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The Glacial History Of Thurstaston, Wirral, And The Surrounding Areas

A study into the glacial processes around these areas in the north west looking at physical signals for the movement and depth of the glaciation.

Date : 08/08/2014

Michael

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Uploaded by : Michael
Uploaded on : 08/08/2014
Subject : Geography

The aim of this report is to use the information gained from the sites in Thurstaston and the Wirral to calculate if the area was indeed covered by a glacier and if so from which direction did the glacier approach the area. The methods used were clast fabric analysis including their size and shape, erratics, till structures, field sketches and analysis used with information from further data sources about the glacial history of the area and the geology of the UK. This paper will try determine if there is a relationship between the rocks found in the till at the sites in the Wirral and the direction of the glacier which once cover this area and that is these two match. There may be issues with some of the data such as the face of the cliff area which was being tested could have slumped or malformed over time meaning the particles inside of this face area would have moved out of their original position giving either a false positive or just a negative result. This is why the data which was gathered needs to be compared against other research and why on one test site in one day you cannot guarantee a high level of accuracy due to the limitations of time and equipment. There were several different methods that were used at Thurstaston beach (Site 1) and at Thor`s rock and Thurstaston common (Site 2). In site one the methods that were used are photographs, sketching and logging the sections along the cliffs at the sites. This meant that we had both records of the cliff using photographs which would not be influenced by memory, sketching that meant anything that was drawn to the eye during our initial look at the cliff face would have been recorded and therefore not missed when later reviewing the photographs and the log sections which give a more detailed sediment analysis as a whole on part of the cliff. This meant that in conjuncture with the photographs and sketches the sediment logs could be compared with them highlighting any differences in grain size and boundaries. Till fabric analysis was also conducted on the cliff face which was photographed and logged so that the angle of orientation and the angle of the dip would be accurately recorded. The minimum amount of clasts that should have been recorded was 20 but in better conditions 30 would be a better amount to have surveyed. Clast size and shape analysis was also recorded so that the sorting of the till could be taken this was important as the variation in the sorting of the till could lead the investigation on another route of the areas movement from glacial pressure (Benn, 2007). The roundness and clast texture was important also because the roundness of the clasts could indicate how much pressure they have been under and how long they have been in that situation. This could indicate how these clasts have ended up in this area. Clast lithology is a crucial input because using this there can be some interpretations of were these rock types had come from. For example if this rock type was not native to the local area it must have been transported in and if you can match the rock type to a different area you can hypothesize that there might be a relation. At site 2 on Thurstaston common the main method was observation and recording 8392690 any features that were visible to use such as fissures and differences in the rock. Also striations which were visible to the eye but this was less reliable because of the weathering of any exposed rock with may make the striations less prominent. Another method that was used on top of Thor`s rock was taking measurements of all the channels which were engraved into the rock. To record these the channels included direction, angle, width and length this meant at the end of the process there was a full sketch with detailed measurements providing a map of the channels which can be found in figure 6. Thurstaston Beach (Site 1) Figure 1 (The data collected from Thurstaston beach) Sample Dip direction Dip angle Axis A (CM) B C Roundness Striated Y or N Lithology 1 218 51 2.5 2.3 1.8 Sub-rounded N Mudstone 2 29 48 5.4 4.3 3.5 Angular N Mudstone 3 31 39 3.2 3 2.8 Sub-Angular N Sandstone 4 30 37 3.3 2.9 1.1 Sub-rounded Y Granite 5 45 47 4.7 3.5 1.2 Sub-rounded N Sandstone 6 210 50 4.7 2 0.3 Angular Y Quartz 7 220 48 7.3 5.7 3.1 Rounded N Quartz 8 200 52 2.3 1.7 1 Sub-angular N Sandstone 9 211 55 7.8 4.3 3.3 Angular N Mudstone 10 278 42 30 23 18 Sub-angular N Basalt 11 279 61 44 32 26 Sub-angular N Mudstone 12 64 6 85 69 67 Sub-rounded Y Basalt 13 120 4 125 85 45 Sub-rounded N Andesite 14 125 8 20 18 12 Rounded N Quartz 15 150 80 32 15 12 Sub-rounded Y Sandstone 16 190 28 50 35 28 Sub-angular N Mudstone 17 145 22 30 38 17 Sub-angular N Mudstone 18 143 12 32 18 8 Sub-rounded N Sandstone 19 140 3 45 38 14 Sub-rounded Y Sandstone 20 125 24 35 24 12 Rounded N Quartz 21 130 46 54 38 24 Sub-rounded N Basalt 8392690 Figure 2 (Tri plot shape) Figure 3 (Stereo 32 data in projection display) c : a b : a (a - b) / (a - c) 0°90°180°270°Equal angle projection, lower hemisphereN total = 21n=21 (linear)Stereo32, Unregistered Version 8392690 Figure 4 (Stereo 32 density projection of data) Figure 5 (Stereo 32 rose diagram of data) 0° 90° 180° 270° Equal angle projection, lower hemisphere N = 21 Maximum density = 4.71 Minimum density = 0.02 Mean density = 1.00 Density calculation: Cosine sums Cosine exponent = 20 Contour intervals = 10 From minimum to maximum Stereo32, Unregistered Version 0° 90° 180° 270° Dip direction (10° classes) N = 21 Maximum = 3.0 Stereo32, Unregistered Version 8392690 Photo 1 (Picture of the cliff where the study took place) Photo 2 (A large boulder in till on the beach) 8392690 Discussion Figure 1 shows the dip direction and angle of the clasts along with the clast sizes also the roundness of the clasts the dip direction doesn`t show a strong correlation amongst the different clasts. This might be because of a cliff slump or collapse or because of differences in how accurate the clasts were measured by different members of the same team. Figure 2 shows the tri plot of the data from figure one the tri plot appears to show a frost shattering correlation (Benn, 2007) which may just show that the rocks have just been exposed and then weathered. Figure 3 is the stereo 32 interpretation of the data from figure 1 showing all of the data entry points exactly. There does appear to be a small correlation towards the south east but there is also a cluster towards the south east and the north west. Figure 4 and 5 are just alter versions of the stereo 32 interpretation of the data to show a more visual aspect of the way the data relates to each other. On Figure 4 you can see how `hot` areas are with the amount of data points and in figure 5 you can see the amount of data points which indicate direction from the input. This makes the data easier to interpretation. Photo 1 shows the cliff face which was sampled for the study this shows the different layers and sizes of the clasts in the cliff face. There are several different layers and textures of sediments that are visible in the photo. Photo 2 shows a boulder which is suspended in the till this shows the poorly sorted nature of some of the areas of the till. At site 1 there was a mixture of different lithology found from mudstone and sandstone to harder crystallised substances such as quartz and granite (figure 1). This means that there has been some transportation from other areas around the area. Most likely these igneous rocks were transported from either the Scottish highlands or the north of Ireland as these two areas both have a high amount of native igneous rocks (OpenLearn, 2006). This could be an indication of the direction of movement of the glacier either from the north from Scotland to the south or from the northwest from Northern Ireland towards the South east. This could be used in conjuncture with the figures 3, 4 and 5 which show and slight preference to south east movement which may show a relationship in the movement of rock type which is from northern Ireland and the seabed in the Irish sea. This also corresponds with other studies which show that glacial movement over the area came from across the Irish sea in a south east direction (Eyles, 1989). There are also though other areas of clasts which show different dip angles which could indicate that a cliff movement has happened or that the clasts have somehow become dislodged in the till which has sent all of the clasts off from their original position this could either give a false positive or a negative (Millar, 2001). This could mean that these clasts provide no useful data or it could means that there has been a slight movement in the clasts which could be expected as the surrounding rebound from the lack of weight which the glacier once provided (Sharp, 1982) which could gently shift some of the clasts which were not firmly in place. Also as the only clasts which were tested were the ones which were already exposed on the cliff face there could of been some slipping from weather such as rainfall lubricating and softening the till making some of the larger clasts move slightly. In photo 1 you can see the amount of 8392690 variation in layers on the till. This could mean that there has been several different periods of glaciations in this area (Lee, 2014) or that when the area was uncovered by the glacier it was either underneath the sea which would explain the sand layers and sediment layers in the till but there has defiantly not been any glaciations in recent geological history as the large layer of natural material which is at the top of the till which must have collected over some time from natural decaying material such as trees and other plant matter creating this large muddy layer at the top of the till. In this section there is also very few clasts which shows a low amount of transportation in this period another property of material decaying instead of transportation action from rivers or glacial movement for example. Thurstaston Common and Thor`s Rock (Site 2) At the second site the data collection was different because instead of finding clasts in the cliff face there was geological formations at surface level such as striations in the rocks at ground levels and marking on Thor`s rock such as channels and scallops which are curved indents on the side of Thor`s rock. These could be scallops or they could be the starting points of more channels as scallops are normally much bigger than the ones found on Thor`s rocks. On Thor`s rock there are only scallops on the southern side of the rock formation which could mean that this is the direction the water was moving over the rock this means the glacier must of been relatively warm and melted on regular occasions only to be replaced by extra snow impaction in the winters (Glasser, 1998). Figure 6 shows all the Nye channels on Thor`s rock which mainly seem to feed into the plunge pool at the top of the rock. There are many different theory`s of how the channels were created in Thor`s rock the main accepted theory is that whilst underneath the glacier high pressure melt water moving under the glacier hit Thor`s rock and created these channels going over the rock. As the water was under so much pressure it could also move against the force of gravity which is why the channels do not always move downwards which is shown on figure 6 this would also mean that there must of been a large amount of ice above the area because the amount of pressure the water would of had to have been under would of been very large indicating a large amount of glaciations above this area. Another theory of how the channels were created at Thor`s rock is that children have created these channels by climbing over the rock. This is possible because the area has been popular with people trying to climb up and around the rock for many years which could of caused some erosion on the rock. The channels are very small in areas which is why the theory states that it would of been children climbing over the rock to create these channels. There any many problems with this theory such as the fact how children would of been able to climb around and over this rock if there were no already excising channels to gain a foothold upon. Also these channels do not follow the easiest routes of climbing this rock. Also why the channels would dip up and down even when the surrounding rock is flat. If these were indeed channels created by children walking around on them it would be 8392690 relatively flat. The idea of children walking on the rock would also not explain the plunge pool in the centre of the rock which could only be created by the children if they were all stood together rubbing their feet on the floor for a very long period of time that they spent on the rock which would seem an unreasonable thing to do. Also the reasoning of high grip trainers as the reason for the high rate of erosion caused by the children (Flinn, 1998) seems to be a poor reasoning as most trainers are actually quite low grip as they are not actually designed for running or exercise but simple for style. Figure 6 (Annotated drawing of Thor`s rock) 8392690 Limitations There are several limitations of this report most of which result from the gathering of the data. None of the students taking this data were experts nor experienced in taking this data which means inaccuracies would be common. This means that some students may have better data taking skills then others but because the data gathering was done in groups this means that the data could be mixed and therefore data which gives a good result would be mixed in with other data which was poorly taken. Another problem was the weather on the day of data gathering which was very poor as it was constantly raining as most of the data gathering was done outdoors and using paper and pens the data gathering was compromised as a lot of the recording were damaged by the rain which may mean that some of the data was misread. Other limitations were the graffiti on Thor`s rock which could mean certain areas were eroded out by people writing their names on the rock. This means the channel dimensions and depth could also of been further eroded from their original state but not significantly. Conclusion The area of the Wirral was clearly under several periods of glaciations over time. The most recent of which must have come from either the north or the north west which was shown in the data both from the rocks which had been transported and from the loose coordination of the dip angles from the clasts in the till on Thurstaston beach. Also because of the alignment of the channels in Thor`s rock also shows a north west approach from the glacier which would also indicate the glacier moving over the marine area of the Irish sea from northern Ireland and then onto the Wirral and the rest of Ireland. Also the amount of pressure required from the melt water to gouge the channels into Thor`s rock must mean there was a large body of ice above the area which was applying the pressure on the water. Although there was no strong correlation in the data which was collected looked at the minor links in the data which was collected in conjuncture with other research on the glacial history of the area there are links to be made such as the glacier moving over the Irish sea to the Wirral area. 8392690 References Benn, D. (2007) `Glacial landforms, sediments.` Clast Form Analysis, 4(1), p904-p909. Eyles, N . (1989) THE LATE DEVENSIAN. Quaternary Science Reviews. 6 (1), p307-p351. Flinn, D. (1998) `Subglacial meltwater channels at Thurstaston Hill, Wirral and their significance for Late Devensian ice sheet dynamics.` Proceedings of the Geologists` Association, 109(1), pp. 139-148 Glasser, N. (1998) `Subglacial meltwater channels at Thurstaston Hill, Wirral and their significance for Late Devensian ice sheet dynamics.` Proceedings of the Geologists` Association. 109(1), pp.139-148 Lee, J. (2014) Ice ages and ice sheets. Available: http://www.bgs.ac.uk/research/climatechange/environment/iceAges.html. Last accessed 20/03/2014. Millar, S. (2001) `Sampling-surface orientation and clast macrofabric in periglacial colluvium.` Earth Surface Processes and Landforms, 26 (11), pp.523-529 OpenLearn Team. (2006) UK rocks by region. Available: http://www.open.edu/openlearn/science-maths-technology/science/geology/uk-rocks-region. Last accessed 20/03/2014. Sharp, M. (1982) `Modification of clasts in lodgement tills by glacial erosion.` Journal of Glaciology, 28(100), pp.475-481 Word count: 2835

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