Fig.5.
Waves are very good at finding weaknesses in the cliff face. Any crack or line of weakness will be remorselessly attacked by destructive waves. It is at the vertical cracks in the cliff face that the waves form the next feature of coastal erosion. They grind away at the crack constantly chipping pieces off until a cave is formed. This cave grows bigger and bigger and once it is of a large enough size the waves are able to block off the entrance as they hit, trapping air inside. This creates great amounts of pressure inside the cave and eventually the back of the cave is pushed through into perhaps another cave formed in the same way on the other side of the headland and an arch is formed across the headland. The arch is then gradually enlarged by the effect of erosion and some weathering, until it can no longer support itself and collapses. This leaves behind a freestanding pillar of rock; a stack. As this process then begins again in the cliff face, the stack itself is attacked by the waves from all sides. It is reduced in size becoming a stump and eventually collapses into the sea (see figure 6).
Fig.6.
Study Site:
Marloes Sands is a beach area located on the south west coast of Wales in Pembrokeshire (see figures 7 and 8).
Fig.7. (Diagram of Wales with Marloes indicated) Fig.8.
The beach itself is about 3000km long from Gateholm Island (shown in figures 9 and 10) at the north west end of the beach to Hoopers Point at the south east end. There are three other notable features along the beach which are Matthew's Slade, Three Chimneys and The Valley access where the stream meets the beach. These will be shown on later figures.
Fig.9. Fig.10.
Most of the cliffs at Marloes are made up of just three types of rock, these are shales, mudstones and sandstones. These were formed between 395 and 440 million years ago during the Silurian age. The rocks at either end of the beach (Old Red Sandstones, Marls and Conglomerates) were formed as the ancient North America and Europe collided. The mountains that were formed here, called the Caledonian mountains run across from south west to north east across what is now the peninsula of St David's head as shown in figure 11. The material eroded from these mountains was deposited further south.
Fig 11. Approximate Location of ancient mountains relative to present day Pembrokeshire
0 10km
N
About 290 million years ago the African Plate collided with the European plate forming the Armorican mountains (also shown in figure 11). These crossed today's Wales below the Caledonian mountains from northwest to southeast. This resulted in the rocks in the area being folded further. This upturned the beds of Old Red Sandstone and distorted the older mudstones, shales and sandstones so that the Old Red sandstones seem to surround them. This is shown at Marloes where the cliff face is distinctly red at the ends and lighter brown in the middle (as shown in figure 12) This makes it look as though the old red sandstone was deposited both before and after the other rocks but in reality what can be seen is just a part of a much larger fold which has been worn away by erosion as shown in figure 13. This shows that the Old Red Sandstone and conglomerates at the ends of the beach are relatively more resistant.
Fig.12.
The most recent mountains formed were the Alpine mountains around the plates of southern Europe but have only slightly folded the Pembrokeshire Mountains further.
Fig.13. The possible effect of folding during the formation of the Armorican Mountains
(Hypothetical cross-section looking NE around Marloes Sands)
Expectations:
After looking in detail at my theory and the model of erosion alongside the geology of the study site I can now state what I think I will find there. I think that the general size of the stacks on this beach would be smaller in comparison to stacks on a beach on the east coast due to the strength of the wind. There may be more stacks nearer the cliff at the ends of the beach because the older rocks in the middle seem to be eroded faster than the old red sandstone at the ends. This would also mean that there would be more, smaller stacks at the ends as well. The stacks furthest out from the cliff face will be shorter than those further in all along the beach in general as this will occur in any circumstance. The smallest stacks out from the cliff will be at the southeast end due to the wind shadow from Gateholm Island though. The shortest stacks should be the ones which have been exposed to wind and wave action for the longest but due to the circumstances on this beach there may be stacks at the northwest end of the beach which came into existence at the same time as stacks at the opposite end of the beach but now still exist when the southeast ones don’t.
Hypothesis:
The main hypothesis is, " The further from the cliff the shorter the stacks". (Figure 14) This taken from the model of coastal erosion as the last feature formed which then shrinks as it is attacked by wave power.
Fig.14.
This goes deeper than just this though, if proved correct there must be more reasons than one for some stacks being shorted than others and there will be variation from one end of the beach to the other. These factors may include the resistance/composition of the rock and amount of bedding and jointing (discontinuities) within them.
Methodology:
To test this hypothesis mainly primary data and some secondary data will be used. Evidence will be collected on the beach as primary data and the background information was researched on the Internet as secondary data.
The data needed to prove or disprove this hypothesis will be;
- distance of stack from cliff,
- height of stack,
- hardness of rock,
- average discontinuities per meter,
- location.
To find the distance of stacks from the cliff face a tape measure will be used to begin with. This may prove to be inaccurate due to the uneven ground. This may happen as the tape measure would follow the uneven surface and read a longer distance. A pacing method may then be more accurate. The number of paces to 100m will be recorded and the same person will then pace from each stack to the cliff (figure 15). This will then be converted to meters.
(Neither method was significantly more accurate but the pacing technique was quicker so this was used so as to fit more stacks in to the time given)
Fig.15.
To find the height of stacks a clinometer will be used to measure the angle. Standing 6m from the stack on level ground the angle will be measured using the clinometer (figure 16). The same person will measure every stack and will stand as straight as possible every time to keep the results accurate. This angle and the 6m distance will then be substituted into the trigonometry "Tan" ratio to work out the height (figure 16). The height of the person measuring (up to eye level) will then be added to give the actual height of each stack (figure 17).
Fig.16.
Fig.17.
To find the hardness of each sample point four objects will be used in a scratch test. A fingernail, a copper coin, an iron nail and a steel blade will be scratched on the surface of each stack. The dust will then be rubbed away and if a mark has been left it will be recorded on a tick sheet (figure 18).
Fig.18.
To find the average discontinuities per meter a stretch of each stack will be measured in meters (with a 2m ranging pole) and recorded, the number of beds and joints will be counted and recorded also (figure 19).
Fig.19.
To sample the stacks information will be taken from every stack available, as they cannot be sampled in places they do not exist. When a stack is found, the distance across the beach from the last stack will be measured using pacing and this will be recorded. This will then be easy to translate to a position on the map later on. Each stack will be given a number as it is sampled and this number will be written on the map next to the stack in question.
Flow Diagram Of Our Process of Work
Data Presentation and Analyses:
Fig.20.
This table is an accumulation of all of our data. It has the data for each stack in line with its corresponding number.
Following are a collection of graphs drawn from the results collected in the table above, they present the data in different ways to show different things. The graph (figure 21) overleaf covers a double page spread and shows the position of all but the last 5 stacks in the table. These were not included in this graph because they were so far away from the rest of the stacks that the graph would not show the positioning very accuratly, as the scale would be so small. The 0 line in the middle of the graph shows the point at which we came onto the beach at Matthews Slade and then the axis run right and left from there showing the distance to the north-west and to the south-east of matthews Slade. The Y axis shows the distance of each stack from the cliff face. Each stack is marked on the graph using two keys. The colour of each point indicates the height interval that it falls into and the shape indicates the average number of discontinuities interval. The graph clearly shows that there are more stacks further from the cliff closer to the south-east end of the beach, there are two other large groups of stacks as the graph moves to the north-west and there are gaps with few or no stacks in between these groups. Most stacks seem to be red coloured showing that most stacks are taller than 0m but shorter than 2m. There is a relative scattering of orange stacks but all stacks over 4m (blue or purple) are found less than 34m from the beach where the axis stretches to 80m.
Fig.21. Location of Stacks
This set of graphs (figure 22) shows the height of the stacks, to the southeast of Matthews Slade, in relation to their distance from the cliff face. The five graphs show the stacks in groups which relate to their position along the beach.
Graph 1 (0-100m) shows that all stacks in this area are less than 50m from the cliff and the tallest stack, at 4.25m is the second closest to the cliff. The shortest stack, at 1.34m is the furthest from the cliff. This proves well my theory that the stacks further from the cliff should be shorter than those closer to it. The stacks inbetween the tallest and shortest follow the pattern to some extent. There is rough negative correlation between the variable with one or two anomalies such as the closest stack to the cliff face being only 1.97m high.
Graph 2 (101-200m) shows the same correlation as shown in the graph preceding it but in a clearer light with fewer anomalies. The tallest stack (again second from the cliff) is 5.3m and the stack before it (closest to the cliff) is 4.88m, only slightly shorter. The furthest stack from the cliff face is not here the shortest. There are in fact quite a few shorter in this section but the general view of it shows that this may merely be an anomaly as the others all seem to fit in line with a rough line of best fit.
Graphs 3 and 4 (201-300m, 301-400m) each have only one data point on them. There seems to have only been two stacks present over this 200m stretch of beach. Refering back to the data tables above, I can see that there are in fact two stacks at 37m from the cliff and 1.45m high on graph 4 but this still only leaves 3 stacks over this great expance of beach. Thinking back to the study site there was a large area of flat sand between the center group of stacks and the group to the south-east end of the beach (shown in figure 21). It therefore concludes that there were no stacks to be recorded in this area rather than that there were stacks, which were not included in the data.
Graph 5 (401-500m) seems to include the large group of stacks at the southeast end of the beach (shown in figure 21). The pattern shown does not seem to fit in with my theory at the moment. In fact there does not seem to be a specific pattern at this point on the beach. The distribution is random. The tallest stack is 33m from the cliff and the shortest is19m from the cliff. All other stacks shown range between these, in both height and distance from the cliff, with no sign of relation to each other. This does not fit my theory regarding the length of timethe stacks had been venerable to wind/wave action but may fit in with the theory about resistance/composition of rock. This may explain the abundance of taller stacks in this section. As the coastline is discordant it would seem logical to assume that the bed of rock between 401 and 500m southeast from Matthews Slade is more resistant than those before it. So therefore there are more stacks in this area.
Fig.23. Height. vs. Distance Across Beach
This set of graphs (figure 23) shows the height of the stacks against their relative distances southeast from Matthews Slade. This is separated into the four graphs each showing a section of the beach of a specific distance from the cliff. These distances are split into 0-20m, 21-40m, 41-60m and 61-80m.
Graph 1 (Cliff-20m) shows that there are a lot of stacks in this area closest to the cliff. This shows that these stacks, not having been exposed to the wave power for as long as ones further out would have been have lasted where those that were further from the cliff havn't. This graph also shows that the stacks are in two main areas at either end of the graph. These areas relate to the central group of stacks near to Matthews Slade and the next large group of stacks to the southeast (shown in figure 21). Five of the stacks on this graph are more than 3m high.
Graph 2 (21-40m) shows fewer stacks than in the graph before as it is further from the cliff face. The stacks on this graph still fall into the two groups mentioned above but the groups are not obvious as the stacks are so much fewer and far stretched. They could just as easily be positioned all over the graph with no pattern but we know that they follow the pattern because we can relate it to the graph above. Three of the stacks on this graph are taller than 3m.
Graph 3 (41-60m) shows about the same number of stacks at this distance as at the distance before. These are much more clumped together though; there is one major clump around 150m across from Matthews Slade and another just after 400m. These correlate with the groups shown in figure 21 again. There is only one stack taller than 3m on this graph.
Graph 4 (61-80m) shows only four stacks. These are collected at the southeast end of the graph. These stacks are furthest out from the cliff and are collected in one area again supporting the theory on resisance and rock composition. This shows that there is probably a band of more resistant rock in this part of the coastline. There are no stacks taller than 3m on this graph.
The specific notes taken on the number of stacks over 3m high from the four graphs show that the number of stacks over 3m high begin high closest to the cliff and then get lower as the graphs move further from the cliff.
