited sediments from braided river channels. During the Carboniferous period, warm equatorial seas containing corals and crinoids became abundant in the area. Peat delta swamps and periodic sea flooding meant that many landforms were created during this time. By the end of the Carboniferous, folding and faulting was the result of continents colliding with one another. The Permian, Triassic and Jurassic did not introduce any flooding to the area, but fossils from adjacent seas can be found in South Wales. The Cretaceous period however brought globally high sea levels and the deposition of chalk to many areas. The St David area gives many examples of plateaus, which are the result of marine erosion. In the Tertiary period, widening of the North Atlantic resulted in block uplift. This creation of new landmass is what can be seen at the present day. Marine erosion has contributed to shaping the landscape into what can now be seen. Finally the Quaternary period showed very cold conditions and glaciation in Britain. In the most recent glaciation, Britain was completely covered by ice. When conditions were cold, the sea level could have dropped 50m below today’s levels. Whereas when sea levels were warm, the may have been 5m higher than today’s. Huge amounts of meltwater resulted in lakes and channels being formed.
by
dannyboakes (student)
