Vale of Glamorgan Field Trip Report

Vale of Glamorgan Field Trip Report

Our first location was at Bwlch y Gro near the village of Ogmore by Sea on the South Glamorgan coast. Here the cliffs are composed of Carboniferous Limestone, the High Tor Limestone, overlain by Triassic sediments. The limestones are within the tidal zone and so they have been wave polished which means that structures and fossils within the rock can be seen. The High Tor Limestone is described as a monomict framework supported limestone – which means that it is composed of a single rock type, - limestone. The rock has a grey colour indicating calcite but there are some patches coloured pink indicating that here it has been dolomotized. Within the rock there are also patches of Galena and Barytes. There are Rugose corals (Syringopora, Siphonophyllia, Palaeosmilia, Michelinia, Lithostrotion) brachiopods – Giganto productus, and gastropods. There are also trace fossils of Thalassinoides, Zoophycos and Planolites. The sediments are poorly sorted with a variety of fragment size and many of the fossils are broken or distorted. This type of assemblage is thought to have been deposited in a storm dominated marine environment. The larger rock fragments could result from older limestone deposits – sub-marine, tidal channel or off shore shelf, disturbed and then redeposited during a storm event.

Burrow / Crinoids

The sediments at this first location are generally poorly sorted. There is however a general fining upwards sequence consistant with a storm environment easing and a calmer environment returning. The lower sediments are described as packstones which grade up into wackestones. Clastic limestones such as those found at this location are graded into three types – Grainstones, Packstones and Wackestones. Grainstones are solely composed of framework supported clasts – either bioclasts or ooids with no micrite or limestone mud matrix. They are deposited in high energy limestone rich environments such as tidal channels or barrier bars. Packstones are like grainstones, composed of bioclasts or ooids but these are held in a micrite or limestone mud matrix. Finally Wackestones have at least 10% matrix supported grains within a limestone mud or micrite matrix. Where the percentage of grains falls below 10% then this is known as a carbonate mudstone or lime mudstone.

Further along the cliffs, Triassic sediments have been deposited unconformably on the High Tor Limestone. This unconformity represents a gap of about 90 million years. There was uplift of the High Tor Limestone during the Armorican Orogeny followed by erosion during the Permian and early Triassic. These Triassic sediments are a limestone breccio –conglomerate. Conglomerate is used to describe rock fragments that are rounded, where as breccio means rough or angular, so the rock here has elements of both, implying that some of the rock components have travelled some distance in a fluvial system prior to deposition. In places large blocks of High Tor Limestone have been detached and moved a few metres. This is indicative of erosion of a scree slope and some lateral fluvial movement in an arid or semi arid environment. Gareth assured us that it is possible to measure the imbrication of the detached blocks – the angle of clasts to gauge the direction of flow. Within the High Tor Limestone there are Neptunian Dykes – fractures that have been infilled with Triassic sediments.

There is yet another unconformity further along, this time between the High Tor Limestone and the Sutton Stone which is in the Lower Lias –the Jurassic. Standing on the depositional surface of the High Tor Limestone there are slight undulations which is interpreted as tidal scour. On this surface there are also Trypanites burrows – a animal that can bore into the hard rock. The Sutton Stone was then deposited on this surface. It is composed of limestone clasts with lots of fossils.

Unconformity underlaying High Tor scoured surface and overlaying Sutton Stone

It was deposited during a period of rising sea level and as such it is trangressive – as level rises further, the sea moves inland. Above the Sutton Stone are the Southerndown Beds, composed of bioturbated limestones and mudstones with a fossil fauna that includes bivalves, gastropods and corals.

Dunraven Estate

Our next locality was at Seamouth, a popular beach backed by limestone cliffs. At the base of these cliffs are the Dancing Stones, a series of gently inclined limestone bedding planes with cracks at right angles through the bedding plane. Here we are standing on the top of the Southerndown beds which contain angular fragments of chert. Above is the Porthkerry formation which has been sub-divided into units A, B, C, and D. The beds here are quite uneven in thickness and this is thought to be due to Thalassinoides burrows. The cliffs are an alternation of limestones and marly clays caused partly by diagenesis and partly by Milanhovic cycles. There is however no change in the fossil fauna, particularly the micro-fossil fauna. About one-third of the way down the cliff are some more massive limestones, about 3 to 4 metres in depth – this is the Seamouth Limestone which equates to Unit C of the Porthkerry formation.

Seamouth / Cliff fall over the anticline

On the east side of the beach is Trywn-y-Witch or Witches’ Point. Here the unconformity between the carboniferous limestone and the Sutton Stone can be seen again. There is also a series of complex folds and faults that can be seen, although the tight anticline that features in Gareth’s book is now obscured by a cliff fall. The Sutton Stone is a freestone, a rock which has no dominant bedding plane that would cause it to cleave and so it is good for carving. It has been used in some on the ornamental stonework at the Houses of Parliament.

We then returned to the top of the beach walking through the gardens of the former DunravenCastle to the viewpoint overlooking DunravenBay. Here there is a fantastic view of the beach – a wave cut platform with cliffs behind which comprise the Porthkerry member of the Blue Lias. In the lower part of the cliff there is more clay present which gives them a grey colour, whilst higher up the cliffs are more orange due to a higher limestone content. From the view point a small plunging anticline can clearly be seen on the beach.

Nash Point / The group walking across the beach

Nash Point

A couple of miles to the east was our final location of the day – Cwm Marcross and Nash Point. Cwm Marcross is a small cove with a pebble beach above a limestone platform. The cliffs that surround the cove have a broad anticline and are formed by Unit B of the Porthkerry member. Some of the beds within this unit are distinctive, particularly one called the Sandwich Bed which has a slightly harder limestone. It is this bed which forms the roof of many caves that occur from Nash Point eastwards. In the cliffs either side of the cove we saw the development of coastal features – rock stacks and arches where caves have worn through from both sides.

BarryIsland

Our first locality was on the western side of the peninsula of Little Island on the western side of BarryIsland. This is the type locality of the Friars Point Limestone which forms the top half of the Black Rock Limestone. The Black Rock Limestone forms the base of the Pembroke Limestone Group in the Dinantian series. In the stratigraphic column these rocks lie some way below the High Tor Limestone seen the day before.

The Friars Point Limestone is a bioturbated bioclastic limestone composed of corals, broken crinoids and bryozoans. Above, there is an angular unconformity with Triassic rocks – the same scoured scree slope deposit that we saw the day before. As before the Triassic was a breccio conglomerate with clast size varying from boulder to pebble. Finer sediments had been deposited on top of the Triassic and these are consistent with mudstones deposited during the initial stage of a lake expansion. These are the marginal facies of the Mercia Mudstones.

Angular unconformity – Little Island / Unconformity with brecciated rock

On the eastern side of the peninsula is WhitmoreBay a sandy beach with a holiday park behind and on a hot day must be crowded with people. Fortunately on a cool day in May less people meant we were able to examine the cliffs easily. The low cliffs at the rear of the beach are the Branscombe Mudstone Formation, (previously known as the Keuper Marl) at the base of the Mercia Mudstones in the Upper Triassic. Within the rock are nodules of Gypsum, white to pink in colour, and which have suffered from dissolution and now appear hollow. In some cases some calcite has been deposited in the resultant void. They are known locally as Potato Stone. Further up the cliff is the Blue Anchor formation (previously called the Tea Green Marl) which is a pale green colour compared with the lower Branscombe which is red. The red colour is derived from the oxidised iron in the rock whereas in the green rock the iron has been reduced. This is consistent with intense evaporation of a receding lake following the deposition of the mudstone. There is a small fault in the cliff with a down throw of 1.5 metres.

Gypsum nodule / Red Lower Branscombe and green Blue Anchor formation at the top

SullyIsland

This is a small island, accessible only on a falling tide and slightly east of BarryIsland. The causeway to the island consists of flat rock that can be very slippery and takes 10 to 20 minutes to cross depending upon agility! On the southern side of the island there is an unconformity between the Carboniferous Limestone and the Triassic succession. At the top of the Triassic sediments are Sand Volcanoes or Teepee structures. These are formed in sediments that are deposited rapidly due to mass flows. They are formed when water escapes upwards through vents causing the sediment layers to be deformed upwards.

/ Many thanks to Wendy for organising another very successful field trip and to Gareth George for showing us some very interesting geology.
Tim Lawrence
Teepee Structure

Lavernock St Mary’s Well Bay

We then drove to St Lawrence Church where we found a plaque commemorating this site as the place where the first radio message was sent over water by Marconi and Kemp in 1897. Following the footpath we found away down onto the beach to see a complete succession of the Triassic Penarth Group up into the Blue Lias Formation. This represents a transition from arid to marine sedimentation. It was a pulsating transgression. The first thing we noted was the gypsum known as the Penarth Alabaster in cliffs in the distance. There are few faults in this area. The photo shows the sequence in the cliffs with the Paper Shales forming the wave cut platform overlain by the Bull Cliff Member [the Triassic ].We identified the Dual Bed which marks the top of the Triassic locally [200ma] over lain by the Planorbis Mudstones and the Lower Laminated beds. The succession records a progressive rise in sea level and a change in climatic conditions. The Mercia Mudstone Group was deposited in a shallow hypersaline lake which evaporated leaving cracks, gypsum and other sabhka We found some fibrous selenite on the beach.

At the top of the Blue Anchor Formation it was becoming marine. The Westbury Beds show transgressive events with deepening upwards cycles. The Blue Lias was a time of regional transgression. The Blue Cliff member fauna indicates slow sedimentation with a firm base whilst the St Mary’s Well member shows increased sedimentation with well oxygenated bottom conditions. This was followed by a rise in sea level during the Lavernock Shale deposition from nodular limestone to shale. The Porthkerry Member shows shallowing.

At the end of the afternoon we went fossil hunting along the beach. The cliffs are not very stable so are best looked at from a distance. We saw some dessication cracks in the marl, ripple marks, straight crested symmetrical ripples indicating waves , fibrous selenite, septarian nodules called beetle stones and three sets of jointing. Echinoderm spines, Protocardia lyostria, shelly layers, ammonite pieces in pebbles amongst other things. Gareth told us to look for pieces of bone but no one found an ichthyosaur! A possible piece of bone was found – by me - what a surprise but Gareth has it and has not yet confirmed that it was bone.

Mostly by Tim Lawrence, last bit by Wendy Hamilton; photo’s by Wendy Hamilton except SullyIsland by Tim Lawrence