Virtual Field Trip from

Locality 3.2 - Barry Island

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? Barry Island lies on the Glamorgan coast southwest of Cardiff. Study the area in Google Maps and Streetview.

Barry is a port town, and one of its claims to fame is that it has the second or third highest tidal range in the world - around 15 metres difference between high and low spring tides - because of the funnelling effect of the Bristol Channel. Where in the world would you go to find higher tidal ranges?

Barry has other claims to fame: its pleasure beach, its slightly old-fashioned British seaside resort atmosphere, fish and chips, ice cream, and of course Gavin and Stacey. Barry Island's world class geology is, however, not as well known as it should be. The unconformity here is superbly exposed in 3-D.

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, which includes a map showing the location of the sections at Points A and B. We are particularly concerned with the nature of the contact between Lower Carboniferous and Upper Triassic rocks, and the sedimentology of the Upper Triassic.

Further reading: these rocks were described in detail in this article by Tucker (1977)


A. Whitmore Bay, Barry Island, viewed at low tide from the south eastern end of the promenade. In the left background is the landward end of Little Island (a promontory): we will
study sections on the W side of Little Island (Point A, not in view) and the W end of Whitmore Bay - the low red cliffs below the trees (Point B).

From © Copyright M J Roscoe and licensed for reuse under this Creative Commons Licence.


Field evidence - Point A: Foreshore and low cliff exposures on the west side of Little Island

Describe what you see in the outcrop photos, and use them as evidence on which to base your interpretations. Pay particular attention to field relationships - contacts between units, how units change as you trace them laterally, etc.

B1. This pair of photos shows the complex contact between the Lower Carboniferous limestones, dipping to the right (south) and the overlying Triassic strata, which are essentially horizontal.

B2. Annotated version of B1

C1. A closer view of the relationships seen at top right of B1 and B2

C2. Annotated version of C1

D1. Another view of the unconformity. Note that the Trias rests on the Carboniferous at different heights in the outcrop.

D2. Annotated version of D1

E. Close-up of D2, showing detailed field relationships.

F. The top surface of the Carboniferous limestones, immediately below the contact with Triassic sediments.

What is this feature? What happened to these limestones in the long time between their uplift and the deposition of the Trias?

G. The erosion surface of the Carboniferous limestones at the point where the scale is located is the actual Triassic land surface.

H1. To the right (south) of G, looking at the same contact.

H2. Annotated version of H1. Look carefully at the area in the box. Can you see a 'frozen' geological process here? What was happening?

I. Horizontally bedded Triassic sediments consisting largely of limestone debris of various sizes. These sediments lie immediately above the unconformity.

J. Hand specimen of a sediment very similar to that seen in Photo I.


Field evidence - Point B: Cliff and beach exposures at the west end of Whitmore Bay

Describe what you see in the outcrop photos, and use them as evidence on which to base your interpretations. Pay particular attention to field relationships - contacts between units, how units change as you trace them laterally, etc.

K. Gently dipping red siltstones and marls. Pale interbeds of micritic dolomitic limestone, and a thick bed (pale) of calcareous sandstone.

L. Closer view of the cliff

M. A cut and polished sample from one of the thin dolomitic limestone beds.

N. A close-up of the thick sandstone, which in place shows cross bedding (not obvious in this photo). Note the large hollow structures above the geologist's head.

O. Note the structures at the horizon marked by the arrow.
We look at similar material in more detail in the next 3 photos.

P. From a lower bed, at beach level. These irregular-shaped structures are locally known as potato stones, contained in red siltstone. They are not transported clasts.

Q. Close-up of one of the features in Photo P. They now mostly consist of calcite, but it probably replaces an earlier mineral. Which mineral?

R. Cross section of a potato stone. Some of these are partly hollow. If they are not clasts, how may they have formed? What does this tell us about environmental conditions?

Here be dinosaurs...

[The text and images were originally compiled by Dr Laura McLennan]. At Bendrick Rock, on the coast east of Barry around grid reference ST144677 Google Maps, Lower Carboniferous marine limestones are unconformably overlain by clastic terrestrial sediments of the Upper Triassic, as at Barry Island. Bendrick Rock geology

The footprints found here were made by several different dinosaurs. The smaller footprints and tracks are assigned to the ichnogenus Grallator and were probably made by a small, 1m tall, theropod dinosaur such as Coleophysis, a small predatory dinosaur that may have hunted in packs.

photo: Laura McLennan

photo: Roger Suthren

image: Dr. Jeff Martz/NPS via Wikimedia Commons
Coleophysis footprint and reconstruction of the dinosaur which may have been responsible for these prints.

The larger three toed theropod footprints are rarer and are attributed to the ichnogenus Anchisauripus.

Anchisauripus footprint (photo from Laura McLennan) and reconstruction (Wikimedia)

Prints left by large four-toed dinosaurs are also found. These probably belonged to the ichnogenus Tetrasauripus. They were most likely made by a fairly large, 7-8m long, herbivorous dinosaur such as Plateosaurus. These dinosaurs were protosauropods, four legged dinosaurs which were capable of walking on their hind legs only if required.

image: Nobu Tamura via Wikimedia Commons
Plateosaurus footprint (photo from Laura McLennan) and reconstruction


Interpreting the field evidence

You have now acquired a variety of different types of data on the outcrops at Barry. This includes lithology, sedimentary structures and field relationships seen in photographs and samples. Your task now is to interpret the data, and write a short account in your virtual notebook


Just for fun...


End of Day 3

Make sure you've completed all the work for this locality. That's the last stop for today. Let's meet for today's beach picnic and Q & A session in Blackboard Collaborate at 5pm today (only available if you're a University of Derby student doing this field trip in real time on Wednesday 29 April, 2020!).


This page is maintained by Roger Suthren. Last updated 29 April, 2020 1:40 PM . All images © Roger Suthren unless otherwise stated. Images may be re-used for non-commercial purposes.