Virtual Field Trip

Virtual Field Trip

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 recognizable stratigraphic sequence. These sequences had in most cases been re-orientated, folded and faulted, but still super positional youngling 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.

Originally during the Ordovician, shale's 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 folding and faulting at one point. This is a result of the collision between Scotland, south and north Britain during the late Silurian and the early Devonian.

The next period to consider is the Silurian. This was studied at Marloes Sands. The coast here was studied at five main sites, A to E. Along the coast here the strata has been turned on it's side, so the bedding planes are almost perpendicular to the ground surface. However this stretch of coast has been faulted to an extent during the late carboniferous, through the collision of Europe and Britain, so the strata do not systematically from north to south.

However there is only minor faulting from location B to E. Location B is of an igneous rock type. There are several pieces of evidence for this. Firstly small dark spheres in the rock here indicate gas bubbles that were trapped when then rock solidified, and then were infilled by other minerals. The second piece of evidence is the colouration of some of the rock. Although the majority of the rock is dark green, some patches were red/brown. The green colour originates from the mineral epidote, however the red results from the oxidation of the iron bearing minerals to haematite mineral. This weathering and the fine grain of the rock indicate that the rock is basaltic, and therefore volcanic.

The next younger rock in the Silurian sequence can be found at location C. This is also volcanic, but of a ...