Report of the Geology Studied in South Wales on the First Year Field Trip

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Report of the Geology Studied in South Wales on the First Year Field Trip

            The geology studied on the field trip to South West Wales was based on the coastal locations. This is because the coast consists of exposed rocks due to marine erosion. The coast therefore provides a cross section into some geological structures and stratigraphical sequences. Study of the coast showed a large variety of different rock types which can be grouped into rocks of similar geological period. These different rock types could in many places be seen in a recognisable stratigraphic sequence. These sequences had in most cases been re-orientated, folded and faulted, but still super positional younging in one direction was still valid. I will therefore describe the geology in order of geological age. This will mean the may jump from site to site, as the whole geological record of the area could not be viewed in one trip. The location of the sites is shown on the map at the end of this report.

            The ages of the rocks studied varied from the most recent period in the area, the Carboniferous to the most ancient in the area, the Precambrian.

            No close up examination of Cambrian or pre-Cambrian rock landscapes or beds took place. However at Settling Nose, just north of Broadhaven Cambrian and pre-Cambrian rock could be observed to the north, stretching west (Cambrian) to east (pre-Cambrian). These are volcanic rocks which form the majority of the headland to the north, St. David’s Head. However several boulder sized Pre-Cambrian rock pieces were found at Settling Noses. These rocks were green and had rhombic crystals under examination. The rock is therefore classified as rhom porphyry. This rock is described as an erratic as it is so dissimilar to the rock type of the landscape it is found in. A glacier has most likely moved it, as the only other possible mechanism for movement is a river, which is unlikely, as it would have been sorted to the bottom of a fluvial deposit. From Settling Nose a dark conical rise can b seen someway into the headland to the north. This is a resistant gabbro intrusion, that has now been exposed. This is of Ordovician age, and is part of a large gabbroitic intrusive system than spreads through St. David’s Head. This area was studied the second day of the trip.

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            Originally during the Ordovician, shales were layered down. Later intrusion of basic igneous material experienced slow rates of cooling, resulted in the formation of gabbro. In the field this was represented by a visible contrast in rock type, with grey, black (when freshly exposed) large grained gabbro against very fine (non visible through eye lens) grained and thinly bedded shale. Also the shale in contact with the gabbro had a baked appearance, which points to liquid igneous intrusion into the shale. Comparison of dip of several areas of shale on St. David’s Head shows large variations, which points to ...

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