From the 1st-4th July we studied a geological area based around parts of Somerset, around Bath and Bristol areas. One specific site that we studied was Sand Bay, north of Weston-Super-Mere, (ST 3305 6596, which is located on the map above. The purpose of this investigation was to construct two Graphic Log Sections, one on the eastern exposure (ST 3375 6645) and one at the western exposure (ST 3245 6605) of the Carboniferous Limestones and volcanic rock exposed in the low, coastal cliffs. Then to compare the two graphic logs (similarities and differences) and interpret the geological condtions that led to the formation of the sequences observed.
In South Wales and the South-West of England in the Lower Carboniferous times, several hundreds of metres of limestone and calcareous mudstones passed southwards into a deformed sequence. A shallow south sea in the north bordered a deeper marine area to the south that revealed limited supplies of course sediment, in which was some volcanic activity.
We collected data from two locations. We had to predict if there was volcanic activity present in the area in the Carboniferous times and if so where was it in relation to the two locations. This was done by studying the beds and what was in them internally. We collected our data in groups, which has its advantages and disadvantages. For example we completed the work quickly, we had many people looking at the same thing making our descriptions more detailed and we could also discuss each type of rock with each other. A disadvantage could be that everyone will get to work out dips, strikes and measuring so will not learn practical fieldwork.
How it was Collected
Dip and Strike
The instrument that we used to measure the dip and strike of each bed was called compass clinometers.
The problem was that the surface of the rocks could be uneven from erosion causing the data to be not very accurate.
With a hammer and wear safety goggles and a hard hat, the colour of the rock was found out by chipping off a small bit from the main bed. This uncovered and clean surface colour free from erosion
There were very little problems with this collection; however, some of the rocks were very hard to chip away at.
The same technique was used when finding out the colour. A hand lens was also used for accuracy.
A tape measure was used to measure the beds. We then used a formula to calculate the true thickness. T=WxSin T=True Thickness, W=Width of Outcrop, =Angle of Dip
Tape measures are not very accurate, and we only measured to .0 and finding where the bed started and finished was also difficult because some beds were very small.
To recognise limestone we used acid-Hydrochloric acid (HCl), which fizzed when there was a reaction. Other types of rock where easily recognisable such as sandstone, it id fine grained and easy to crumble in your fingers. Tuff was usual fairly fined grained and buff or dark brown in colour.
Easy to recognise. A compass was sometimes used in the direction that cross bedding was going to and from.
Some fossils were easy to identify eg crinoid stem and bivalves. Yet, on other fossils we had to use a book to make sure they were correct.
Some had been eroded and weathered away making it difficult to recognise them
How we collected our fieldwork data is shown in the table below.
Analysis and Conclusions
Eastern Exposure, Middle Hope (ST 3375 6645)
Rock Type/ Colour/ Grain size
Sedimentary Structures/ Fossils/ Other
Explanation on Formation
Limestone rock that reacts with HCl could be bioclastic limestone. Grey and fine-course grained
Fragments of shale. Some jointing and lamination. Fossils include crinoid stem and calcite veins also noticeable
Laminations caused by changes in mineral compostion formed during high rapid current flow eg flash flood. Bioclastic limestone with calcite veins; bioclastic formed by a composition of broken fragments of pre-existing limestone, of shell or calcite crystals. Calcite has also being precipitated out forming calcite veins. Crinoid stem present suggests bed was near a reef, in shallow water.
Limestone with some tuff emerging into joints. Tuff also includes calcite veins. Various colours, very fined grained
Jointing, Calcite veins.
Tuff is consolidated ash, which could mean that this bed was near a volcano. Jointing indicates that there has been some surface movement.
Volcanic tuff, buff coloured and fine grained.
Bivalves, sea creatures, faulting and calcite veins.
Tuff, fine grained could mean that the bed was far from the volcano as small particles of rock travel further in the wind or waves.
Tuff, fine grained and dark brown.
Tuff, from a volcano, fined grained indicating far from it.
Tuff, buff in colour.
Cross bedding Northeast from present day and graded bedding.
Cross bedding produced by wind or water current action moving sediment into a series of angled layers. Graded bedding produced when the energy of a current decreases allowing sediment grains to be deposited, with the largest at the bottom and the finest at the top. Both of these sedimentary structures could have been formed at a delta (where a river meets the sea) or submarine fan (at the base of a continental shelf from a result of turbidity currents).
Conglomerate, 25cm=largest boulder and 7-9cm=smallest boulder
Conglomerates are consolidated pebble, gravel or boulder beds, which accumulate along sea or lakeshores and in rivers. They are indicative of shallow water sedimentation and vigorous currents.
Limestone, fine grained, light grey to buff.
Symmetrical ripples, bioturbation and lamination.
Symmetrical ripples in the limestone are formed by two directional current movement over sediment causing sediments to move to and fro creating ridges. These are formed on tidal flats and shoreline sands. Laminations are formed during high/rapid direction of current conditions also which happen on tidal flats. Bioturbation is the disturbance of a sediment by the activities of organisms, often destroying its structure (lamination) see Figure 1
Tuff, Lapilli (volcanic fragments and deposits) course-fine grained.
Banded and quite hard rock.
Consolidated ash is tuff. It can contain fragments up to 64mm in diameter, which are called lapilli. Lapilli may be angular but are commonly spherical. Banding shows differing layers in texture of colour.
Corals, bivalves and slikenslides.
This limestone was probably formed in relatively shallow water as a reef. The evidence for this is, the fossils that are found within it, corals and bivalves. Slickensides happen when the surface of the rock has become polished or striated from the grinding or sliding motion of an adjacent rock mass. Corals live in shallow, clear, oxygenated warm water = reef.
Western Exposure, Middle Hope (ST 3245 6605)
Rock Type/ Colour/ Grain size
Sedimentary Structures/ Fossils/ Other
Explanation on Formation
Bioclastic Limestone, fine to medium grained and grey in colour.
Crinoids, coral, calcite veins, jointing, chert nodules and bands of iron staining
Bioclatic limestone is organic limestone consisting of shell remnants or of calcite depositions. The calcite deposits could explain the veins found in the rock. Crinoids and corals mans the bed could be by a reef in a shallow sea. Chert nodules which are extremely hard microcrystalline consisting of mainly interlocking quartz crystals. Iron staining could be the result of volcanic activity.
Tuff, fine with larger fragments and buff.
Tuff from a volcano. In this bed there are larger fragments indicating it was nearer to the volcano, heavier fragments cannot be carried as far by wind or wave. Weathered tells us that the bed had been left exposed for a period of time.
Lime mud/clay, grey with red discolouration, very fine.
Laminations, calcite veins, weathered shale.
Very fine particle rock with laminations formed thin layers of sediment due to high/rapid current flow conditions.
Crystalline limestone, red.
Large crystals in limestone could have been derived from animal skeletons such as crinoid plates.
Tuff-some layers, fragments. Rock colour is buff.
Tuff, volcanic matter.
Limestone, dark grey and very fine grained.
Hard lamination, bioturbation and calcite veins present.
Hard Lamination could have formed as a product of flash floods on tidal flats. Bioturbation indicates organisms lived nearby, maybe on a reef.
Tuff, dark buff, fine to course grained.
Fine to course grained tuff indicates that the volcano is nearer than other location. The minerals eg calcite or quartz are derived from fluids and are deposited in faults or fractures. See figure 2
Bioclastic limestone, dark grey and fine to course in grain size.
Laminations, fragments of shell and jointing.
Larger grain sizes and fragments of shell in limestone shows near a reef. Lamination also can be caused by flash floods could be in shallow water.
Tuff, fine to course grain size and buff in colour.
Bryozoan, 5cm limestone bed within main bed, calcite veins also present.
Variation of grains in tuff -near volcano. Bryozoan which are marine type creatures that can live at a range of depths, from tide to abyssal. The organism’s skeleton may be calcium carbonate that could describe the calcite veins present.
Limestone, dark grey, very fine (hard rock).
Banded tuff, fossils such as crinoids and bryozoans. Lipilli, jointing and calcite veins as well.
Lipilli, which is ejected from volcanoes, is found here suggesting it must have been near the site (fairly large structure). Jointing indicates that the surface could have moved and crinoids and bryozoans found as fossils could indicate that the bed was near a reef.
Tuff, red in colour, fine grained and softish.
Includes calcite a vein, also rock is weathered.
Tuff from a volcano, weather could mean that the rock had been left for some time. Calcite veins, precipitation of calcite from eg an organism.
Tuff, medium-course and grey.
Too weathered to tell sedimentary structures, but there is bioturbation.
Medium to course grained tuff, closer to the volcano. Another bed that could have been exposed for some time. Bioturbation, creatures living in the environment.
Limestone, fine, medium grained, grey/buff colour.
Calcite veins formed from hydrothermal solutions at a range of temperatures and pressures. The minerals eg calcite or quartz are derived from fluids and are deposited in faults or fractures.
Limestone, mud, soft sandstone. Red and soft and very fine grained.
Laminations, chert nodules/bands. Faulting and asymmetrical ripple marks.
Faulting, the surface has moved. Asymmetrical ripple marks could have been formed by a meandering river or delta. Unidirectional current movement over sediment causing sediments to build up into ridges which tend to have one long shallow slope and one short steep downstream slope. Laminations can also be formed at a delta and chert nodules deposited. The mud and soft sandstone also is deposited near a delta. Or near a tidal flat as there is no coal nearby for it to be a delta.
Limestone, grey/fine-medium grained.
Slumping, Jointing and calcite.
Slumping occurs when ‘sliding or slumping of wet, recently deposited sediment down a slope on the sea floor’. Jointing could have occurred when slumping happened.
Unconsolidated tuff, dark grey.
Unconsolidated tuff means that the tuff is not that well compact, no pressure. The minerals eg calcite or quartz are derived from fluids and are deposited in faults or fractures.
Well-consolidated tuff, dark grey.
Well-consolidated tuff means that some pressure has been forced to make it more compact, could be some faulting in earlier beds. The minerals eg calcite or quartz are derived from fluids and are deposited in faults or fractures.
Basalt, dark in colour.
Pillow lavas formed underneath the surface of water and near a volcano where magma is produced.
Bioclastic limestone, crystalline, grey in colour, medium to angular shaped grains
Jointing, slickensides, corals, bivalves, shell fragments, cross bedding, laminations, amidoles.
All fossils are associated with a reef. Cross bedding produced by wind/water current action moving sediment into a series of angled layers with a main sedimentary bed. Can be caused at a delta where lamination can also occur. Medium to angular shaped grains there has not been much erosion from the elements on the grains. Slickensides are parallel striations on rock surfaces produced by relative motion across opposite sides of fault planes.
The similarities between both log sections are that both contain tuff. Tuff is consolidated ash from a volcano, which tells me that both beds were near the same volcano. One difference is, location two has more beds of tuff than on location one, which suggests that location two was nearer the volcano. The grain size also backs up this theory. Location two contains more variety of grain sizes, from fine to larger. The larger particles are heavier therefore cannot travel as far with waves or wind. Finer grains are deposited further away as they are lighter and more easily carried (by suspension and solution). Another similarity is both sets of beds contain calcite veins running within them that are precipitated directly from seawater. This suggests both beds could have been submerged in water.
Both sets of beds contain lamination showing at one point or several points in the making of these beds that there could have been some kind of flash floods in the area or possibly they were on the same tidal flat.
Differences include: Location one identifies cross bedding and graded bedding in bed 5 and symmetrical ripples in bed 7. This shows me that there could have been a two directional current as that is what is need to form both symmetrical ripples and cross bedding. However, Location two only shows asymmetrical bedding and unidirectional current is needed for that to be produced. Another difference is Location two has pillow lavas in bed 18. This gives more evidence to show that it is nearer to the volcano. Also in bed 19 there is evidence of a slickenside, which could only be produced if a shelf was present.
In Location two there was more bioturbation in the beds, for example bed 6 and bed 12 than in Location one where it was only located in bed 7. This could have because the environment in Location two could have been more susceptible for burrowing to happen. In general Location two shows more evidence of fossils within the beds, this could imply that it was nearer to a reef or a at the base of a slope where organisms fell from. The reef also could have started growing and then get submerged in ash from the nearby volcano and then re-grow, creating more beds with fossils included within them. Location two in beds 2, 3 and 12, also show more weathering. This could suggest that the rock here was softer than rocks in beds in Location 1 where is seems that harder rocks dominate.
This is a diagram of how I would have expected the locations to be positioned.
The type of explosion that could have protruded from the volcano could have been a Phreatic Eruption. “Phreatic” (or steam-blast) eruptions are driven by explosive expanding steam resulting from cold ground or surface water coming into contact with hot rock or magma. The distinguishing feature of phreatic explosions is that they only blast out fragments of preexisting solid rock from the volcanic conduit; no new magma is erupted. Phreatic activity is generally weak which could explain evidence of a none violent volcano. However, they can be quite violent in some cases, such as the 1965 eruption of Taal Volcano, Philippines, or Mount St. Helen’s April 10th 1980.
The good aspects of the fieldwork were that we collected primary data ourselves making it individual. This also makes sure that it was all collected the same way. The negative aspects were we could have got more detail on some of the rock description, as it would have been easier to produce a more exact log.
The things that hampered my data collection were some rocks were difficult to break as they were so hard, some were also being obstructed by larger rocks and it was difficult to get a proper look. Weathered rocks were sometimes difficult by seeing what colour they were and how big the grain size was.
I found some things difficult to do because of some of the weather conditions it was quite hard to identify some rocks, for example because it was a sunny day at location two, light could have been shining on them differently creating shadows. Some locations were difficult to get to. On one of the last beds on location two, we had to climb over the bed to measure the width accurately and also the dip and strike correctly. You had to make sure that you were measuring the angle that there was most off, not a weathered part of the bed. Weathering also made identifying the sedimentary structures quite difficult, for example Location two, bed 2 I didn’t manage to recognize any sedimentary structures.
The only anomalous result I can really spot was Location two and bed 19 with the slickensides. Slickensides are parallel striations on rock surfaces produced by relative motion across opposite sides of fault planes (suggest a phase of early reverse movement was followed by normal brittle fault movement). I don’t think this really affected my final conclusion, however, because that was the only one I saw in both the locations, maybe further on there would be more evidence of them.
I think my data was as accurate as possible. As I did all the dip and strike, I repeated them all the same. However, I could have repeated them a second time to find out the average dip and strike. I could have been more accurate in how the fossils were placed in the bed. For example the percentage amount in which they took up, if there were a few grouped together and well-preserved then it could be exceptional preservation, conservation.
I think my data is fairly reliable. If I were to go back to the same location I would probably gather the same information. The dips and strikes could vary a little because of the different place I would take them from.
The limitations of the techniques used included bumpy surfaces where we might have no t got the right measurements. Not enough measurements where we could have taken an average on our findings. This would have made it more reliable. On the first day at Location One the weather was fairly cold and sometimes drizzly. This could affect our results by rushing them and not measuring or looking at the beds correctly.
If I had the chance to change my investigation I would make the techniques I used better. For example making my results more accurate, repeating them to make sure they are the same. Also better logging techniques, maybe recording my about the structure of the beds.
If I were to extend my study I would repeat the measurements making them more accurate. I would also include more locations to find out where the volcano did actually sit in the Somerset countryside.
Minerals, Rocks and Fossils….W.R Hamilton, A.R Wooley and A.C Bishop
Collins Dictionary Geology….Dr James MacDonald and Dr Christopher Burton
British Regional Geology Bristol and Gloucester District…..G.A Kellaway and F.B.A Welsh