Virtual Field Trip from

Locality 3.1 - Caswell Bay

It is important that you access this field trip on a laptop or desktop PC. Click on any image to enlarge it.

Where are we? Today we've driven east to the South Glamorgan coast, on the beautiful Gower Peninsula near Swansea. Study the area in Google Maps and Streetview.

At this location, our logged section takes us through several major units of the Pembroke Limestone Group (Lower Carboniferous), in stratigraphic order from the base.

Handout: refer to the handout for today, which contains essential diagrams and information. Don't forget to use the information in the Field Guide too. Both are available in Course Resources.

Tasks: See today's handout. Record all your observations in your virtual notebook.


A. General view of the logged section (foreground) on the east side of Caswell Bay


B. Part of the logged section. Strata young and dip steeply to the right (north)


C. Our goal is to study ancient carbonates, but here are some modern carbonate-producing organisms in the intertidal zone at this location

What are the organisms shown? Try to classify them. Which of them are geologically important as sediment formers?


D. What are these organisms? What are their closest relatives?

E. The rocks here show strong Variscan deformation, including folding and thrusting. On a small scale, much of the deformation is brittle, as we would expect in competent, rigid limestones. Here are some fine sets of tension gashes.

F. Minor faults and shear zones.

There's a lot of interesting, detailed geology here, especially for limestone fans, and we usually spend 3-4 hours logging this section and admiring the view.

As always, the aim of this exercise is to synthesize information of a variety of different types - for example lithology, thickness and geometry of sedimentary units, sedimentary structures and palaeocurrents, fossils and trace fossils (or absence thereof), lateral and vertical changes etc. From these observations, presented on your log, you should be able to attempt interpretations of the the processes, conditions and environments involved in the formation of the various units.

The Caswell Bay log.

A higher resolution version is provided in the Base Maps file for you to complete your log.

Black Rock Limestone Subgroup (0 to 40m on log)

We last saw the Black Rock Limestone (BRL) at the top of the section at Skrinkle Haven, above the Avon Group. The upper part of it is exposed here, but we don't see the base in this section.

G1. General view of the BRL. Bedding is vertical and strikes roughly east-west, from bottom left to top right in the photo. Younging is to the right. There are also strong sets of joints which could be confused with bedding.

G2. View along vertical bedding in the BRL. Younging to the right. White patches are irregular vugs filled with calcite.

G3. BRL showing dark, muddy limestones with silicified fossils and irregular vugs filled by quartz (replacing calcite?) standing out on the weathered surface. Some parts of the limestones do not react readily with dilute HCl.

G4. Laminated fine muddy limestones and mudstones are interbedded with coarser more fossiliferous limestones. Vertical dip, younging towards top of photo.

G5. Identify the fossils in this muddy, poorly sorted limestone. Comment on their preservation. What do they tell us about conditions of deposition?

G6. Describe the rock, using the hand specimen description scheme, and identify the fossils.

G7. Thin section of one of the coarser beds in the BRL. Plane-polarized light; scale in mm. Arrow shows way up.

Describe the rock. What is the main bioclastic component? What are the highly irregular thin dark features crossing the slide, and what process do they represent?


Caswell Bay Oolite Formation (also known as Gully Oolite) (40 to 86m on log)

To study this formation in detail, we are going to take two short, interactive virtual field trips. Each consists of 6 images, from broad scale views down to microscope views. Proceed through the field trip using the arrows. The icon will load a higher magnification view of the image; the icon will take you back to the home page. Study the images and answer the interactive questions. Make notes and images in your virtual notebook as you go through the trips.

Additional images and information from the Caswell Bay Oolite (CBO):

H1. View along the sharp, roughly planar contact between the dark BRL (right) and the lighter CBO (left). Beds are vertical and young to the left (north)

H2. CBO dips steeply north (right). Note the distinctive light grey colour of the CBO.

H3. Bedded CBO. What are the coarser components likely to be?

H4. Identify these structures. What do they tell us about sediment transport?

H5. This is a section through the basal contact of one of the coarser beds seen within the CBO.

Describe the contact. Identify the components of the coarse bed. What structures are present in the bedded oolite below?

H6. Another coarse bed within the CBO.

What is the main fossil component?

H7. Large clast in one of the coarser beds.

Identify it as accurately as possible, and comment on the sedimentary processes involved.

H8. Another large clast in one of the coarser beds.

Identify it as accurately as possible, and comment on the sedimentary processes involved.

Caswell Bay Mudstone Formation (86 to 92m on log)

This unit is quite different from any other limestones in this section. Some of the beds do not react with acid, suggesting they are dolomitized. Macrofossils are largely absent from these beds, unlike other units in the logged section. What might this tell us about environmental conditions?

J1. Beds dip steeply right (north). The feature on the left is the highly irregular dip slope at the top of the CBO. The contact with the overlying Caswell Bay Mudstone Formation (CBM) is 1m to the left of the geologist with the pink sweater.


J2. The discontinuous basal unit of the CBM - this rock infills hollows in the irregular top of the underlying CBO. Above this coarse rock, laminated limestones are seen at the top of the photo.

J3. Laminated micritic limestones of the CBM.

J4. There are interbeds of mudstone, weathering back, within these laminated micritic limestones.

Look carefully at the laminations in this photo and J3.

J5. Steep dip and younging to the left.

Try to account for the nature and origin of the laminations in this micritic limestone.

J6. Thin section of a laminated limestone bed in plane-polarized light (PPL). Scale in mm. Arrow shows way up.

Describe the depositional and diagenetic features you can see.

J7. PPL; scale in mm; arrow shows way up.

Describe the two lithologies. What are the grains in the coarser laminae?

J8. PPL; scale in mm; arrow shows way up.

What are the light-coloured features in this micritic limestone? How and when did they form?


High Tor Limestone Formation (above 92m on log)

In this rather coarse-grained unit, macrofossils again become abundant. All beds react reasily with dilute HCl.

K1. View of the Caswell Bay Mudstone Formation and its sharp, more or less planar contact with the overlying High Tor Limestone (HTL). The contact is in the narrow gap under the overhang. The dip is now less steep, still to the north.

K2. A closer view of the HTL, 50 cm above its base.

Give as full a description as possible, using the hand specimen scheme. Identify the fossils present.

K3. Comment on the grain size, sorting and fossil content of this limestone, in the bed above K2.

K4. Thin section of the rock shown in K3. PPL; scale in mm; arrow shows way up.

What additional information can you obtain from this view?


Interpreting the field evidence

You have now acquired a variety of different types of data on the sedimentary sequence shown in the log. This includes lithology, sedimentary structures, fossils and field relationships seen in photographs and samples. Once you have completed your log, your task is to interpret the data.

  1. For each major unit or significant surface on the log, write a brief interpretation of the processes and conditions of formation. Include in your interpretation information such as environmental energy, flow regimes, salinity, oxygen levels, depth etc. There must be evidence to support all of your interpretations.
  2. Suggest a depositional environment for each unit.
  3. Write a short interpretative account of changes in processes, conditions and environments from the base to the top of the logged sequence.

Just for fun...

I recently discovered the Welsh composer Grace Williams on the radio. She was born in Barry, our next location. Listen to some of her music and see what you think - perhaps start with Hiraeth. If you like this, check out her YouTube channel.

Since we're in Glamorgan, today's recipe is for Glamorgan sausages - possibly the world's first vegetarian sausages, made with cheese and leeks. This recipe is from the Hairy Bikers.


Next locality

Make sure you've completed all the work for this locality. Now we can get back on the virtual coach and head off to our next stop at Barry Island


This page is maintained by Roger Suthren. Last updated 13 January, 2021 12:08 PM . All images © Roger Suthren unless otherwise stated. Images may be re-used for non-commercial purposes.