Rocks & Pebbles near Twlc Point

Broughton Bay is a wide sandy expanse on the north shore of the Gower Peninsula in South Wales, facing the Loughor Estuary or Burry Inlet. A small promontory called Twlc Point at the western end of the beach has an interesting geology with an exposure of Hunts Bay Oolite from the Carboniferous Period. I have written about these strata in earlier posts such as:

Rocks on the west side of Broughton Bay Part 1

Rocks on the west side of Broughton Bay Part 2

Rocks on the west side of Broughton Bay Part 3

Brachiopod fossils in Hunts Bay Oolite at Broughton Bay

On this particular visit I was content to appreciate the way that pebbles of many types and colours on the upper shore were clustered around outcrops and boulders of the limestone which were often pink-tinged and sometimes fossiliferous.

Rip-Rap and Storm Beach Rocks at Broughton Bay

Detail of a boulder on the beach used as a defence against coastal erosion

Storms battering the coast can drive substantial boulders up-shore where they remain at the top of the beach, frequently above extreme high tide level. By this natural phenomenon, a strong barrier is incidentally formed, that protects the upper shore, the base of dune systems, and cliff strata from the extremes of erosion by undercutting and removal of sediments by the sea.

The knowledge that loose arrangements of weighty boulders, with plenty of space between the individual rocks, can absorb and dissipate the energy of the waves, is used in the construction of artificial sea defence systems such as rip-rap for reducing or preventing coastal erosion.

There is evidence of both kinds of barrier, the accidental and the artificial, at Broughton Bay on the Gower Peninsula in South Wales. The rocks are mostly local with the majority comprised of local Carboniferous Limestone – with the occasional boulder of Old Red Sandstone conglomerate or individual mass of brick-wall debris. Many boulders are beginning to be colonised by lichens of various kinds and colours; and the rip-rap offers the opportunity to get up close to the rocks to study their natural textures and fracture patterns.

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Rocks on the west side of Broughton Bay – Part 3

Rock textures and fracture patterns in Gower Carboniferous Limestone

Some more images showing the mixed-up nature of the upper rocks in the exposure of Hunts Bay Oolite Subgroup of the Carboniferous Limestone in the cliffs at the west end of Broughton Bay on the Gower Peninsula – illustrating the irregular fracture patterns, rough textures and combinations of rock type within the formation. I am considering whether these messed up rocks represent a storm event way back at the time they were forming?

According to Willoughby, C (1996) Environments of Deposition in the Carboniferous Limestone of South  East Gower:

In the sequence of the Hunts Bay Oolite there is also further evidence of storm events with the appearance of bends and lenses of coarse laminated packstones*. These are associated with breccias* comprised of angular fragments of coarse oolites in the packstone matrix, indicating hardground formation before these have been ripped up in storm events and rapidly redeposited within reworked packstones. In places the junctions between these two lithologies are gradational, probably indicating that the sea floor at this time was uncemented and soft allowing mixing to occur.

I visualise that scenario as resulting in something that looks like pack ice,  where a solid sheet of ice has been broken up by some force of nature into many angular fragments that then refreeze into a solid form with the pieces ‘cemented’ together by newly formed sea ice.

Definitions

* Breccia is coarse, clastic, sedimentary rock, the constituent clasts* of which are angular. Breccia literally means ‘rubble’ and implies a rock deposited very close to the source area.

* Clasts are particles of broken-down rock. These fragments may vary in size from boulders to silt-sized grains, and are invariably the products of erosion followed by deposition in a new setting.

Packstone is defined by the Dunham Classification as a limestone characterised by a grain-supported texture, together with a lime-mud matrix.

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

COPYRIGHT JESSICA WINDER 2013

All rights reserved

Rocks on the west side of Broughton Bay – Part 2

Rock textures and fracture patterns in Gower Carboniferous Limestone

Some more images showing the mixed-up nature of the upper rocks in the exposure of Hunts Bay Oolite Subgroup of the Carboniferous Limestone in the cliffs at the west end of Broughton Bay on the Gower Peninsula – illustrating the irregular fracture patterns, rough textures and combinations of rock type within the formation.

In the British Geological Survey (BGS) Lexicon of Named Rock Units regarding Hunts Bay Oolite Subgroup, the rock is described:

Lithological Description:

Mainly ooidal limestones, with subordinate skeletal, peloidal and oncolitic limestones and calcite mudstones. Mainly dolomitised on the southeast and east crop of the South Wales Coalfield.

Definition of the Lower Boundary:

The base of the Subgroup is gradational and taken at the incoming of a predominantly oolitic grainstone sequence, locally with units of calcite mudstone, above the skeletal packstones or skeletal/peloidal/oolitic packstone/grainstones of the underlying High Tor Limestone Formation.

Definition of the Upper Boundary:

In the Vale of Glamorgan the top is taken at the sharp contact, defined by a palaeokarstic surface, between the heterolithic lithologies of the Subgroup and the overlying partially dolomitised sandy skeletal packstones and the calcareous sandstones of the Pant Mawr Sandstone Member [Oxwich Head Limestone Formation]. In the Gower and Pembrokeshire it is taken at the palaeokarstic surface separating the heterolithic lithologies of the Subgroup from the mottled and pseudobrecciated skeletal packstones of the overlying Oxwich Head Limestone Formation.

What do all these geological terms mean? I’m away to consult with the Oxford Dictionary of Earth Sciences!

Reference

Waters C N, Waters R A, Barclay, W J, and Davies J R (2009) A lithostratigraphical framework for the Carboniferous successions of southern Great Britain (onshore), British Geological Survey Research Report RR/09/01, NERC, Keyworth, Nottingham.

Willoughby, C (1996) Environments of Deposition in the Carboniferous Limestone of South  East Gower, Bsc Geology Thesis, University of Wales, Aberystwyth.

Wilson, D, Davies, J R, Fletcher, C J N, and Smith M. (1990) Geology of the South Wales Coalfield, Part VI, the country around Bridgend. Memoir of the British Geological Survey, Sheets 261 and 262 (England and Wales).

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

Rock textures and fracture patterns in Gower Carboniferous Limestone

COPYRIGHT JESSICA WINDER 2013

All rights reserved

Rocks on the west side of Broughton Bay – Part 1

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

These rocks are not pretty but they are interesting! This is the project I am working on at the moment. I am trying to understand the rocks on the west side of Broughton Bay on the Gower Peninsula. I know that they are Hunts Bay Oolite Subgroup of the Carboniferous Limestone from looking at the geology map – but that is just the beginning of the understanding.

Not all the rock looks the same. There seem to be two main parts. The upper part of the cliff looks a bit of a mess, a bit of a jumble, coarse sediments and lots of fractures, ?faults, and various lenses and bands of material. Sometimes it looks like some sort of breccia. It is mostly examples of the appearance of the rock in the upper part of the cliff that are featured in this post. By comparison, the lower part of the cliff tends to be a different colour, is more solid, massive, and continuous, with fewer fractures, and containing obvious fossils.

In this and subsequent posts I am just putting down my thoughts as I read more about the subject. I have already found a couple of good references to help me. What I really need to do is visit the “type location” for this particular rock type, from which this particular subgroup of the Carboniferous Limestone was first properly identified and described. Perhaps the next time I am in Gower. Maybe it will all become a lot clearer then. Meanwhile, I will continue my attempts to identify the components of the rock and understand their origins.

First of all, it should be noted that in the photographs shown here, the orange, yellow, and black patches are modern lichens living on the surface of the rock and are nothing to do with the rock itself. The white colour on the rock close-ups is usually crystalline calcite. The top picture in this post shows the general location where the following photographs were taken, from just before Twlc Point to Foxhole Point on the west side of Broughton Bay. The boulders on the beach at the foot of the low cliff are covered with living barnacles attached to mussels, and by green seaweed.

The Hunts Bay Oolite Subgroup (HBO) is the most recent nomenclature for this type of rock but it was first named and defined as the Hunts Bay Group by Wilson et al (1990). The type section is the cliffs and foreshore reefs between Bacon Hole and Pwlldu Head, Southgate, Gower, South Wales (SS 5604 8674 – 5728 8632).

This is a work in progress and  I will amend and annotate the text and pictures as I learn more, as well as adding new posts on the subject! If you, as a reader and expert geologist, know that I as an amateur have made a mistake in my understanding about these rocks, please do drop me a line and put me right.

I am currently reading:

Waters C N, Waters R A, Barclay, W J, and Davies J R (2009) A lithostratigraphical framework for the Carboniferous successions of southern Great Britain (onshore), British Geological Survey Research Report RR/09/01, NERC, Keyworth, Nottingham.

Willoughby, C (1996) Environments of Deposition in the Carboniferous Limestone of South  East Gower, Bsc Geology Thesis, University of Wales, Aberystwyth.

Reference

Wilson, D, Davies, J R, Fletcher, C J N, and Smith M. (1990) Geology of the South Wales Coalfield, Part VI, the country around Bridgend. Memoir of the British Geological Survey, Sheets 261 and 262 (England and Wales).

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

Rock texture in the cliffs on the west side of Broughton Bay, Gower, South Wales.

COPYRIGHT JESSICA WINDER 2013

All rights reserved